QUEENSLAND UNIVERSITY OF TECHNOLOGY

SCHOOL OF PHYSICAL AND CHEMICAL SCIENCES

MEASUREMENT OF 222Rn EXHALATION RATES AND

210Pb DEPOSITION RATES IN A TROPICAL

ENVIRONMENT

Submitted by Cameron Lawrence (B. App. Sc., M. App. Sc.) to the School of Physical and Chemical Sciences, Queensland University of Technology, in partial fulfilment of the requirements of the degree of Doctor of Philosophy.

March 2005 Key Words

Radon, exhalation, emission, Lead-210, deposition, excess, redistribution, budget, Kakadu, Ranger, uranium, mining, radionuclides, isotopes, soil moisture, radium, activity concentration, land application, soil erosion, atmospheric transport, geomorphic landscapes, tropics, Alligator Rivers Region, environmental radioactivity, Jabiru, atmospheric dispersion, soil profile, diurnal, seasonality, wet season, dry season, precipitation scavenging, aerosol transport, aerosol removal, Hadley circulation, water inundation

i Acknowledgements I owe great thanks to my supervisor Dr. Riaz Akber for all his support and effort during the course of this project, his assistance and direction has been invaluable. Many thanks also go to my external supervisor and the other support staff of the Enrad group at eriss, Dr. Paul Martin, Dr. Andreas Bolhöfer, Mr. Bruce Ryan, Mrs. Therese Fox and Mr. Peter Medley, for their countless hours of assistance, sample analysis and data retrieval. I also owe many thanks to the remainder of the eriss team, especially the Jabiru Field Station, for their support during my time in Jabiru. Eriss provided my accommodation and all work facilities for the 20 months of my stay at Jabiru and for that I am gratefully appreciative. Major parts of this project would not have been possible without the assistance of ERA personnel, specifically Mr. Ian Marshman, for arranging access to the Ranger sampling locations. Special thanks go to my father, Eoin Lawrence, for all his support during my studies over the years. His support has fantastic providing me with sound advice in all the major decisions I’ve had to make. I only hope that I can continue to live up to his expectations as I enter the next phase of this life. Most of all I am pleased to have the support and love of my partner Saski who has a direct understanding of the personal commitments required to complete this work. Her support over the last year in all things has been phenomenal and I look forward to providing her the same support in all matters in her life.

ii Abstract This thesis provides the measurements of 222Rn exhalation rates, 210Pb deposition rates and excess 210Pb inventories for locations in and around Ranger Uranium Mine and Jabiru located within Kakadu National Park, Australia. Radon-222 is part of the natural 238U series decay chain and the only gas to be found in the series under normal conditions. Part of the natural redistribution of 222Rn in the environment is a portion exhales from the ground and disperses into the atmosphere. Here it decays via a series of short-lived progeny, that attach themselves to aerosol particles, to the 210 210 long lived isotope Pb (T1/2 = 22.3 y). Attached and unattached Pb is removed from the atmosphere through wet and dry deposition and deposited on the surface of the earth, the fraction deposited on soils is gradually transported through the soil and can create a depth profile of 210Pb. Here it decays to the stable isotope 206Pb completing the 238U series. Measurements of 222Rn exhalation rates and 210Pb deposition rates were performed over complete seasonal cycles, August 2002 – July 2003 and May 2003 – May 2004 respectively. The area is categorised as wet and dry tropics and it experiences two distinct seasonal patterns, a dry season (May-October) with little or no precipitation events and a wet season (December-March) with almost daily precipitation and monsoonal troughs. November and April are regarded as transitional months. As the natural processes of 222Rn exhalation and 210Pb deposition are heavily influenced by soil moisture and precipitation respectively, seasonal variations in the exhalation and deposition rates were expected. It was observed that 222Rn exhalation rates decreased throughout the wet season when the increase in soil moisture retarded exhalation. Lead-210 deposition peaked throughout the wet season as precipitation is the major scavenging process of this isotope from the atmosphere. Radon-222 is influenced by other parameters such as 226Ra activity concentration and distribution, soil porosity and grain size. With the removal of the influence of soil moisture during the dry season it was possible to examine the effect of these other variables in a more comprehensive manner. This resulted in categorisation of geomorphic landscapes from which the 222Rn exhalation rate to 226Ra activity concentration ratios were similar during the dry season. These results can be extended to estimate dry season 222Rn exhalation rates from tropical locations from a measurement of 226Ra activity concentration.

iii Through modelling the 210Pb budget on local and regional scales it was observed that there is a net loss of 210Pb from the region, the majority of which occurs during the dry season. This has been attributed to the fact that 210Pb attached to aerosols is transported great distance with the prevailing trade winds created by a Hadley Circulation cell predominant during the dry season (winter) months. By including the influence of factors such as water inundation and natural 210Pb redistribution in the soil wet season budgeting of 210Pb on local and regional scales gave very good results.

iv Contents

Chapter 1: Introduction 1 1.1 Overview 1 1.2 Alligator Rivers Region 6 1.3 Project objectives 9

Chapter 2: Literature Review: Previous research in relation to radon emanation, migration, exhalation and 210Pb deposition 10 2.1 Overview 10 2.2 Radon emanation 10 2.2.1 Introduction 10 2.2.2 Radon emanation and radium distribution 12 2.2.3 Radon emanation and soil moisture 14 2.2.4 Radon emanation, soil porosity and grain size 17 2.2.5 Radon emanation, pore size and number 18 2.2.6 Radon emanation and soil temperature 20 2.2.7 Variations in emanation coefficients for radon isotopes 21 2.3 Radon migration, exhalation and soil gas concentration 22 2.3.1 Introduction 22 2.3.2 Radon exhalation measurements techniques 24 2.3.3 Radon exhalation surveys 24 2.3.4 Radon migration, exhalation, soil gas concentration and soil moisture 29 2.3.5 Radon exhalation, soil gas concentration and atmospheric pressure 31 2.3.6 Radon exhalation, soil gas concentration and temperature 32 2.3.7 Radon exhalation, soil gas concentration and wind speed 33 2.3.8 Radon diffusion theory 33 2.3.9 Radon exhalation temporal variations 36 2.3.10 Radon migration, exhalation and soil gas concentration summary 38 2.4 Pb-210 deposition 39 2.4.1 Introduction 39 2.4.2 Pb-210 depositional rate studies 41 2.4.3 Pb-210 soil studies 45 2.4.4 Pb-210 deposition and geographical location 49 2.4.5 Pb-210 atmospheric concentration studies 50 2.4.6 Pb-210 summary 51

v 2.5 Chapter summary 52

Chapter 3: Project location, site selection and measurement schedules 55 3.1 Overview 55 3.2 Exhalation from open ground – Investigation of physical parameters [226Ra activity concentration, distribution in grains, grain size and porosity] 55 3.2.1 Ranger operations 55 3.2.2 Ranger site selection 60 3.2.3 Ranger measurement schedule 63 3.3 Seasonal and diurnal radon exhalation [moisture, pressure and temperature] 66 3.3.1 Site selection 66 3.3.2 Seasonal site measurement schedule 70 3.3.3 Diurnal measurement schedule 71 210 3.4 Excess Pb soil sampling 72 3.5 Pb-210 deposition sampling 76

Chapter 4: Methodology 77 4.1 Overview 77 4.2 Available techniques for radon exhalation measurements 78 4.3 Radon exhalation measurement with charcoal canisters 79 4.3.1 Charcoal canister counting system, calibration & efficiency 82 4.4 Radon emanometers 83 4.4.1 Emanometer calibration 87 4.4.2 Associated emanometer measurements 88 4.5 Soil moisture readings 89 4.6 Soil activity concentration measurements 91 4.6.1 Geofizika GS-512 portable gamma detector 91 4.6.2 Determination of 226Ra from gamma dose rates 93 4.6.3 Soil sampling and preparation 94 4.6.4 Excess 210Pb analysis of soil samples 97 4.7 Pb-210 deposition measurement 97 4.8 HPGe gamma spectroscopic system 99 4.8.1 Calibration of spectroscopy system for project samples 102 Chapter 5: Radon sources 107

vi 5.1 Overview 107 226 5.2 Rn-222 exhalation rate and Ra activity 107 5.3 Diurnal measurements of radon exhalation 128 5.4 Seasonal measurements of radon exhalation 135 5.5 Chapter summary 143

Chapter 6: Lead-210 deposition and excess 145 6.1 Overview 145 210 6.2 The Pb story 145 6.3 Pb-210 deposition 147 6.3.1 Seasonal 210Pb results 147 6.3.2 Annual depositional rate, average values and residency time 153 6.3.3 Pb-210 deposition summary 156 6.4 Pb-210 excess in soil samples 156 6.4.1 Pb-210 inventories 156 6.4.2 Penetration half depth 161 6.4.3 Excess 210Pb summary 163 6.5 Magela Land Application Area 164 6.5.1 Introduction 164 6.5.2 Uranium-238, 226Ra and 210Pb depth profile inventories 164 6.5.3 Experimental plot inventories 168 6.5.4 Radium-226 and 210Pb distribution 170 6.5.5 Magela Land Application Area summary 172 6.6 Chapter summary 173

Chapter 7: Lead-210 budget 175 7.1 Introduction 175 7.2 Hadley circulation 175 210 7.3 Local area Pb budget 176 7.3.1 Fate of Ranger 222Rn 177 7.3.2 Determination of 222Rn exhalation rates from 210Pb deposition and excess 210Pb inventories 177 7.3.3 Determination of excess 210Pb inventories from 210Pb deposition 179 7.3.4 Determination of 210Pb deposition and inventories from 222Rn exhalation rates 182 7.3.5 Local area 210Pb budget summary 183 210 7.4 Regional Pb budget 184 7.4.1 Kakadu dry season 222Rn emission 184 7.4.2 Kakadu wet season 210Pb budget 188 7.5 Chapter Summary 191

vii Chapter 8: Conclusions and future directions 193 8.1 Project outcomes 193 8.2 Future directions 197 8.3 Conclusions 198

viii List of Figures Figure 1.1: The uranium and actinium natural decay series with radon isotopes highlighted ...... 3 Figure 1.2: Global population weighted average of human exposure to natural sources of radiation, total 2.4 mSv.y-1 (UNSCEAR 2000) ...... 4 Figure 1.3: Alligator Rivers Region, Northern Territory, Australia, curtesy Supervising Scientists Division ...... 7 Figure 2.1: Fate of 222Rn nucleus just after 226Ra-222Rn transmutation, R is the recoil range of 222Rn nucleus in solid material. A: Recoil and embedding in same grain. B: Recoil, ejection from grain and stopping in interstitial space C: Recoil, ejection from grain, crossing air gap and embedding in neighbouring grain. D: Recoil, ejection from grain and embedding in neighbouring grain. E: Recoil and stopping in water in the interstitial space. .12 Figure 2.2: On right scanning electron micrograph of monazite (top) and zircon (bottom). On left thorium distribution in the same grain (Holdsworth and Akber 2004) ...... 14 Figure 2.3: Emanation coefficient as function of increasing water content for sample of till sieved into various grain sizes. (Adapted from Markkanen and Arvela (1992)) ...... 16 Figure 2.4: The ratio of the saturated emanation coefficient to dry emanation coefficient for increasing moisture (Emission ratio). (Adapted from Sun and Furbish (1995)) ...... 16 Figure 2.5: Emanation coefficient for increasing grain size and differences between radium distribution for natural samples. Surface Ra has thickness equal to the recoil range (40 nm). (Adapted from Greeman and Rose (1995))18 Figure 2.6: Radon emanation, migration and exhalation...... 23 Figure 2.7: Typical 210Pb soil profiles for various soil uses (adapted from Walling et al. (2003))...... 48 Figure 3.1: Ranger Uranium Mine, numbers indicate approximate sampling locations used for this project...... 57 Figure 3.2: Flow chart of Ranger processing (ERA 2005)...... 58 Figure 3.3: Original Magela Land Application Area (MLAA)...... 64 Figure 3.4: Map of region displaying seasonal sites...... 69 Figure 3.5: Dry season (April-October) wind rose for Jabiru East (data courtesy of Australian Bureau of Meteorology) [26 years averaged]...... 73 Figure 3.6: Map of Jabiru and Ranger, numbers indicate approximate locations of selected sites for soil samples ...... 74 Figure 4.1: Charcoal canister ...... 80 Figure 4.2: Radon emanometer ...... 84 Figure 4.3: Cutter used to accurately place emanometer saucer...... 85 Figure 4.4: Schematic of radon/thoron emanometer...... 86 Figure 4.5: Set up for emanometer calibration ...... 88 Figure 4.6: Default calibration curve for Diviner 2000 soil moisture probe ...... 90 Figure 4.7: Geofizika Brno NaI(Tl) GS-512 gamma spectrometer in use at Rangers waste rock dump ...... 93 Figure 4.8: Base of discs used for soil samples...... 96 Figure 4.9: 210Pb deposition collector deployed at Oenpelli ...... 98 Figure 4.10: eriss detector room, Darwin (Photograph by Bruce Ryan)...... 100

ix Figure 4.11: Calibration curves and equations for pressed disc soil samples, energy unit is keV...... 104 Figure 4.12: Efficiency calibration for resin samples ...... 105 Figure 5.1: Plot of 222Rn exhalation rates vs. 226Ra activity concentrations for all sampling sites ...... 122 222 226 Figure 5.2: Ratio of Rn exhalation rate to Ra activity concentration (RE-R) for locations during dry conditions ...... 124 Figure 5.3: Plot of 222Rn exhalation rate vs. 226Ra activity concentration for all sites categorised by geomorphic groups ...... 127 Figure 5.4: Diurnal variations of atmospheric pressure observed at Jabiru East (Data courtesy of Australian Bureau of Meteorology) ...... 129 Figure 5.5: Diurnal variations in atmospheric and soil temperatures at Jabiru East (Data courtesy of Australian Bureau of Meteorology) ...... 129 Figure 5.6: Normalised 222Rn exhalation rate for all sites vs. time of day of measurement ...... 131 Figure 5.7: Normalised 220Rn exhalation rate for all sites vs. time of day of measurement ...... 131 Figure 5.8: 222Rn exhalation rate vs. soil temperature...... 132 Figure 5.9: 220Rn exhalation rate vs. soil temperature...... 132 Figure 5.10: 222Rn exhalation rate vs. change in atmospheric pressure ...... 133 Figure 5.11: 220Rn exhalation rate vs. change in atmospheric pressure ...... 133 Figure 5.12: Seasonal variations of 222Rn exhalation rates and cumulative rainfall...... 136 Figure 5.13: Seasonal variations of 222Rn exhalation rates and cumulative rainfall continued ...... 137 Figure 5.14: Averaged 222Rn exhalation rates and atmospheric concentrations for sampling periods at Mudginberri...... 139 Figure 5.15: 2002-2003 wet season moisture profiles for Jabiru East ...... 141 Figure 5.16: 2003 dry season soil moisture profiles for Jabiru East ...... 141 Figure 5.17: Mirray soil moisture profiles, all readings...... 143 Figure 6.1: Jabiru East 210Pb deposition and cumulative rainfall...... 148 Figure 6.2: Oenpelli 210Pb deposition and cumulative rainfall ...... 149 Figure 6.3: Relationship between excess 210Pb and 40K...... 160 Figure 6.4: Relative cumulative excess 210Pb versus depth for soil scrapes.162 Figure 6.5: Relative cumulative excess 210Pb versus depth for soil cores.....162 Figure 6.6: Inventory depth profile for 2cm sectioned cores from irrigated TM1 and non-irrigated TM2 ...... 165 Figure 6.7: Inventory depth profile for 5cm sectioned cores, irrigated (core 2) and averaged non-irrigated cores...... 165 Figure 6.8: U-238 inventory depth profile for irrigated core TM1...... 166 Figure 6.9: U-238 inventory depth profile for irrigated core 1 and core 2...... 166 Figure 6.10: Inventory depth profile for scrape 1 from experimental plot...... 169 Figure 6.11: Inventory depth profile for scrape 2 from the experimental plot 169 Figure 6.12: Activity concentration depth profile for core collected by J. Storm 1994 (fine grains, <2mm only)...... 170 Figure 6.13: Distribution of 226Ra in soil fractions for 0-10cm ...... 171 Figure 6.14: Distribution of 210Pb in soil fractions for 0-10 cm ...... 172 Figure 7.1: Global Hadley circulation model (curtesy Australian Bureau of Meteorology) ...... 176 Figure 7.2: Natural redistribution of 210Pb...... 181

x Figure 7.3: Radium-226 versus 238U to 10 cm to demonstrate 226Ra/238U disequilibrium ...... 181 Figure 7.4: Geomorphic landscapes of the Kakadu region (Adapted from Lowry and Knox (2002))...... 186 Figure 7.5: Wetland area of Kakadu (Santos-Gonzalez et al. 2002) ...... 189

xi List of Tables Table 2-1: Worldwide reported 222Rn exhalation rates...... 26 Table 3-1: Classification of ore grades at Ranger...... 56 Table 3-2: Number, name and position of sites selected at Ranger, refer to Figure 3.1...... 60 Table 3-3: Measurement dates for Ranger Mine sites ...... 64 Table 3-4: Number of Ranger measurements ...... 65 Table 3-5: Seasonal measurement sites, names and locations ...... 68 Table 3-6: Sites and dates of diurnal measurements...... 72 Table 3-7: Sites for excess 210Pb soil samples...... 75 Table 3-8: Soil collection dates and samples taken...... 75 Table 4-1: Emanometer calibration check results...... 88 Table 4-2: Standards for Pressed Disc Geometry ...... 103 Table 4-3: Standards for resin...... 103 Table 4-4: Corrections factors applied to activity concentrations of soil samples...... 106 Table 5-1: Dry season values of 222Rn exhalation rates from all measurement sites ...... 108 226 Table 5-2: Ra activity concentrations and RE-R ratio ...... 110 Table 5-3: Analysis result of geomorphic clusters ...... 126 Table 5-4: Correlation coefficients for diurnal measurements ...... 134 Table 6-1: Results from Jabiru East 210Pb deposition collector ...... 149 Table 6-2: Results from Oenpelli 210Pb deposition collector...... 150 Table 6-3: Seasonal and annual 210Pb depositional rates and rainfall ...... 154 Table 6-4: Total inventories of excess 210Pb...... 158 Table 7-1: Measured and estimated seasonal and annual 222Rn exhalation rates for Jabiru East...... 179 Table 7-2: Estimated and observed excess 210Pb inventories...... 180 Table 7-3: Estimated 210Pb deposition rate and excess 210Pb inventory...... 183 Table 7-4: Dry season 222Rn emission from Kakadu National Park...... 187 Table 7-5: Wet season kappa for various geomorphic landscapes ...... 188 Table 7-6: Wet season 222Rn emission from Kakadu National Park...... 190 Table 7-7: Estimated Kakadu region 222Rn exhalation rate, wet season 210Pb deposition rate and total excess 210Pb inventory...... 191

xii Statement of Original Authorship

The work contained in this thesis has not been previously submitted for a degree or diploma at any other tertiary educational institution. To the best of my knowledge and belief, the thesis contains no material previously published or written by another person except where due reference is made.

Signed ______

Date______

xiii 1

Introduction

1.1 Overview Discovery of 222Rn gas emanating from samples of 226Ra is attributed to German physicist Friedrich Ernst Dorn in 1900 who was performing work in relation to the recent discovery of the three types of radioactivity by Ernest Rutherford in 1899 (Romer 1964). Radon-222 is present in nature as the only gas found in the natural 238U radioactive decay series, Figure 1-1; it is the direct progeny of 226Ra, has a half- life of 3.82 days, is colourless, inert, the major radon isotope, a noble gas, which is the heaviest gaseous element. Radon-222 is soluble in water (bulk solubility = 1.95*10-4 at standard temperature and pressure) and highly soluble in organic solvents (Chemical Rubber Company 2004; UNSCEAR 1982). Two other isotopes of radon exist in nature and are members of the 232Th and 235U natural radioactive decay series, they are 220Rn (thoron) and 219Rn (actinon) respectively, Figure 1.1. The half-lives of 220Rn (55.6 s) and 219Rn (3.96 s) are much shorter than that of 222Rn and as a result they are not as useful for environmental studies although exposure to 220Rn and its progeny is considered for radiological dose assessment in certain industries. The radon isotopes, specifically 222Rn, are reported as contributing the largest component of human exposure to natural radiation (UNSCEAR 2000), Figure 1-2. Trace quantities of 238U, 232Th, 235U and their progeny are found in all natural rocks and soils. As a result radon isotopes are emitted in some quantity from every natural and a number of man-made surfaces. Exposure via inhalation of the radon isotopes and their progeny has been associated with an increase in the risk of lung cancer (Smith 1988; Jacobi 1988; Lubin and Boice 1997). Concentration measurements of 222Rn and its progeny for the determination of radiation doses to occupationally exposed individuals and members of the public living in proximity to supervised radiation areas are standard practice. The International Commission on Radiological Protection (ICRP 1991) recommendations for limits to ionising radiation from man- made sources are 20 mSv.yr-1, effective dose, for occupationally exposed workers

1 and 1 mSv.yr-1 for members of the public; this does not include medical exposure as a patient or natural background. Occupational exposure to 222Rn is common in mining and milling industries, predominantly in the uranium mining industry. Sand mining of 232Th and 238U rich sands for minerals like zircon, rutile, monazite and ilmenite also increases exposure to 220Rn and 222Rn. Other than mining and milling a number of areas are subject to accumulation of 222Rn and 220Rn thus increasing exposure of people working or living in such environments, these include poorly ventilated natural caves, basements and cellars. Accumulation of 222Rn in poorly ventilated houses, that enters the living area from crawl spaces or basements, has been identified as a health risk in a number of countries around the world (UNSCEAR 2000). Eventually 222Rn decays through a series of short-lived progeny, with half-lives ranging from 104 μs to 26.8 minutes, until it reaches the “long-lived” isotope 210Pb with a half-life of 22.3 years. A portion of the natural radon isotopes are emitted into the atmosphere from the rock and soil surfaces around the globe in a process known as exhalation. The depth from which exhalation occurs depends on a large number of factors including the isotope half-life, soil porosity, soil moisture, soil temperature, air pressure, air temperature and wind speed. Having a longer half- life, 222Rn is exhaled from greater depths than 220Rn that in turn is exhaled from greater depths than 219Rn. In normal soils 222Rn is exhaled from the top few metres, 220Rn from the top few decimetres and 219Rn from the top few centimetres. Radon-222 entering the atmosphere mixes as a gas and is subject to atmospheric transport. The progeny of 222Rn are electrically charged, having been stripped of electrons during the decay process, a majority of them attach to aerosols existing in the atmosphere.

2 Uranium Decay Series Actinium Decay Series

U-235 U-234 7.04*108y U-238 β 2.46*106y 4.47*109y Pa-231 Pa-234m α α β 3.28*104y α β 1.17m Th-227 Th-230 Th-231 α 98.6% 18.7d Th-234 7.54*104y 25.5h 24.1d β α Ac-227 α 21.8y Ra-226 Ra-223 1600y α 1.4% 11.4d

β α Fr-223 α 21.8m Rn-222 3.82d Rn-219 3.96s α Po-210 Po-214 β α Po-218 β 138d 104μs Po-211 3.10m 0.3% Bi-210 α Po-215 0.516s Bi-214 α β β α β 5.01d 1.78ms 19.9m Pb-206 Pb-210 Bi-211 α Stable α β Pb-214 22.3y 2.14m 26.8m Pb-207 Pb-211 α Stable 36.1m β Tl-207 4.77m 99.7%

Figure 1.1: The uranium and actinium natural decay series with radon isotopes highlighted

3 Thorium Decay Series

Th-228 Th-232 β 1.91y 1.41*1010y Ac-228 α α β 6.15h Ra-224 Ra-228 3.66d 5.75y α

Rn-220 55.6s

α 64.1% Po-211 Po-216 β 0.516s 0.145s Bi-211 α 2.14m α β Pb-207 Pb-212 α Stable 10.6h β Tl-207 4.77m 35.9%

Figure 1-1 continued: The thorium natural decay series with thoron highlighted

21%

Terrestial Cosmic 50% 17% Ingestion Inhalation

12%

Figure 1.2: Global population weighted average of human exposure to natural sources of radiation, total 2.4 mSv.y-1 (UNSCEAR 2000)

4 Aerosols are removed from the atmosphere by two processes; the first is dry deposition and the second is precipitation scavenging. The latter of these removes the larger amount of aerosols from the atmosphere over a shorter period of time. Removal of radioactive particles in the atmosphere therefore comes through their wet and dry deposition. The deposition of attached and unattached 222Rn progeny occurs on all surfaces over the globe. The portion that is deposited on land is either washed into the water systems or is transported, mixed and covered by more new soil creating a depth profile of deposition history. Profiling excess 210Pb and examining its behaviour in soils is a major component of this project. The natural phenomenon of radon exhalation, its atmospheric mixing and the deposition of its progeny back onto the surface of the globe have numerous applications. Measurements of radon exhalation rates from the ground and radon and radon progeny concentrations in the atmosphere have been used for the following applications: − For indirect measurement of radium isotopes; − Ground fault line identification and earthquake prediction; − Radiological dose assessment; − Tracers for determination of atmospheric transport; − Investigation of the aerosol attachment process; − For studies of the prevailing conditions that control the exhalation. Measurement of 210Pb in soils has the potential for use in soil redistribution studies. The man-made fission product 137Cs is commonly used for these types of studies (Zapata 2003) but cessation of above ground nuclear testing means there is no replenishment of 137Cs into the atmosphere. The natural exhalation of 222Rn into the atmosphere replenishes 210Pb almost continuously. Also there was a greater amount of 137Cs released into the atmosphere of the northern hemisphere than the southern during the above ground nuclear tests. This makes using 137Cs in the southern hemisphere even more limited. Another surface deposited natural radionuclide, 7Be, can be used but its short half-life of 55 days limits potential applications. As the half- lives of 210Pb and 137Cs are comparable, 22.3 years and 30.1 years respectively, 210Pb can be used for the same environmental applications as 137Cs along with providing new applications, these include;

5 − Use in soil sedimentation studies for agriculture and land care management; − Transport and bioturbation of soil particles; − Soil erosion studies; − Indicator of deposition history; − Indicator of 222Rn progeny transport in the atmosphere. An aim of this project is to provide potential for development of further applications using 210Pb. 1.2 Alligator Rivers Region This project’s main aim is to budget the 210Pb within the Alligator Rivers Region of tropical northern Australia. This project was developed due to the current lack of knowledge and understanding of the behaviour of 222Rn and 210Pb within tropical regions. Budgeting involves identification of the sources and sinks of 210Pb within a region and producing a model of the exhalation and deposition process that matches observed values at selected locations. The area selected was the Alligator Rivers Region of the Northern Territory, Australia shown in Figure 1-3. This region encompasses the world heritage listed Kakadu National Park and includes the operational Ranger Uranium Mine (RUM), owned by Energy Resources of Australia (ERA). The region also hosts a number of former uranium mine sites and undisturbed uranium deposits. This area was selected as it lies within the tropics, is representative of a complete geomorphic system, and the Ranger Uranium Mine acts as a localised 222Rn source. At Jabiru East there is a government field station laboratory (eriss) with which the project was performed in collaboration with. They provided full support during the time spent in the region.

6

Figure 1.3: Alligator Rivers Region, Northern Territory, Australia, curtesy Supervising Scientists Division

7 The region is classified as wet and dry tropics with those two distinct seasons in the year. The wet season is usually from November to March and the dry season is from May to September. The months of April and October are transitional periods between these seasons. The local Aboriginal people have categorised eight seasons in the area corresponding to temperature and humidity changes that occur throughout various times of the year. The effects that this contrast in weather conditions has on the exhalation and deposition rates of 222Rn and 210Pb have received very little attention over the years. Hence a primary reason for selection of a wet-dry tropical location was to study the influence of season on the exhalation and deposition rates of these radionuclides. Average annual rainfall for the region is 1485.3 mm per annum, occurring mostly in the wet season and the average annual pan evaporation rates are 2591.5 mm per annum (ABM 2005). The prevailing winds for the region are easterly to south-easterly during the dry season and northerly to north-westerly during the wet season (ABM 2004). In the Alligator Rivers Region the wet season deposits large amounts of rain over a period of 4-5 months. While several rain-producing systems have been identified in northern Australia they all fall into the two main categories of convective or cyclonic systems. A convective rain event is intense but short-lived (minutes to hours) and covers relatively small areas (1-100 km2). The contrast is the cyclonic rain events and monsoon troughs created as low-pressure systems passing across the area. Cyclonic and monsoon rains last for a few days to weeks and cover much larger areas. Jabiru is the only township located within Kakadu National Park while Oenpelli is an Aboriginal township located in Arnhem Land in the Alligator Rivers Region. A number of tourist accommodation resorts, ranger stations and Aboriginal settlements are also located within the region. Jabiru lies about 12 km west of the Ranger mine, some 260 km east of Darwin. It has a population of about 1200 people that includes mining personnel, eriss personnel, local Aboriginals, park rangers, resort staff, people employed in associated community facilities and families of all the above. Jabiru is a closed township and most people leave after their term of employment has ceased. Gunbalunya, also known as Oenpelli, is located 60 km north-north-east of the Ranger mine; it is an Aboriginal township 15 km inside Arnhem Land from Cahill’s Crossing with a population of approximately 750, the

8 population increases slightly in the wet season as a number of people move from the surrounding region to the town for shelter and facilities. Tourist accommodation centres are located within Kakadu National Park at Cooinda, South Alligator River, Jabiru and Ubirr. Each centre varies in the number of people that they can accommodate and the peak tourist influx is during the dry season from May to September. On average some 200,000 people visit Kakadu every year and spend an average of 4 days in the region. Aboriginal settlements are located throughout the Alligator Rivers Region and vary in population throughout the year depending on the season, hunting opportunities and traditional ceremonies. 1.3 Project objectives The aim of this project is to investigate 222Rn exhalation, 210Pb deposition and excess 210Pb inventories within the wet and dry tropical region of Kakadu National Park, Northern Territory, Australia. The specific objectives for the project are outlined as follows: − Investigate the principal contributing meteorological, geographical & geological factors that affect the exhalation of 222Rn and deposition of 210Pb; − Measure the seasonal variations of 222Rn exhalation rates at several sites; − Measure the seasonal variations of 210Pb depositional rate in both wet and dry deposition; − Study the transport of 210Pb through the surface layers of the soil through the measurement of radionuclides in soil samples; − Model the 210Pb budget in the Kakadu region. This project was performed in collaboration between eriss and QUT and a work plan was established at an early stage of the project. The details of site selection and schedule of measurements are found in Chapter 3 while information regarding all equipment and methodology used throughout the project is covered in Chapter 4.

9 2

Literature Review: Previous research in relation to radon emanation, migration, exhalation and 210Pb deposition

2.1 Overview The aim of this chapter is to provide details on the type of work previously reported in this field and establishes that the work presented in this project is unique. Since its discovery radon emanation, migration, exhalation and the deposition of its progeny has been studied extensively. A comprehensive review on the process of radon emanation and migration through soils and rocks has been performed previously (Tanner 1964; Tanner 1980). These reports are referred to in relation for work performed up to 1980 while this review mainly covers research performed after that date. Studies of 210Pb deposition date back to the early 1960’s and a compilation of global results are maintained and updated by the Laboratory of Glaciology and Geophysics of the Environment, France (Preiss et al. 1998). They provide results for the majority of 210Pb deposition studies performed over the last 40years along with a list of all relevant publications. 2.2 Radon emanation 2.2.1 Introduction Emanation of radon atoms is defined as their ejection from their source material sometime after the radioactive decay of the parental radium isotope. The process of emanation for the isotopes 220Rn and 219Rn is the same as that for 222Rn, with recoiling ranges and diffusion lengths differing due to different alpha energies and half-lives. Studies and models of radon emanation have been performed by a number of researchers with some work concentrating on physical factors shown to affect its emanation such as soil moisture and porosity; pore size and number; radium concentration and distribution; grain size and shape; atmospheric and soil temperature as well as atmospheric pressure (Markkanen and Arvela 1992; Baixeras et al. 2001; Morawska and Phillips 1992; Morawska and Jefferies 1994; Mosley et al. 1996; Schumann and Gundersen 1996; Semkow 1990; Fleischer 1987; Menetrez et al. 1996; Amin et al. 1995; Gan et al. 1986; Greeman and Rose 1995; Sun and

10 Furbish 1995; Maraziotis 1996; Iskander et al. 2004; Grasty 1997; Strong and Levins 1982; Goh et al. 1991; Barton and Ziemer 1986; Misdaq et al. 1998; Holub and Brady 1981; Nazaroff and Nero 1988; Megumi and Mamuro 1974; Landa 1987a). A large number of studies have been performed that simply report measurement techniques and radon emanation rates for various materials including uranium ores, tailings, construction materials, rocks, soils and minerals (Rogers and Nielson 1991; Ferry et al. 2002a; Martino and Sabbarese 1997; Bossew 2003; Ho and Weng 1981; Quindos et al. 1994; Singh and Ghuman 1988; Howard et al. 1995; McCorkell 1986; Funtua et al. 1997; Holdsworth and Akber 2004; McCorkell et al. 1981; Barillon et al. 1991; Beckman and Balek 2002; Klein et al. 1995; Chao et al. 1997; Chao and Tung 1999; Burke et al. 2003; Bigu and Hallman 1993; Landa 1987b; Zahorowski et al. 1994; Sonter et al. 2002; IAEA 1992). This review focuses on radon emanation from natural materials but the process of emanation is similar for all materials. Radon is created after the radioactive disintegration of its parent, an isotope of radium. Under normal conditions radium, and all the isotopes in the decay chain before it, are in a solid phase. While some of these isotopes are slightly soluble in water in general radium will be produced at the site of the original uranium or thorium atom. If this is deep within a medium then radon will be produced within that medium and remain trapped. For radium close to or on the surface of a medium there is the chance that some of the radon produced might escape into the interstitial space with the momentum of recoil from radioactive decay. The fraction of radon atoms that escape from a medium into the interstitial space is known as the “emanating power” or “emanation coefficient”, this is dimensionless being the fraction of emanating radon atoms to total radon atoms. For soils Tanner (1964, 1980) describes that upon the creation of a radon atom one of the following three things are likely to occur as shown in Figure 2.1; − It will travel a short distance within the grain and become embedded within the grain (A); − It can travel across the interstitial space between grains and become embedded in another grain (C & D); or − It is released into the interstitial space where diffusion and transport mechanisms migrate the radon (B & E).

11 R A

B E C D

Figure 2.1: Fate of 222Rn nucleus just after 226Ra-222Rn transmutation, R is the recoil range of 222Rn nucleus in solid material. A: Recoil and embedding in same grain. B: Recoil, ejection from grain and stopping in interstitial space C: Recoil, ejection from grain, crossing air gap and embedding in neighbouring grain. D: Recoil, ejection from grain and embedding in neighbouring grain. E: Recoil and stopping in water in the interstitial space.

The recoil range of a 222Rn atom after the disintegration of 226Ra are given as 30-50 nm for solids, 95 nm in water and 64000 nm in air (Tanner 1980; Greeman and Rose 1995). The diffusion coefficient of radon through solid materials is of the order of 10-20 cm2.s-1, so considering 222Rn with a 3.8 day half-life any produced deep within a soil grain are most likely to disintegrate close to its position at the end of recoil after 226Ra transmutation. This means that 222Rn will only emanate from a thin layer (30-50 nm) on a soil grains surface as recoil after 226Ra transmutation is the dominant emanation mechanism. 2.2.2 Radon emanation and radium distribution The distribution of radium in soil grains are described as the key factor that affects radon emanation (Greeman and Rose 1995; Semkow 1990; Schumann and Gundersen 1996; Morawska and Jefferies 1994; Morawska and Phillips 1992; Holdsworth and Akber 2004; Nazaroff and Nero 1988; Landa 1987b; Landa 1987a).

12 Original models of radon emanation assumed that radium distribution was homogeneous throughout the grain and that these grains were also spherical (Morawska and Phillips 1992). This does not represent the common situation where uranium and radium mobilization result in a higher proportion of radium deposited on the surface of soil grains (Morawska and Phillips 1992; Schumann and Gundersen 1996; Greeman and Rose 1995). For materials with a homogenous distribution of radium it is expected that the emanation coefficients would be low compared to those with a surface distribution. Materials exhibiting emanation coefficients higher than expected for a homogenous distribution supports the conclusion that radium in those materials are most likely close to surface distributed. In a study on the emanation of 220Rn from samples of monazite and zircon Holdsworth and Akber (2004) provided an example of the variation in 222Rn emanation rates from different 226Ra distributions. As may be seen in Figure 2.2, thorium (and hence 228Ra) distribution of the two types of grains are different, monazite has a uniform distribution while zircon has specks of thorium mainly on smaller sized grains. The resulting experimental emanation coefficients for the two minerals was reported as (9.0±2.6)*10-4 and (4.1±1.9)*10-2 for monazite and zircon respectively. Fractures and fissures on the surface of a grain, referred to as pores, from previous radioactive decays, chemical or weathering effects, effectively increase the surface area of the grain and can increase its emanation coefficient by up to a factor of two (Schumann and Gundersen 1996; Semkow 1990; Morawska and Phillips 1992). These pores provide more surface area and radium trapped within the soil grain may now be in close proximity to the interstitial space created by these pores. These pores will have a greater effect on the emanation coefficient for material with a homogenous radium distribution compared with those with a surface distribution. Further details of the effect of these pores are covered in section 2.2.5.

13

Figure 2.2: On right scanning electron micrograph of monazite (top) and zircon (bottom). On left thorium distribution in the same grain (Holdsworth and Akber 2004)

While radium distribution is certainly the key factor affecting the radon emanation coefficient of materials other factors such soil moisture, porosity, temperature, grain size and atmospheric pressure are all known to influence it. The combined effect of all these variables explains large variations in the emanation coefficients observed for various materials. 2.2.3 Radon emanation and soil moisture After radium distribution the soil moisture content has been described as the next most important factor affecting the radon emanation coefficient. Water filling the pores of grains and the interstitial space between grains shortens the range of radon atoms exiting the grain. This will stop radon atoms from embedding themselves into neighbouring grains or crossing pores to embed in the same grain. Since radon is soluble, water may also aid in the release of radon trapped in pores on

14 the surface of a grain caused by atoms embedding into the grain (B and D in Figure 2.1) (Fleischer 1987; Tanner 1980; Morawska and Phillips 1992; Semkow 1990). The effect of moisture content on radon emanation coefficients has been reported extensively (Tanner 1964; Tanner 1980; Menetrez et al. 1996; Greeman and Rose 1995; Markkanen and Arvela 1992; Fleischer 1987; Grasty 1997; Sun and Furbish 1995; Megumi and Mamuro 1974; Mosley et al. 1996; Strong and Levins 1982; Goh et al. 1991; Barton and Ziemer 1986; Semkow 1990; Nazaroff and Nero 1988). All work on the subject has shown that small amounts of moisture increase the radon emanation coefficient but continual increase in moisture eventually reduces the measured emanation coefficient (Schumann and Gundersen 1996; Menetrez et al. 1996; Fleischer 1987; Markkanen and Arvela 1992; Megumi and Mamuro 1974; Barton and Ziemer 1986). This effect is shown in Figure 2.3 and Figure 2.4 taken from two of the studies on the topic. The amount of moisture that produces the maximum level of emanation is dependent on the type of soil, grain size and its porosity. It needs to be noted that the reduction in emanation reported by most literature and displayed in Figure 2.3 and Figure 2.4 is actually an artefact of the measurement technique. Measurement of radon in these studies requires it to be in its gaseous phase and not in a dissolved state. The emanation coefficient is the proportion of radon atoms liberated into the interstitial space regardless of the medium filling that space. So in essence any increase in moisture increases the emanation coefficient till it reaches a steady state and the reductions experimentally observed are a measurement limitation. Moisture ensures that there is less radon available in the gas phase and this will have a direct impact on the radon concentration and migration. There is general agreement on the topic that it only requires a small amount of moisture filling the interstitial space to increase emanation. The reduction in emanation coefficient seen with higher moisture contents is attributed to a reduction in the diffusion of radon trapped in the water.

15 50 0.074mm 0.125-0.25mm 45 1-2mm 2-4mm 2-4mm 40

35

30

25

20

15 Emanation Coefficient (%) 10

5

0 0 5 10 15 20 25 30 Water Content (%)

Figure 2.3: Emanation coefficient as function of increasing water content for sample of till sieved into various grain sizes. (Adapted from Markkanen and Arvela (1992))

10 9 8 Rn-222 7 6 5 4

Emission Ratio 3 2 1 0 0 102030405060708090100 Moisture Saturation (%)

Figure 2.4: The ratio of the saturated emanation coefficient to dry emanation coefficient for increasing moisture (Emission ratio). (Adapted from Sun and Furbish (1995))

16 2.2.4 Radon emanation, soil porosity and grain size Both soil porosity and grain size are known to affect radon emanation (Misdaq et al. 1998; Markkanen and Arvela 1992; Amin et al. 1995; Maraziotis 1996; Baixeras et al. 2001; Landa 1987b; Schumann and Gundersen 1996; Tanner 1980; Morawska and Phillips 1992; Barton and Ziemer 1986; Semkow 1990; Greeman and Rose 1995; Sun and Furbish 1995; Nazaroff and Nero 1988). Porosity is defined as the ratio of empty space volume to solid material volume. It should be noted that grain size and interstitial space are interrelated as grain size increases so does interstitial space, however this does not necessarily increase porosity as the increases may balance out. Smaller sized grains may also have larger than expected values of porosity especially if the grains are filled with pores providing more interstitial space. Porosity also depends upon the amount of compaction of the material. An increase in interstitial space, hence porosity, means that radon has a greater chance of stopping in the interstitial space after ejection from a grain. It has been shown in models and experiments that the radon emanation coefficient is directly proportional to porosity (Sun and Furbish 1995; Misdaq et al. 1998; Maraziotis 1996). While increasing grain size increases the interstitial space, proportionally less radon will escape from large grains due to the smaller surface area to volume ratio. Hence larger emanation coefficients are observed for smaller grain sizes. Published results on the topic from both modelling and experiments shows that the radon emanation coefficient is inversely proportional to the grain size (Morawska and Phillips 1992; Semkow 1990; Amin et al. 1995; Markkanen and Arvela 1992; Greeman and Rose 1995; Maraziotis 1996; Barton and Ziemer 1986; Tanner 1964; Baixeras et al. 2001). In their models Maraziotis (1996) made the assumption that radium was distributed homogenously while Semkow (1990) looked at surface distribution. Morawska and Philips (1992) modelled both aspects while Greeman and Rose (1995) calculated that surface distributed radium provided higher emanation coefficients compared with same sized grains with a homogenous distribution, shown in Figure 2.5.

17 100 Surface Ra Homogeneous Ra 80

60

40

20 Emanation Coefficient (%)

0 0.01 0.1 1 10 100 Particle Diameter (mm)

Figure 2.5: Emanation coefficient for increasing grain size and differences between radium distribution for natural samples. Surface Ra has thickness equal to the recoil range (40 nm). (Adapted from Greeman and Rose (1995))

All work on the topic agrees that regardless of radium distribution an inverse relationship between radon emanation and grain size exists. The recoil range of 222Rn in solid materials is approximately 40 nm, which may be smaller than the thickness of the 226Ra surface distribution of up to a few micrometres. Perhaps it is because of this reason that authors have observed decreasing emanation coefficients with increasing grain sizes even for situations where 226Ra is distributed more on the grain surface. 2.2.5 Radon emanation, pore size and number To date there has been no published quantitative experimental work examining the effect of granular pores on radon emanation. This is likely due to difficulties in quantifying the size and number of pores of various types of material. Three publications have studied the effect through various forms of modelling and examined pore size, number and type of radium distribution (Morawska and Phillips 1992; Semkow 1990; Maraziotis 1996). Granular pores increase the amount of surface area on a grain providing radon with more opportunity to escape thus increasing the emanation coefficient (Semkow 1990; Morawska and Phillips 1992). The model produced by Semkow (1990) examined the effect that the number and distribution of pores had upon emanation. This was performed through a comprehensive analysis of various pore sizes and their

18 distribution on a grain. The results showed that high emanation coefficients were proportional to high pore densities as pores provide more open space for radon to stop in the interstitial space (Semkow 1990). Radium distribution is related to the effect that surface pores have on radon emanation. If radium is homogenously distributed throughout a grain, then granular pores will provide access that allows radon to emanate. If the radium is surface distributed then pores have minimal effect on the emanation coefficient unless the radium is surface distributed within the pores as well (Morawska and Phillips 1992; Semkow 1990). Morawska and Philips (1992) produced four models examining variations in radium distribution and the effect of inner particle porosity (grain with cracks and fissures). The first two models examined variations in the radium distribution with no inner porosity while the latter two examined the same two types of radium distributions but this time in the presence of inner particle porosity. This study also examined the effect of grain size. Results from the work agree well with those published by Semkow (1990) in that particles with higher inner porosity (pores) had higher emanation coefficients. This was seen to be consistent over a range of grain sizes. Maraziotis (1996) produced a model based upon homogeneous distribution of radium within a grain using cylindrical pores. They were particularly interested in the relationship between particle radius and the size of pores on the grains. Their results show that as the pore radius increases the emanation coefficient decreases for constant particle radius. However as the distribution of the pores increases, thus increases the porosity of a grain, the emanation coefficient also increases which is in agreement to the work performed by both Semkow (1990) and Morawska and Phillips (1992). The real situation may be entirely different to the results of the models above since it has been shown that radium distribution is generally on the surface, that pores on a grain may vary in shape and size and the distribution of pores may vary from one grain to another even in the same material. It is clear that for a comprehensive understanding of emanation from samples that measurements of the grains average size, shape, radium distribution, pore size and pore distribution should be performed (Morawska and Phillips 1992).

19 Radon atoms that embed themselves into neighbouring grain produce recoil pore up to 30-50 nm depth and 1 nm in diameter. The energy involved in the recoil is enough to melt the solid material in the pore. It was originally thought that the radon atom would be trapped in this pore, as the thermal conductivity coefficient for cooling was several orders of magnitude greater than the diffusion coefficient. However Tanner (1980) continues by mentioning that experiments show that there is a brief period of time where the solid material is vaporized and the thermal conductivity coefficient is comparable with the diffusion coefficient so it is possible that the radon will escape back into the interstitial space. 2.2.6 Radon emanation and soil temperature Radon emanation from soils is proportional to temperature (Stranden et al. 1984; Gan et al. 1986; Markkanen and Arvela 1992; Iskander et al. 2004; Goh et al. 1991). This occurs because the physical sorption of gases onto solids is temperature dependent. As temperature increases gases desorb from solid materials and become freely available. This effect has been clearly demonstrated for radon absorption on charcoal. Markkanen and Arvela (1992) noted that the variation in emanation coefficient due to temperature was much greater in dry samples than samples with moisture in them. Desorption of radon as a result of temperature increase will release it into the interstitial space, as it is soluble in water the measured increase in emanation from moist samples is less than that of dry samples although the actual increase would be the same. It has been noted (Stranden et al. 1984) that the effect of temperature is not as large as the effect of moisture on the emanation coefficient. That is, for the range of ambient conditions, moisture in the interstitial space increases emanation greater than desorption by an increase in temperature. The most recent of these studies (Iskander et al. 2004) provides a function for the determination of emanation coefficient across temperature range (-20 oC to 40 oC). A result of the work performed by Iskander et al. (2004) is a function usable for both dry and wet soils, provided in Equation 2-1. E = 0.21T + 14.8 (R2 = 0.98) Equation 2-1

20 Where E – Emanation coefficient (%) T – Temperature (oC)

2.2.7 Variations in emanation coefficients for radon isotopes Most of the work mentioned in Sections 2.2.2-2.2.6 are concerned with 222Rn although a small amount includes 220Rn (Howard 1987; Howard et al. 1995; Martino and Sabbarese 1997; Sun and Furbish 1995; Greeman and Rose 1995; Megumi and Mamuro 1974). No work has been performed for the emanation of 219Rn due to measurement difficulties given the isotope’s short half-life (3.96 s). Other than recoil and diffusion ranges the process of radon emanation should be similar for the three natural radon isotopes. Studies that have examined both 220Rn and 222Rn emanation found that 220Rn emanation coefficients are approximately 10% less than those of 222Rn (Megumi and Mamuro 1974; Greeman and Rose 1995; Sun and Furbish 1995). We know that 220Rn atoms have more kinetic energy as a result of the decay process compared to 222Rn atoms. As a result 222Rn atoms have a larger range in all media. Tanner (1964, 1980) reports that the range in air of the various radon isotopes is 64 μm for 222Rn, 83 μm for 220Rn and 92 μm for 219Rn. The chances of a 220Rn or 219Rn atom embedding into a neighbouring grain after being expelled from its original grain are much greater than a 222Rn atom. However greater range means that there should be a greater contribution from deeper within the grain and that the two effects should balance out. This would be the case for homogeneously distributed radium but as it has been pointed out previously that radium is generally surface distributed in natural materials. This is one explanation why 220Rn emanation coefficients are smaller than those for 222Rn in soils; similarly 219Rn emanation rates would be even smaller. The situation would be further complicated if there are differences in the soil grain distribution of 224Ra and 223Ra compared to 226Ra but this has not been studied. The model by Sun and Furbish (1995) predicted that an increase in porosity increases 222Rn emanation greater than 220Rn emanation but moisture would have a similar affect on both. Increased porosity provides more surface area for emanation but the increased range of 220Rn atoms mean they are more likely to cross a pore and embed into the other side. However the reduction in recoil range due to the presence of water is enough to stop both isotopes and increase emanation similarly. They also made a comparison of their theoretical results to the existing experimental results and found a good correlation between them.

21 Overall the large amount of studies performed examining radon emanation rates can be grouped; soils, rocks and man made materials. Typically soils have 222Rn emanation coefficients between 0.1-0.3 while rocks are generally less, 0.01-0.1, as a result of their denser packed structure and low porosity, man made materials vary greatly depending upon the characteristics of the material. 2.3 Radon migration, exhalation and soil gas concentration 2.3.1 Introduction The fraction of radon atoms that reach the interstitial space become subject to diffusion and transport mechanisms at work within the soil. The type of mechanism that predominates depends on the type of soil, its moisture content and the underlying geological conditions. Transport is a pressure driven flow of soil gases that can be described by Darcy’s law while diffusion is the flow of soil gases due to a concentration gradient and is described by Fick’s law. Radon close to the soil surface boundary will diffuse into the atmosphere. This process is known as exhalation. The depth from which radon is removed from the soil into the atmosphere depends on the type of soil, its moisture content, the isotopes half-life and underlying geology. For 222Rn the depth is generally about 1-2 m in unsaturated soils, deeper for sands and shorter for saturated and compacted soils (Tanner 1964; Holdsworth and Akber 2004). Diffusion depths for 220Rn and 219Rn are greatly reduced due to their shorter half-lives. The process of radon emanation, migration and exhalation is sketched in Figure 2.6. For unsaturated, undisturbed rocks and soils, diffusion is the dominant mechanism for radon migration as once released into the interstitial space it follows the concentration gradient and moves towards the surface (Tanner 1964). Later Tanner (1980) describes that in unsaturated, fractured rocks and disturbed soils, transport can become the dominant mechanism for radon migration but it is dependent on the geological conditions such as the amount of fracturing or disturbance. In saturated rocks and soils, transport is the dominant mechanism for radon migration as radon is dissolved in the water and moves with it.

22 Soil surface 3: Rn exhalation

Soil grain 2: Rn migration 2: Rn migration Interstitial space

1: Rn emanation

Figure 2.6: Radon emanation, migration and exhalation There have been a large number of studies published involving measurements of radon exhalation and its soil gas concentration; they can be divided into the following main categories: − Equipment for exhalation measurements; − Exhalation rate surveys; and − Studies examining the influencing physical factors and temporal variations etc. The following physical factors are shown to influence the exhalation of radon into the atmosphere: − Soil moisture; − Atmospheric pressure; − Soil temperature; − Atmospheric temperature; and − Wind speed. Radon-222 migration and the physical factors that influence it can be studied comprehensively by simultaneously taking 222Rn soil gas concentration measurements along with measurements of the physical factors mentioned previously. A number of studies have reported this type of work and are mentioned in section 2.3.4.

23 2.3.2 Radon exhalation measurements techniques There is a large amount of published literature devoted to various types of radon exhalation measurement techniques (Harley 1992; Escobar et al. 1999; Keller and Schutz 1988; Martino et al. 1998; Kearney and Krueger 1987; Davey 1995; Abu- Jarad 1989; Barillon et al. 1993; Bigu and Elliot 1994; Bland and Norlander 1988; Dave and Lim 1982; Kirichenko 1970; Lehmann et al. 2003; Ielsch et al. 2002; Nikezic and Urosevic 1997; Gan et al. 1983; Countess 1976; Akber et al. 2002; Bigu 1984; Abdelrazek 1984; Savvides et al. 1985; Zahorowski and Whittlestone 1996; Labed et al. 1994; Balcazar et al. 1999; Lawrence 2001; NCRP 1988; Khan et al. 1980; Akber et al. 1980; IAEA 1992). Techniques are firstly classified as either passive or active. Passive systems have no electrical components and rely upon the natural properties of radon and/or its progeny. Active systems do have electrical components and are based upon scintillation chambers coupled to photomultiplier tubes or silicon surface barrier detectors. Radon-220 is known to contribute significantly in measurements of radon exhalation and a variety of methods exist to either distinguish or eliminate 220Rn contribution. Any contribution of 219Rn is considered to be negligible since most measurement techniques are designed in such a way that 219Rn will decay before the measurement takes place. It is not the aim of this review to provide a detailed analysis on the techniques that are available for radon exhalation measurements. Details of the techniques used for the measurements performed in this project are found in Chapter 4. 2.3.3 Radon exhalation surveys The exhalation rates of 222Rn and 220Rn from various sources such as soils, mineral sands, rocks, construction materials, uranium tailings and uranium ores have been the subject of numerous studies (Todd and Akber 1996; Mason et al. 1982; Hart and Levins 1986; Schery et al. 1989; Morris and Fraley 1989; Hutter 1996; Nielson et al. 1996; Whittlestone et al. 1996; Denagbe 2000; Jha et al. 2000; El-Dine et al. 2001; Sengupta et al. 2001; Sroor et al. 2001; Shweikani et al. 1995; Kerrrigan and O'Connor 1990; El-Amri et al. 2003; Sharma et al. 2003; Kumar et al. 2003; Evangelista and Pereira 2002; Al-Jarallah 2001; Somashekarappa et al. 1996; Whittlestone et al. 1998; Abu-Jarad and Fremlin 1982; Cosma et al. 1996; Cheng and Porritt 1981; Labed et al. 1996; Singh et al. 1999; Tufail et al. 2000; Oufni and

24 Misdaq 2001; Ramola and Choubey 2002; Oufni 2003; Koarashi et al. 2000; Ferry et al. 2002b; Jovanovic 2001; Andjelov and Branjnik 1996; Carrera et al. 1997; Ielsch et al. 2001; Schery and Petschek 1983a; Fleischer et al. 1980; Kvasnicka 1990; Todd 1998; Bollhöfer et al. 2003; Wilkening et al. 1974; Auty and Preez 1994; Schery and Whittlestone 1986; Megumi and Mamuro 1974; Sonter et al. 2002; Lenzen and McKenzie 1999). This project is primarily interested in 222Rn exhalation from natural soils and uranium ore. A global average value for 222Rn exhalation rates from natural soils is given (UNSCEAR 2000) as 16 mBq.m-2.s-1 taken from work performed by Wilkening et al. (1974). This survey covered a range of sites throughout New Mexico, Texas, Hawaii and Alaska but the average is weighted towards the readings obtained from New Mexico, as this was the largest sample set. Results from a number of 222Rn exhalation rate surveys, including Wilkening et al, (1974), and others performed globally are shown in Table 2-1. The large variation in the values reported indicate that while the UNSCEAR (2000) average value is a reasonable estimate of global 222Rn exhalation rates it cannot account for all geomorphic types and is not valid for all seasons. True 222Rn or 220Rn exhalation rates from an area can only be obtained through direct measurement.

25 Table 2-1: Worldwide reported 222Rn exhalation rates Reference Location No.+ Technique* Average Reference Location No.+ Technique* Average Rn-222 Rn-222 Rate# Rate# (mBq.m-2.s-1) (mBq.m-2.s-1)

Wilkening et al. New 180 Acc, Flow, 24±12 Whittlestone Tasmania, Not Eman 31 (1974) Mexico Vert et al. (1998) Australia reported

Wilkening et al. Texas 9 Acc 10±4.3 Whittlestone Tasmania, Not Eman 15 (1974) et al. (1998) Australia reported

Wilkening et al. Hawaii 29 Acc 11±18 Nielson Florida, 882 CC 15 (1974) (1996) Undeveloped

Wilkening et al. Alaska 18 Acc, Flow 6.9±1.8 Whittlestone Hawaii 34 Eman 2.6±4.8 (1974) et al. (1996) UNSCEAR All 236 All Above 16±13 Nielson Florida, 112 CC 48 (2000) Above (1996) Developed

Somashekarappa Kiaga, 12 Acc 31±19 Auty and Jabiluka Not CC 25±5 et al. (1996) India duPreez Eastern reported (1994) Decline

Sengupta et al. Bihar, 15 SSNTD 1.4±1.1 Auty and Jabiluka Not CC 46±31 (2001) India duPreez Mine Valley reported (1994)

26 Reference Location No.+ Technique* Average Reference Location No.+ Technique* Average Rn-222 Rn-222 Rate# Rate# (mBq.m-2.s-1) (mBq.m-2.s-1)

Todd et al. (1998) Jabiru, 22 Eman 64±25 Evangelista King George 41 NTD 1.6±1.5 Australia et al. (2002) Is.

Ielsch et al. France 89 Eman 45±22 Schery et al. Australia 105 Eman, Acc 22±4.4 (2001) (1989)

*Techniques: Acc-Accumulator, Flow-Flow through scintillometer, Vert-Estimation from vertical profile, Eman-Emanometer, SSNTD-Solid state nuclear track detector, NTD-Nuclear track detector, CC-Charcoal canisters +Number of measurements #Average 222Rn exhalation rate with reported error

27 There is only a handful of literature that reports 222Rn exhalation rates from ambient Australian soils (Todd 1998; Schery et al. 1989; Schery and Whittlestone 1986; Whittlestone et al. 1998; Lenzen and McKenzie 1999; Auty and Preez 1994). Of these, the report by Schery and Whittlestone (1986) details 222Rn and 220Rn exhalation rates from a number of locations throughout Australia and is the most comprehensive Australian study of 222Rn exhalation to date. Their report provided Australian averages of 222Rn exhalation rates and was used to estimate Australian exposure to natural radiation by UNSCEAR (UNSCEAR 1993; UNSCEAR 2000). The work performed by Todd (1998) was performed in the Alligator Rivers Region in the Northern Territory, Australia. This work reported a range of 222Rn and 220Rn exhalation rates from a number of locations around the Ranger uranium mine. The work carried out by Schery and Whittlestone (1986) also contained readings from around the Ranger mine and Jabiru region. However both of these studies have only provided a snap shot of 222Rn exhalation rates from the area. Todd (1998) has provided diurnal readings but mentioned that further work was required to understand the parameters affecting radon exhalation rates from a tropical environment. Auty and duPreez (1994) performed a survey of 222Rn exhalation rates from North Ranger (Jabiluka). The uranium deposit here is between 20-200 m deep and mostly covered with sandstone, as 222Rn diffusion lengths are only a few metres the work can be considered to be on ambient soils. They reported 222Rn exhalation rates of 46±31 mBq.m-2.s-1 and 25±5 mBq.m-2.s-1 for the mine valley and eastern decline, respectively. These studies are important as they were performed in the same region where this project was based. Whittlestone et al. (1998) examined 222Rn exhalation rates from a number of sites in Tasmania and Victoria they reported winter and summer values to observe seasonal variations of 222Rn exhalation rates. More details of temporal variations are covered in section 2.3.9. A number of studies of 222Rn exhalation rates have been performed from rehabilitated uranium mines, undisturbed ore-bodies, operational uranium mines and tailing and ore samples from Australia (Mason et al. 1982; Kvasnicka and Auty 1994; Davy et al. 1978; Kvasnicka 1990; Bollhöfer et al. 2003; O'Brien and Whittlestone 1981; Hart 1986; Strong and Levins 1982; Akber et al. 2002; Harris and Chandler 1992; Davey 1994; Sonter 1987; Martin et al. 2002). The report by Mason et al. (1982) for the 222Rn exhalation from Ranger mine only performed measurements from the waste rock dump and tailings dam wall. Readings were also

28 +96 -32 -2 -1 performed at Nabarlek and Rum Jungle. A value of 49 mBq.m .s /%U3O8 was reported for waste rock material at the three sites. This value corresponds to +0.910.47-0.31 mBq.m-2.s-1(222Rn)/Bq.kg-1(226Ra). Kvasnicka and Auty (1994) were able to perform measurements on the ground above ore body 3 before mining commenced but the sample set of three charcoal cups provides poor statistical analysis. They obtained average pre mining 222Rn exhalation rates of 4.1 Bq.m-2.s-1 and 2.5 Bq.m-2.s-1 for ore bodies 1 and 3 respectively obtained from calculations derived from experimental results. This report provides the only pre mining 222Rn exhalation rates for Ranger ore body 3. Previously Kvasnicka (1990) used the results reported by Mason et al. (1982) to model the exhalation rates from Ranger for an atmospheric dispersion model. The other studies listed were performed at Nabarlek, Northern Territory (Bollhöfer et al. 2003; Martin et al. 2002), Koongarra, Northern Territory (Davy et al. 1978), Lake way, Western Australia (O'Brien and Whittlestone 1981), Olympic Dam, South Australia (Akber et al. 2002; Davey 1994; Harris and Chandler 1992; Sonter 1987) or were laboratory based using Australian samples (Hart 1986; Strong and Levins 1982) Measurement of the 222Rn exhalation rates from various sources at Ranger uranium mine is important for the modelling of the 210Pb budget within the region. To date most reports of 222Rn produced by the Office of the Supervising Scientist have concentrated on the measurement of 222Rn and 222Rn progeny concentrations within the Jabiru and Jabiru East region as a means of determining mining related doses received by members of the public in the region (Akber et al. 1991; Akber et al. 1994b; Akber and Pfitzner 1992; Akber et al. 1994a; Whittlestone 1992; Akber and Pfitzner 1994; Kvasnicka 1990; Akber et al. 1993; Peterson et al. 1993) 2.3.4 Radon migration, exhalation, soil gas concentration and soil moisture Similar to radon emanation the most important factor affecting radon migration and exhalation is soil moisture. Numerous authors have examined the effects of soil moisture or precipitation on 222Rn exhalation rates and soil gas concentrations (Tanner 1980; Tidjani 1988; Kraner et al. 1964; Grasty 1993; Washington and Rose 1990; Shweikani et al. 1995; Hutter 1996; Tanner 1964; Thomas et al. 1992; Luetzelschwab et al. 1989; Asher-Bolinder et al. 1990; Whittlestone et al. 1996; Jha et al. 2000; Schery et al. 1989; Ferry et al. 2001; Graaf

29 et al. 1992; Megumi and Mamuro 1974; Stranden et al. 1984; Owczarski et al. 1990; Koarashi et al. 2000; Ferry et al. 2002b; Hart 1986; Nazaroff and Nero 1988; Todd and Akber 1996). Laboratory and field studies show that small amounts of soil moisture increase 222Rn exhalation, while further increasing the moisture content, reduces it (Owczarski et al. 1990; Todd and Akber 1996; Megumi and Mamuro 1974; Stranden et al. 1984; Schery et al. 1989). Peak 222Rn exhalation rates were noted to coincide with same saturation levels that produce peak 222Rn emanation (Owczarski et al. 1990; Stranden et al. 1984). Stranden (1984) provided an explanation that the addition of small amounts of water increases emanation. Some of the additional 222Rn atoms that stopped in the pore space will escape from the water and diffuse to the soil surface. It may be likely that the exhalation to emanation ratio may be smaller at these saturation levels than for dry soils but this has not been investigated. Addition of more water to the soil traps 222Rn within the soil and reduces the exhalation rate. Todd and Akber (1996) study of a monazite sample showed that three days after simulated rainfall 220Rn exhalation rates increased. The increase lasted for four days and exhalation was 20% higher than the recorded initial dry exhalation rate. This behaviour was likely due to the sample surface layers drying out following the simulated rain event and the moisture content of the sample reaching a level that increases exhalation as mentioned above. The ability for soil to retain moisture primarily depends upon the soil porosity. Soil porosity plays the dominating role in the length of time that 222Rn and 220Rn exhalation rates remain reduced at a site after precipitation. The sooner water penetrates deeper into the ground or is evaporated from the soil the faster the recovery to normal radon exhalation rates The remainder of the studies performed on the topic of radon exhalation and moisture are based upon field work and most of them are more concerned with precipitation rather than the soil moisture content (Washington and Rose 1990; Thomas et al. 1992; Luetzelschwab et al. 1989; Jha et al. 2000; Ferry et al. 2001; Grasty 1994; Kraner et al. 1964; Shweikani et al. 1995; Koarashi et al. 2000; Ferry et al. 2002b). The work of these authors has shown that 222Rn and 220Rn exhalation rates decrease directly after moderate to heavy rain events while soil gas concentrations increase. The rain events examined in these cases increase soil moisture levels to values that retard exhalation. It has been shown that regions with

30 low precipitation have high exhalation rates and regions with high precipitation have low exhalation rates (Wilkening et al. 1974; Washington and Rose 1990; Tidjani 1988; Kraner et al. 1964; Grasty 1993). Soil depth profiles of 222Rn concentration after precipitation have shown that there is a delay in the increase of 222Rn concentration at large depths due to the time it takes for moisture to penetrate through the soil transporting 222Rn with it (Kraner et al. 1964). The depth to which moisture penetrates the soil has a direct influence on the 222Rn exhalation rate and concentration at that site (Kraner et al. 1964). 2.3.5 Radon exhalation, soil gas concentration and atmospheric pressure After soil moisture, atmospheric pressure has been described as being the most important meteorological condition affecting 222Rn and 220Rn exhalation and soil gas concentration (Tanner 1980; Tanner 1964). Studies have shown that increasing atmospheric pressure decreases exhalation rates and increases soil gas concentrations while the opposite is observed for a decrease in atmospheric pressure (Wilkening et al. 1974; Fleischer et al. 1980; Edwards and Bates 1980; Schery and Gaeddert 1982; Schery et al. 1982; Clements and Wilkening 1974; Janssens et al. 1988; Chen et al. 1995; Owczarski et al. 1990; Jha et al. 2000; Koarashi et al. 2000; Kraner et al. 1964; Thomas et al. 1992). It is noted that these effects are observed in short periods of time during pressure changes and that after pressure changes pass exhalation rates return to previous levels. Kraner et al. (1964) concluded that changes in the depth profile of 222Rn concentration levels, due to changes in atmospheric pressure, correlated well with what was theoretically expected from the diffusion of gases from soil media. Changes in atmospheric pressure will create pressure induced fluctuations of air in the soil. An increase in pressure will push air down into the soil retarding exhalation and increasing soil gas concentration. A decrease in pressure draws air out of the ground increasing exhalation and reducing soil gas concentration. It was noted that it is important for detection equipment to allow for changes in atmospheric pressure at the point of measurement. Clements and Wilkening (1974) designed a simple analytical model to examine the effect of atmospheric pressure on 222Rn exhalation and compared it with reported laboratory and field studies. Their model combined the effects of diffusion

31 and pressure induced transport and they concluded that diffusion remained the predominant migration mechanism. The results obtained from their model agreed with those reported by Kraner et al. (1964). They stated that the magnitude of change in 222Rn exhalation was dependent on the magnitude and rate of the pressure change. That is pressure changes over short periods of time vary the radon exhalation rate more than similar changes over longer periods. This is because quick pressure changes influence soil gases more than slower changes that may have little or no influence on soil gases. The work performed since these early studies serves to justify the results obtained in them. Some studies have reported a poor correlation for 222Rn and 220Rn exhalation rates with atmospheric pressure changes (Tidjani 1988; Schery et al. 1989; Graaf et al. 1992). They all reported that observed pressure changes were small and that other variables may be dominant. 2.3.6 Radon exhalation, soil gas concentration and temperature It has been mentioned previously in Section 2.2.6 that 222Rn emanation increases with temperature. It has been documented and reported that a good correlation between soil temperature and the 222Rn soil gas concentration also exists (Washington and Rose 1990; Hutter 1996). Washington and Rose (1990) reported that changes in soil temperature have less effect on 222Rn concentrations in dry soils than they do for moist soils. This is due to desorption of radon from solids into the interstitial space, mentioned in Section 2.2.6. If the interstitial space is filled with air radon is freely available to diffuse to the surface however if it is filled with moisture radon will remain dissolved in water. Natural fluctuations in ambient temperature do not have much of an influence on 222Rn and 220Rn exhalation as soil moisture and atmospheric pressure. Their is agreement in studies performed that the increase in exhalation due to temperature is minor if any (Jha et al. 2000; Stranden et al. 1984; Schery and Petschek 1983a; Schery et al. 1989; Tidjani 1988; Hutter 1996; Morris and Fraley 1989). In a number of these studies, where regression analysis has been used, the influence of temperature on exhalation has been reported with low correlation coefficients. Since temperature affects the physical properties known to influence radon exhalation and the diffusion coefficient the net effect that temperature has on radon soil gas concentrations and exhalation rates is likely due to these other dominant parameters being affected.

32 2.3.7 Radon exhalation, soil gas concentration and wind speed Wind speed has been reported as theoretically having an effect on 222Rn exhalation and soil gas concentration (Kraner et al. 1964; Wilkening et al. 1974). Kraner et al. (1964) reported that increased wind speed would deplete 222Rn from the upper layers of the ground, increase the concentration gradient and increase the exhalation rate. Direct measurements for the effects of wind speed are difficult since common measurement techniques involve covering the sampling area reducing wind speed to negligible velocities. Only one recent study has examined the influence of wind speed on exhalation rates (Jha et al. 2000). They reported that the wind speed variations from the region studied are not strong enough to cause any considerable change in the exhalation rate. Also they theorised that even extreme wind speeds would only produce minor variations in exhalation rate through depletion of soil gas from the upper layers of the ground. 2.3.8 Radon diffusion theory Fick’s law of diffusion of gases through porous media covers radon migration through unsaturated, undisturbed rocks and soils (Nazaroff and Nero 1988). It is shown here as Equation 2-2. d 2C λC EλρR Equation 2-2 − + = 0 dx2 D Dε Where C: interstitial concentration of radon isotope (Bq.m-3) x: soil depth measured from the soil-air interface (m) λ: radon isotope disintegration constant (s-1) D: diffusion coefficient of radon isotope (m2.s-1) E: radon isotope’s emanation coefficient for material R: parent radium isotope’s activity concentration of the material (Bq.kg-1) ρ: bulk density of material (kg.m-3) ε: porosity of the material The resultant radon exhalation rate at the soil-air interface can then be determined by ⎛ dC ⎞ Equation 2-3 J = −Dε⎜ ⎟ ⎝ dx ⎠

33 where dC − is determined as the solution from Equation 2-2, using the appropriate dx boundary conditions Equation 2-3 becomes ⎛ H ⎞ Equation 2-4 J = EexpρRλL × tanh⎜ ⎟ ⎝ L ⎠ where H: sample thickness (m) L: diffusion length (m) with

L = D Equation 2-5 λ

Radium activity concentration and distribution in the soil primarily affects radon emanation and has been covered in Section 2.2.2. For 222Rn exhalation, from ambient undisturbed soils, correlation with 226Ra activity concentration has been reported as relatively weak due to the other parameters that play a role (Ielsch et al. 2001; Oufni 2003; Sharma et al. 2003; Sroor et al. 2001). Yet those authors who have studied 222Rn exhalation rates from both uranium bearing minerals and ambient soils have reported differences in 222Rn exhalation rates of orders of magnitude (Todd 1998; Schery et al. 1989). Migration of radon is negligible to the radon soil gas concentration at a given point and it has been shown that the relationship with radium activity concentration is stronger than that seen between radon exhalation and radium activity concentration (Luetzelschwab et al. 1989; Asher-Bolinder et al. 1990; Grant et al. 2001; Choubey et al. 1999) The diffusion length of 222Rn in undisturbed soils is approximately 1-2 m as previously mentioned in Section 2.3.1. As such areas where soil cover is shallow or areas where soil is porous, the underlying geological rock formations may have a direct influence on the 222Rn exhalation rates observed. This is especially the case if the 226Ra concentration of the rock formations differs greatly from the overlying soil. A number of studies have been performed that examine underlying geology and uranium concentration variations and their relationship to the 222Rn exhalation rate and soil gas concentration (Choubey et al. 1999; El-Dine et al. 2001; Ielsch et al. 2001; Ramola et al. 1988; Sengupta et al. 2001; Varley and Flowers 1992; Akber et al. 1980; Khan et al. 1980). These types of studies include the determination of

34 underlying uranium mineralisation and detection of ore bodies through enhanced exhalation rates. Tanner (1980) reported that in fractured rocks and disturbed soils transport mechanisms become the dominant method for 222Rn and 220Rn migration meaning that Fick’s law presented above in Equation 2-2 no longer holds for these conditions. Ielsch et al. (2001) examined the variations in 222Rn exhalation rates and atmospheric concentrations at the soil atmospheric boundary over two transects from coastal to inland France. Variations in geology and uranium concentration over the two transects were recorded and compared with the 222Rn measurements. A number of uranium rich granites were located along these transects and it was found that while increased 222Rn exhalation rates were positively correlated with increased uranium concentration it was not the only contributing factor. Other factors such as fractures, variations in depth of soil cover over granite formations and soil type were also important. Similar studies to this have been performed in India and England and report that uranium concentration is an important, but not the only influencing factor for increased 222Rn exhalation (Varley and Flowers 1992; Sengupta et al. 2001; Choubey et al. 1999). Studies performed over fault lines and fractured rocks are in agreement with the conclusions drawn by Tanner (1980) in that transport dominates and high 222Rn exhalation rates are observed from these areas. This leads into a whole area of study where 222Rn exhalation measurements have been used for the determination of fault lines, as an earthquake precursor and to examine the underlying geological conditions. This topic is beyond the scope of this project and has not been considered in this review. Soils with large porosity have more interstitial space and have greater 222Rn and 220Rn exhalation rates (Shweikani et al. 1995). The increase in exhalation is due to two factors. Firstly larger interstitial space will increase emanation with a reduction in the number of 222Rn and 220Rn atoms that embed into nearby grains. Secondly the larger space in the soil provides an easier diffusion path to the soil surface. A result of this is that 222Rn and 220Rn will exhale from greater depths in these types of soils. Shweikani et al. (1995) noted, using a constant moisture value of 14%, there was a non-linear behaviour in the effect of porosity on the 222Rn exhalation rate compared to the linear relationship expected from Fick’s law, Equation 2-2. They argued that the effect of moisture created the variation in

35 emanation as having the same moisture content and varying porosity would vary the actual amount of moisture layer on each grain. This was in agreement with theoretical emanation modelling reported by other authors (Tanner 1964; Semkow 1990; Maraziotis 1996; Markkanen and Arvela 1992). A model by Owczarski et al. (1990) demonstrated the effect that a variation in soil porosity has upon 222Rn exhalation. Even though this work was primarily about variations in 222Rn exhalation due to changes in soil moisture and atmospheric pressure, their use of five different soil types with different porosity provides a good display of the effect that porosity has on 222Rn exhalation. A variation in exhalation rates between less porous materials (clay) and more porous materials (sand) was evident in their study. 2.3.9 Radon exhalation temporal variations Precipitation, soil moisture, atmospheric pressure and temperature vary temporally. Systematic variations may occur with the time of day and between seasons. A number of studies have been performed that measure 222Rn and 220Rn exhalation rates and soil gas concentrations over time to observe changes resulting from variations of these parameters (Jha et al. 2000; Wilkening et al. 1974; Whittlestone et al. 1998; Segovia et al. 1987; Thomas et al. 1992; Ferry et al. 2002b; Ferry et al. 2001; Washington and Rose 1990; Hutter 1996; Torri et al. 1988; Tidjani 1988; Winkler et al. 2001). Most of these studies however focus on measurement of 222Rn concentrations in soil gas and only three of them are on 222Rn exhalation rates (Whittlestone et al. 1998; Wilkening et al. 1974; Jha et al. 2000). Even though there is typically an inverse relationship between the two the lack of seasonal exhalation studies leaves a gap in the body of knowledge. Only one study (Jha et al. 2000) has been performed within a tropical region, Jaduguda, India, which lies just south of the Tropic of Cancer. The study was performed within a uranium mineralised zone and reports good correlations of 222Rn exhalation rates with soil temperature and atmospheric concentration values. They observed only minor fluctuations in the overall seasonal results and conclude that the composite influence of the meteorological parameters masked the effect of individual parameters. Wilkening et al. (1974) only performed 222Rn exhalation measurements from winter through to spring at a site in Rio Grande valley, Socorro, USA. This was not a long enough time scale to observe any major seasonal variations in the radon exhalation rate. Finally, Whittlestone et al. (1998) performed comparison

36 measurements for different seasons. The study was based on data measured from sites in Victoria and Tasmania. They reported reduced 222Rn and 220Rn exhalation rates in winter. This was related to the fact that the rainfall rates were higher during winter. Winter minima of exhalation rates in US and European studies have been related to snow cover and frozen ground which restricts exhalation from the ground (Washington and Rose 1990; Winkler and Rosner 2000). Temporal variations in soil gas concentrations have been observed at many locations (Wilkening et al. 1974; Washington and Rose 1990; Hutter 1996; Winkler et al. 2001; Torri et al. 1988; Tidjani 1988). Of particular interest is the study by Tidjani (1988) that has been performed in the African tropical country of Senegal. The author did not find seasonal variations in 222Rn soil gas concentrations at the onset of the wet season. They commented this was contrary to what they expected given the reported retardation of 222Rn with soil moisture. Again Wilkening et al. (1974) measurements over a few months aren’t enough to justify any seasonal variations although they did report variations up to 30% that correlated well with barometric pressure changes, these measurements took place in New Mexico. Washington and Rose (1990) observed winter minima and summer maxima of 222Rn soil gas concentrations from three years of data collected from sites in Pennsylvania and New Jersey. This was in contrast to what had been reported by other authors from different locations, such as those performed in New Jersey and Germany (Winkler et al. 2001; Hutter 1996), but it was in agreement with results obtained from an Italian study (Torri et al. 1988). Washington and Rose (1990) concluded that winter minima and summer maxima were due to the seasonal variations in the rainfall, soil moisture, atmospheric pressure and temperature. Another study of 222Rn soil gas concentrations from New Jersey (Hutter 1996) reported spring to summer lows and autumn to winter highs and agree well with exhalation studies. A study of the seasonal 222Rn soil gas concentration in Germany (Winkler et al. 2001) also reported spring to summer lows and autumn to winter highs. Peaks observed in 222Rn soil gas concentrations over the summer months correlated well with rainfall patterns that saturate the soil and prevent 222Rn migration. Of note is that there has been little work performed on seasonal variations in 222Rn and 220Rn exhalation rate from open ground and the main focus has been on soil gas concentration measurements. This is perhaps because soil gas concentration is more related to 222Rn entry into homes. It is most likely that seasonal variations in

37 exhalation rates are also as broad as those observed for soil gas concentration measurements and may be site specific. Revision of the above studies was important for this work as the region this work was performed in experiences distinct wet and dry seasons. From theory and the studies previously published it was expected that 222Rn exhalation rates in the Kakadu region should decrease throughout the wet season. 2.3.10 Radon migration, exhalation and soil gas concentration summary There is wide and varied literature dealing with the subject of radon migration, soil gas concentration and exhalation. Some of this work focuses on measurement techniques but a large portion examines various soil and meteorological parameters known to influence exhalation, migration and soil gas concentrations. There is an agreement in all work that soil moisture plays the most dominant role in affecting radon exhalation because of its solubility and thus exhalation is retarded after precipitation. Moisture has a capping effect and traps radon in the soil before sweeping it through the soil as the moisture passes downwards. The capping effect dominates and radon soil gas concentrations increase after precipitation. The other parameters that have been noted to influence radon are atmospheric pressure; soil and atmospheric temperature; wind speed; radium activity concentration and distribution; soil grain size, porosity, compaction and underlying geological formations. The effect of most of these has been examined through either laboratory or field measurements. It appears that the magnitude of influence of these parameters varies from site to site and they are also seen to combine and mask out the effects of other parameters. It has been noted that there is a lack of knowledge about the seasonal dependency of radon exhalation that needs to be further investigated. There is also a lack of knowledge about the composite behaviour of the influencing parameter that effect radon exhalation. A number of studies examined include measurements of both 222Rn and 220Rn. This is a result of improved detection equipment that provides a means of distinguishing between the contributions of the two isotopes for simultaneous measurements. A number of the studies mentioned above have provided both 222Rn and 220Rn exhalation rates from their work (Schery and Petschek 1983b; Bigu and Elliot 1994; Zahorowski and Whittlestone 1996; Schery et al. 1989; Whittlestone et

38 al. 1996; Schery and Petschek 1983a; Whittlestone et al. 1998; Megumi and Mamuro 1974; Todd and Akber 1996). The same geological, meteorological and seasonal factors that affect 222Rn exhalation rates also affect 220Rn exhalation rates in a similar manner with a variation in the magnitude of the effect only. It can only be assumed that they also affect 219Rn exhalation rates but measurement of this isotope in all but laboratory conditions is difficult. 2.4 Pb-210 deposition 2.4.1 Introduction Radon-222 atoms exhaled from the ground spread into the atmosphere where they remain until they decay. Most of radons direct progeny, 218Po, is electrically charged (~88%), a result of the decay process, and a large proportion (~90%) become attached to aerosol particles (Pagelkopf and Porstendorfer 2003). The processes involved in attachment has been the subject of many studies (Bandi and Phillips 1988; Bouland and Chouard 1992; Cheng et al. 2000; Hopke 1996; Morawska and Jamriska 1997; Planinic et al. 1997; Porstendorfer et al. 2000; Tokonami 2000; Winkler et al. 1998; El-Hussien and Ahmed 1995). It is reported that the attached fraction attaches to aerosol particles with an average size distribution in the range 0.07-1 μm (Winkler et al. 1998; Morawska and Jamriska 1996; Tu et al. 1994; Porstendorfer et al. 2000) while the unattached fraction forms clusters with trace gases and vapours in air with a size spectrum between 0.5-3 nm (Pagelkopf and Porstendorfer 2003). The half-lives of the direct progeny of 222Rn are such that within 4 hours more than 99% have decayed into 210Pb. Meanwhile both attached and unattached fractions are subject to atmospheric transport mechanisms that occur at the place of their formation (Winkler et al. 1998; Koch et al. 1996). It is reported that precipitation is the dominant scavenger of the attached fraction (Preiss and Genthon 1997; Patterson and Lockhart 1964), while dry deposition through plate-out and fall- out removes the unattached fraction (Schery et al. 1992; Lupu and Cuculeanu 1999). Precipitation scavenging can occur as a result of in cloud rainout of particles within the rain clouds, or below cloud washout of dust and particles in the atmosphere. Contrary results have been reported for the mean residency time of 210Pb in the atmosphere, it has been reported as being between a few days or up to a hundred days. Residency times of 5-9 days are typically reported for temperate regions (Beks et al. 1998; Pourchet et al. 2000; Koch et al. 1996; Moore et al. 1977). Even with the

39 broad range of reported residency times they are all shorter than the half-life of 210Pb, 22.3 years. After deposition on the ground 210Pb penetrates into the soil over time to a depth that depends on soil characteristics such as grain size, porosity, moisture content and attachment properties. The natural process of 222Rn progeny attachment, 210Pb deposition and soil transport has a wide variety of applications including the following: − For the determination of aerosol movement and residency times in the lower atmosphere (Preiss et al. 1996; Koch et al. 1996; Winkler et al. 1998; Martin 2003; Lupu and Cuculeanu 1999); − The examination of radon progeny deposition on indoors surfaces for health studies (Lively and Ney 1987; Leung et al. 2000; Morawska and Jamriska 1996; Nikezic and Yu 1999); − Using existing data to develop models of global sources and deposition of 210Pb (Preiss and Genthon 1997; Preiss et al. 1996); − For studies of the prevailing meteorological conditions controlling 210Pb atmospheric concentration and deposition over seasonal changes or individual events (Rangarajan et al. 1986; El-Hussien et al. 2001; Winkler and Rosner 2000; Bonnyman et al. 1972; Rosner 1988; Branford et al. 1998; Kim et al. 2000; Beks et al. 1998; Martin 2003; Melieres et al. 2003; Zahorowski et al. 2004; Todd et al. 1989; Baskaran 1995; Baskaran et al. 1993; Hussain et al. 1990; Turekian et al. 1983); − To investigate the history of 210Pb deposition and its soil transport through analysis of soil profiles (Pourchet et al. 2000; Nozaki et al. 1978; Moore and Poet 1976; Matthews and Potipin 1985; Thomson et al. 2002); − Investigating the effect of soil surface and geomorphic changes on the 210Pb atmospheric concentration and deposition (Branford et al. 1998; Patterson and Lockhart 1964; Lupu and Cuculeanu 1999); − Applied to the measurement of erosion and sedimentation deposition in water systems (Imboden and Stiller 1982; Zapata 2003; Li et al. 2003; Walling et al. 2003; Heijnis 1999; Matthai et al. 1998; Bonniwell et al. 1999; Heijnis et al. 1987; Wallbrink and Murray 1993; Belyaev et al. 2004; Ugur et al. 2004; Zhang et al. 2003; Matisoff et al. 2002; Milton et

40 al. 2001; Whiting et al. 2001; Walling and He 1999; Wallbrink et al. 1999; Ivanovich and Harmon 1992; Ivanovich and Harmon 1982). This study focuses on the deposition of 210Pb. 2.4.2 Pb-210 depositional rate studies Radon-222 exhaled into the atmosphere from land is the only source of 210Pb to the atmosphere. Currently a global database exists containing the results of numerous studies of 210Pb atmospheric concentrations, depositional rates and lake sedimentation rates (Preiss et al. 2003). For depositional rates this database currently holds results from more than 100 sites over all continents and is the most extensive collection of 210Pb depositional rate values available to date. While a compiled list of this work is too large to add to this chapter it is noted that worldwide 210Pb depositional rates range from 2 Bq.m-2.y-1 (Argentina Islands) to 465 Bq.m-2.y-1 (Kitami, Japan) with an average value of 124 Bq.m-2.y-1. Of note is that values reported for the Northern Hemisphere are larger than those for the Southern Hemisphere due to larger landmass, hence 210Pb source, found in the Northern Hemisphere. Some of the major contributors to this database have performed measurements of 210Pb deposition for many years investigating seasonal variations and related them meteorological patterns (Baskaran 1995; Winkler and Rosner 2000; Melieres et al. 2003; Su et al. 2003; Rosner 1988; Baskaran et al. 1993; Turekian et al. 1983; Todd et al. 1989; Beks et al. 1998; Bonnyman et al. 1972; Kim et al. 2000). Others have investigated deposition over shorter periods of time or even through individual rain events (Martin 2003; Caillet et al. 2001; Pettersson and Koperski 1991; Fujitaka et al. 1992). It is best to understand the effects occurring within individual rain events before broadening the concepts to seasonal studies. It has been mentioned that rainfall is the dominant scavenger of 210Pb from the atmosphere. This occurs through two methods, “in-cloud rainout” and “below- cloud washout” (Martin 2003; Caillet et al. 2001). In-cloud rainout refers to the removal of 210Pb attached to aerosols that themselves are attached to water molecules within clouds. Below-cloud washout refers to the removal of attached and unattached 210Pb within the atmosphere collected on rain as it descends. Fujitaka et al. (1992) showed that near surface 210Pb atmospheric concentrations returned to normal levels 3hours after precipitation events that caused significant washout. That study was performed in the Chiba region, Japan (35oN) but it was noted that the region receives

41 monsoonal rain patterns. Caillet et al. (2001) performed measurements of 210Pb deposition from individual rain events at a study site in Switzerland. They noted the heavier rainfall produced more washout compared to light rain events and reported a mean reload time of 1-2 days for 210Pb. A number of measurements from the east coast of the United States have been made around Chesapeake Bay (Todd et al. 1989; Kim et al. 2000). A report published by Todd et al. (1989) examined the atmospheric characteristics of 210Pb along the South Virginia Coast from 1982-1985. Both wet and dry depositional rates were measured and it was found that wet deposition provided more than 90% of the total deposition. Their investigation into seasonal variations found spring maxima and autumn minima for the 210Pb depositional rates. These were attributed to precipitation events and ocean air mixing respectively. The results were in agreement with other reports from coastal regions of the United States. Kim et al. (2000) performed a more recent study of the atmospheric factors that affect the 210Pb depositional rate at Stillpond in the north of Chesapeake Bay. The study measured 210Pb deposition for one year. They also reported a strong correlation between the depositional rate and precipitation and a spring maximum and autumn minimum. They noted that at this location a period of at least two weeks between weak precipitation events was required to obtain similar depositional rates again. Martin (2003) analysed uranium and thorium series radionuclides deposited in rainwater from a number of tropical storms between 1985-1989 at Jabiru and Jabiru East in the Northern Territory, Australia. Samples were collected from a total of 16 storms, 9 at Jabiru East and the 7 in the township of Jabiru. This work was of interest as the area of study was the same as the one used for this project. He noted that most of the 210Pb depositional rate measured in these storms was sourced from in-cloud rainout while a ‘substantial fraction’ of the measured activity concentrations of 238U, 234U, 230Th and 226Ra was due to below-cloud washout of dust transported from the nearby Ranger mine. He also determined 210Pb residency times and reported widely ranging values from 0-70 days. It was noted that residency times were smaller in the mid to late wet season, a result of the constant removal of 210Pb from the atmosphere due to frequent rain events. For six of the storms he collected sequential samples and noted that for three of them there was an initial large deposition of 210Pb that decreased rapidly throughout the remainder of the storm. It was reported as a result of below-cloud washout of 210Pb created from 222Rn decay in the atmosphere

42 rather than washout of 210Pb attached to suspended dust. He noted that sequences of light rain tended to have larger 210Pb concentrations due to partial washout. This finding was in agreement with work previously published (Turekian et al. 1983). In contrast to the work performed by Martin (2003) Pettersson and Koperski (1991) measured the dry deposition of 238U series radionuclides on passive vinyl collectors as a function of distance from Ranger mine. They derived an estimate of the dry deposition of each long-lived 238U series radionuclide at Jabiru East to be about 10 Bq.m-2.y-1. From their work a value of 27 Bq.m-2.y-1 for 210Pb can be determined. The global database includes Australian data of 210Pb depositional measurements performed by Bonnyman et al. (1972) covering six years of sampling. This is the only report available for annual 210Pb depositional rates from Australia including the tropical northern Australia. The report observed a correlation between 210Pb depositional rates and precipitation events. It was also reported that very high depositional rates were recorded in hot, dry inland areas after rare rain events occurred. Lead-210 depositional rates followed the same patterns as those of the fission product Cs-137 and they also followed a seasonal trend that was related to the rainfall pattern. This report includes results from the tropical locations of Darwin and Townsville where the 210Pb deposition rates obtained were 95 Bq.m-2.y-1 and 38 Bq.m-2.y-1 respectively. The group responsible for maintaining the global database noted the lack of results for 210Pb depositional rates from tropical locations (Preiss et al. 1998). They recently reported results from a study site in French Guiana, South America, covering two years of continuous monitoring (Melieres et al. 2003). The site is tropical, experiencing monsoonal, wet season summers and dry season winters with an average annual rainfall of 3010mm most of which occurs in the wet season, January-June. They reported a mean annual 210Pb deposition rate of 163±75 Bq.m-2.y-1 and also observed that deposition was proportional to rainfall but only for rainfall events less than 15 cm over 15 days. This linearity was masked by strong fluctuations when the rainfall increased to higher values. No attempt was made to determine seasonal trends for the data but the information displayed shows depositional peaks occurring during the monsoonal months for both years of study. A long term 210Pb depositional rate measurement project was performed in the Netherlands (Beks et al. 1998). Results from four locations, Groningen, Texel, de

43 Bilt and Bithoven have been reported with the latter two sites being the work of another author. Sampling at these sites was performed from 1987-1994, 1991-1996, 1991 and 1987-1991 respectively. Annual 210Pb depositional rates for the sites are determined as 71±20 Bq.m-2.y-1, 82±33 Bq.m-2.y-1, 56 Bq.m-2.y-1 and 71±38 Bq.m-2.y-1 respectively, where the errors are determined as the standard deviation of annual data. The authors reported large random daily variations related to precipitation events. Overall, lower depositional rates are likely to be a result of land and sea air mixing. Summer depositional rates were higher than winter and related to storms and precipitation events that are more common in the summer months. The large annual variability was reported as being controlled by the number of heavy rain events and thunderstorms each year. Studies of 210Pb depositional rate in Germany have been performed from 1981-1999 the results of which have been reported in two articles (Rosner 1988; Winkler and Rosner 2000). In agreement with other work, they reported a strong correlation between precipitation and the amount of 210Pb deposition. Seasonal fluctuations of 210Pb depositional rate observed were summer maximums and winter minimums related to precipitation events that are more common in summer. Also the ground is capped with ice throughout the winter months that will reduce 222Rn exhalation as a 210Pb source. The site used in this study is 10 km north of Munich and classified as a continental location with a prevalent westerly wind carrying moist Atlantic air. The resultant averaged annual 210Pb depositional rate was 180±42 Bq.m-2.y-1 and was in general agreement with other research performed in Europe. Results of a continuous monitoring study from Taiwan, 1996-2001, have also reported seasonality in 210Pb depositional rates with winter peaks and summer minimums (Su et al. 2003). Winter peaks were related to the southward movement of the Mongolian High when the northeast monsoon brings 210Pb enriched air masses towards Taiwan. During summer the southwest monsoon introduces 210Pb depleted air masses from the South China Sea and western Pacific resulting in reduced 210Pb deposition. The switch of monsoon patterns largely explains the annual cycle of 210Pb deposition. While they did not report annual depositional rates they did observe the typical correlation of deposition with precipitation events. They also related increased deposition with dust storms that come from continental Asia laden with 210Pb.

44 Researchers in the United States have a number of stations monitoring 210Pb depositional rate over many years. Baskaran et al. (Baskaran 1995; Baskaran et al. 1993) have provided reports on the seasonal variability of 210Pb depositional rate from Galveston and College stations in Texas from 1990-1993. In the first report (Baskaran et al. 1993) they did not observe any obvious seasonal fluctuation in the 210Pb depositional rate. However they did report that precipitation was the controlling factor for the amount of deposition. They also examined the amount of deposition after individual precipitation events to measure the contribution of each event to the total deposition. In a second paper Baskaran (1995) reported that a small number of precipitation events (4-6%) contributed a large amount (20-30%) of the total annual 210Pb deposition. This is related to the fact the most washout of 210Pb occurs within the first half hour of precipitation. Rain events lasting for long durations will generally deposit less 210Pb as they continue (Martin 2003). Investigation for seasonal fluctuations continued, since other North American stations had reported spring-summer highs and autumn-winter lows of 210Pb depositional rate. These further investigations resulted in Baskaran (1995) reporting the same seasonal fluctuations from their stations but on a much smaller scale and also heavily dependent on precipitation events. Annual 210Pb deposition rates were 170 Bq.m-2.y-1 and 130 Bq.m-2.y-1 for Galveston and College, respectively. An earlier report by Turekian et al. (1983) measured the 210Pb depositional rate at New Haven in Connecticut and Bermuda. From a model, they expected depositional rates at Bermuda to be quite large as it is downwind from a continental source. Observations showed that the Bermuda depositional rate was 70% of that predicted by the model due to a seasonal high pressure system that deflects continental air. For New Haven the 210Pb deposition rate was reasonably correlated precipitation with but this was not the case at Bermuda. Overall the reported depositional rates are 202 Bq.m-2.y-1 for New Haven and 115 Bq.m-2.y-1 for Bermuda. 2.4.3 Pb-210 soil studies Lead-210 settles upon the surface of the earth through dry and wet deposition and has a strong affinity for soil and sediment particles (He and Walling 1997). Initially 210Pb will attach to soil particles close to the surface of the soil reducing in concentration with respect to depth. It should be noted that 210Pb found in the surface soil can be a result of deposition or decay within the ground from 222Rn. Given that

45 the ground component of 210Pb can be determined from an assumption that it is in equilibrium with 226Ra the excess, deposited, 210Pb can be determined. Measurements of soil profiles indicate that excess 210Pb is redistributed vertically in the surface layers of the soil. He and Walling (1997) explain this that redistribution reflects the physio-chemical and biological processes that operates within the soil including diffusion, convection and bioturbation. Ion exchange is the most dominant absorption method but this is reversible and excess 210Pb can be replaced by other ions leaving it free to re-enter the soil. Lead-210 released from one location may be re-absorbed at another and transported downward with soil moisture, while free in the soil it may also become subject to molecular diffusion. Bioturbation associated with vertical mixing by soil fauna also represents an important mechanism for redistribution of excess 210Pb in the soil. With a mean lifetime of 32 years, excess 210Pb should travel to depths of 10-20 cm in normal soils. It is possible to use measurements of 210Pb in soils, snow or ice as a means of determining an integrated average of previous depositional history for a region. Such measurements can also be used for studies of erosion in surface soils and determination of sedimentation rates in water systems. Measurement techniques, applications and various studies of 210Pb in soils and ice cores have been reported by a number of researchers (Matthews and Potipin 1985; Moore and Poet 1976; Nozaki et al. 1978; Zapata 2003; Walling et al. 2003; Li et al. 2003; Branford et al. 1998; Pourchet et al. 2003; Schulz et al. 2003; Kim et al. 1997; Roos et al. 1994; Graustein and Turekian 1986; Wallbrink and Murray 1993; Thomson et al. 2002; Huh and Su 2004; Heijnis 1999; Heijnis et al. 1987; Imboden and Stiller 1982; Matthai et al. 1998; Bonniwell et al. 1999; Belyaev et al. 2004; Ugur et al. 2004; Zhang et al. 2003; Walling and He 1999; Milton et al. 2001; Matisoff et al. 2002; Whiting et al. 2001; Ivanovich and Harmon 1992; Ivanovich and Harmon 1982). The use of 210Pb for erosion and sedimentation studies has become the most common use for this radionuclide. Until recently, focus lay on 137Cs but this fission product has only been released into the atmosphere during above ground atomic explosions or by an accidental release, such as Chernobyl. As above ground nuclear testing ceased in the early 1960’s there is no longer an input source of 137Cs into the top layers of the soil. Due to redistribution and radioactive decay there is a constant reduction in the 137Cs inventory. Compound this with the fact that equatorial regions and the southern hemisphere received less deposition of 137Cs, 210Pb is a suitable

46 alternative (Walling et al. 2003). Pb-210 is constantly replenished due to continuous constant release of 222Rn from the surface soils across the world. The soil interactions between the two radionuclides are very similar (He and Walling 1997) so researchers of soil erosion will likely turn to 210Pb for future studies as the 137Cs profile disappears. For short-term erosion studies the short-lived radionuclide 7Be has been successfully used, but with a half-life of 55 days its use is limited to these short-term studies and studies are best coupled with measurements that include other radionuclides (Zapata 2003). Moore and Poet (1976) noted that the profile of 210Pb will be different for disturbed soils compared to undisturbed soils and that from analysis of the 210Pb profile a date for the soil disturbance could be determined. They were also able to use their results to determine an estimate of the annual 210Pb depositional rate for the area of study in Colorado. Nozaki et al. (1978), Roos et al. (1994) and more recently Zapata (2003) have also reported that analysis of the atmospheric component of 210Pb in soil profiles can be used to provide the historical depositional rate for about the last 100years. A similar method was used by Pourchet et al. (2003) to determine past 210Pb depositional rates over Antarctica through a measurement of snow and ice core samples. Measurement of 210Pb from salt marshes has also shown it to be useful as a geochronological tool (Kim et al. 1997; Thomson et al. 2002). In Zambia samples were taken from undisturbed and disturbed sites showing the clear distinction in 210Pb profiles as a result of erosion and farming (Walling et al. 2003). Of notice in this work was the ability to accurately determine tillage depths of soils from an analysis of depth profile. The example of soil profiles in Figure 2.7 is from their work. For this work erosion or sedimentation was determined from analysis of the total excess 210Pb inventory compared with an averaged standard undisturbed profile. Where the total inventory was less than the standard, it was concluded that erosion has occurred and where it was greater, sedimentation has occurred.

47 a) Undisturbed soil profile c) Commercial cultivation soil profile with deposition Unsupported 210Pb content (Bqkg-1) Unsupported 210Pb content (Bqkg-1) 0 1020304050607080 0 1020304050607080

0-2 0-2

4-6 4-6

8-10 8-10

12-14 12-14

16-18 16-18 Tillage Depth Depth (cm) Depth (cm) 20-22 20-22

24-26 24-26

28-30 Total inventory = 2602 Bqm2 28-30 Total inventory = 5054 Bqm2

b) Communal cultivation soil profile with erosion d) Bush grazing soil profile with erosion 210 -1 Unsupported Pb content (Bqkg ) Unsupported 210Pb content (Bqkg-1) 0 1020304050607080 0 1020304050607080

0-2 0-2 Tillage Depth 4-6 4-6

8-10 8-10

12-14 12-14

16-18 16-18 Depth (cm) Depth (cm) Depth 20-22 20-22

24-26 24-26

28-30 28-30 Total inventory = 1505 Bqm2 Total inventory = 2000 Bqm2

Figure 2.7: Typical 210Pb soil profiles for various soil uses (adapted from Walling et al. (2003))

In Australia Wallbrink and Murray (1993) examined the potential of 210Pb for erosion studies by setting up an experimental erosion field and generating artificial erosion processes. In conjunction with measurements of 7Be and 137Cs they found that they were able to distinguish various erosion processes based up on measurements of radioactivity in the sedimentation. They found that sediments moved from surface soils had high concentrations of 7Be and 210Pb while those derived from gully erosion had much lower concentrations. Similarly Li et al. (2003) used 137Cs and the 210Pb/137Cs ratio to determine the main sources of erosion in the hills of western China. They were able to determine that gully erosion and not surface soil was the dominant erosion process contributing to sedimentation in the Yellow River. However Huh and Su (2004) noted that 210Pb deposition rates vary spatially and temporally making it more difficult to determine whether varied values are a result of erosion or natural processes. They noted that erosion studies are possible if variations in 210Pb depositional rates are known along with their relationship with precipitation events of the study region. In the past radiochemical techniques were used to determine 210Pb in water and soils. Reports the like of Matthews and Potipin (1985) detail the techniques

48 involved in sample preparation for the extraction of 210Pb fallout from soils for alpha beta counting. More commonly today though, due to the improved technology of low-level germanium gamma detectors, soil samples are analysed through the detection of the low energy gamma ray (46 keV) that is emitted during 210Pb decay. This has reduced the analytical time required to process samples although there are some issue involved with self-absorption of the low-energy gamma ray. Matthews et al. (1985) continued on to determine that the maximum penetration depth for 210Pb in normal soils was 25 cm and that migration distance and time is different to 137Cs. Primarily 210Pb is used for sedimentation studies in fluvial systems to examine the build up of sedimentation through analysis of the radionuclide concentrations (Zapata 2003). There is a large body of work on this topic but it is considered to be beyond the scope of this review. 2.4.4 Pb-210 deposition and geographical location It has been observed in a number of studies that geographical location also has an influence on 210Pb deposition. In some cases it may be a result of the prevailing winds carrying 210Pb rich or poor air from other locations, in other areas it might be due to the interaction between topography that affects the rainfall patterns, such as the effect that mountainous regions have on inducing or preventing rainfall. A number of studies report that coastal locations have less 210Pb deposition as a result of mixing 210Pb poor air from the ocean and 210Pb rich air from the land (Beks et al. 1998; Bonnyman et al. 1972; El-Hussien et al. 2001; Kim et al. 2000; Patterson and Lockhart 1964; Winkler and Rosner 2000; Todd et al. 1989; Baskaran et al. 1993; Hussain et al. 1990; Preiss et al. 1996; Turekian et al. 1983). As has been previously mentioned, Turekian et al. (1983) specifically investigated the depositional rates of 210Pb from an east coast site on continental United Sates and the island of Bermuda. The results obtained for the coastal sites agreed well with observations from similar sites in the United States. However they expected Bermuda to have a high depositional rate as it is downwind from a continental source of 210Pb. The values obtained from Bermuda did not agree with their expectation and further analysis showed that the effect of ocean air mixing was underestimated and that the prevailing weather conditions kept the 210Pb depositional rate at Bermuda at lower levels compared to mainland sites. It should be noted that the prevailing meteorological conditions of an area dominate more than the effect of localised geography even though the two may be

49 linked. This has been reported by Patterson and Lockhart (1964) who showed that the meteorological conditions prevailing in their area of study were much more significant to 210Pb deposition than the geographical location. This study looked at the depositional rates of 210Pb along the 80th meridian in an attempt to observe latitudinal variation. The results showed up to an order of magnitude difference in the depositional rates between the various stations leading them to the conclusion that meteorological conditions prevail. Localised variations in geography and topography have been examined for their effect on the depositional rate of 210Pb (Lupu and Cuculeanu 1999; Branford et al. 1998). Lupu and Cuculeanu (1999) reported that the vegetation cover over an area had a dramatic effect on the vertical profile of the concentration of radon progeny. It was concluded that this would have a major effect on the 210Pb depositional rate at these locations. Branford et al. (Branford et al. 1998) measured 210Pb depositional rates from the coast to inland sites in Scotland, over an area with three distinct mountain peaks. The results showed that deposition was greatest on the ocean-facing face of the first mountain and deposition increased with increasing altitude for this mountain. A weaker, similar observation was made for the second mountain but not for the third. They concluded that the deposition was related to precipitation events and that the results for the ocean facing mountain strongly showed the effect of its influence to induce rainfall that reduces with increasing distance from the coast. In this case a strong correlation was made between the localised geography and 210Pb deposition but this was due to the influence that mountains have on inducing precipitation events. 2.4.5 Pb-210 atmospheric concentration studies There is of course a relationship between 210Pb depositional rates and atmospheric concentrations. This relationship is a result of 210Pb below-cloud washout during rain events that reduces 210Pb atmospheric concentrations. A number of the studies mentioned have also investigated 210Pb atmospheric concentrations, performed correlations with depositional rates and investigated seasonal fluctuations (Rangarajan et al. 1986; El-Hussien et al. 2001; Winkler and Rosner 2000; Bonnyman et al. 1972; Rosner 1988; Beks et al. 1998; Todd et al. 1989; Baskaran 1995; Baskaran et al. 1993). Some of these studies have also examined 7Be and other radionuclide depositional rates as it can be performed during analysis of 210Pb.

50 Until the study by Melieres et al. (2003) the majority of work on 210Pb atmospheric concentrations within tropical regions was from India (Rangarajan et al. 1986). Rangarajan et al. (1986) reported averaged annual readings of 210Pb concentrations from their various collection sites across India over many years of study. They showed a distinctive decrease in 210Pb atmospheric concentrations during the monsoonal period. They also reported that this was a result of dominant sea breezes and below-cloud washout. They observed clearly though that in the short term 210Pb concentrations varied seasonally with dry season peaks and wet season troughs at all of the study locations. These results differ from those reported by Melieres et al. (2003) but noted that differences in the prevalent winds can explain this. Guiana experiences ocean winds for most of the year and occasionally Saharan plumes carried all the way to South America creating occasional 210Pb peaks. In India, especially Bombay, dry season winds come from inland sources carrying more 210Pb while the wet season winds come from the ocean with a lower 210Pb concentration. An article published by El-Hussein et al. (2001) examined the seasonal variation of 210Pb concentration in the surface air at El-Minia, Egypt. They reported summer minimums and winter maximums of the concentration of 210Pb in the surface air. With low rainfall in this region the variation was attributed to an inversion effect of the surface air layers during the different seasons. They also observed daily variations that they attributed to atmospheric mixing. The results reported were in agreement with patterns observed elsewhere around the world. The other studies all show that 210Pb deposition increases with rainfall while atmospheric concentrations decrease due to below-cloud washout. This will reach a point where the replenishment of 210Pb from exhaling 222Rn into the atmosphere is retarded due to capping effect by moisture in the soil so further rainfall will result in reduced deposition rates and further reduced atmospheric concentrations. It has also been noted that locations with high 210Pb atmospheric concentrations initially have large depositional rates. It is evident that below-cloud washout of airborne 210Pb is a significant contributor to the total 210Pb depositional rate. 2.4.6 Pb-210 summary The previous section provided a detailed description of the current state of knowledge of 210Pb deposition and its application to soil studies. It has touched on the topics of geographical depositional studies and its use for geochronology plus

51 broadly examining the relationship between 210Pb depositional rates and atmospheric concentrations. Perhaps the most important information to be drawn from this section of the review is the relationship between 210Pb deposition and precipitation, especially that a small number of rain events can contribute a large amount of the annual deposition rate. Of note is the lack of 210Pb depositional rate measurements from tropical regions, especially from Australia. While Bonnyman et al. (1974) performed a very good study the results focused upon urban locations. Given the application of 210Pb for erosion studies a complete knowledge of depositional rates at specific locations is important for holistic study. Even with the existence of a global database for 210Pb atmospheric concentrations and depositions there has been little attempt to correlate these with 222Rn exhalation rates on regional or global scales. 2.5 Chapter summary This chapter has reported a summary of the current literature available on radon emanation, migration, exhalation and the resultant deposition of 210Pb. The summary shows the lack of current research on these topics within Australia compared to the number of studies that are being performed internationally. Australia is a major source of 222Rn, and thus 210Pb, in the southern hemisphere and the only major source within Oceania. The report by Bonnyman (1972) is the only long-term study of 210Pb depositional rates from Australia and has provided the values reported by Preiss et al. (2003) for their global database of 210Pb concentrations and depositional rates from around the world. There have been more recent reports of radon exhalation rates from a number of ambient Australian soils. Schery et al. (1989) provided Australian averages of 222Rn and 220Rn exhalation rates for the international UNSCEAR reports (UNSCEAR 1993; UNSCEAR 2000) while Todd (1998) examined the rate of 222Rn and 220Rn in the vicinity of the Ranger uranium mine in the Northern Territory of Australia. These are the two most recent works on radon exhalation within Australia and neither examined seasonal fluctuations or provided correlation between radon exhalation rates and geological or meteorological conditions. Only Whittlestone et al. (1998) started to address these issues but that work was also only a snapshot of the overall picture.

52 Other than studies performed on operational or rehabilitated mine sites there is a current lack of 222Rn exhalation studies being performed within Australia. While a number of stations are monitoring 222Rn and 222Rn progeny atmospheric concentrations no correlations have been made between atmospheric concentrations and exhalation rates or 210Pb depositional rates. While a number of studies have been performed on radon exhalation from abandoned and rehabilitated uranium mines and natural ore bodies within the Alligator Rivers Region, to date no substantial survey has been performed to determine the 222Rn source term from the Ranger uranium mine. The reports produced for the Ranger mine source have been incomplete or relied partially on model-based estimates for 222Rn exhalation rates from the various identified sources at the site. Since the mine is a localised source of 222Rn within the region measurements of the source term will be helpful in modelling the 210Pb budget for the region, and for estimates of dose to people from inhalation of 222Rn progeny sourced from the mine site. The work covering the process of radon emanation and its migration is very comprehensive. The models produced in more recent years (Semkow 1990; Maraziotis 1996) provides ideas to solve some of the remaining questions of radon emanation. The Office of the Supervising Scientist (OSS), the Australian Atomic Energy Commission (AAEC), Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) and the Northern Territory Department of Mines and Energy have all carried out measurements of 222Rn exhalation rates and surface air concentrations from various locations within the Alligator Rivers Region of the Northern Territory. Analysis of this data and continued measurements can be used to examine the specific meteorological, geographical and geological conditions that are involved in the process of 222Rn exhalation and 210Pb deposition. Research and reports from tropical areas in the Southern Hemisphere are lacking information on the physical factors that affect 222Rn exhalation its transport and transport of its progeny. Most reports have been made for the important assessment of radiological dose received by members of the public living in the vicinity of operational or abandoned mines. The location of this project, in the wet and dry tropics, is of intrinsic interest for 222Rn exhalation and 210Pb deposition rate measurements. Periods of several months without rainfall are extremely useful to observe the effect of influencing parameters, such as 226Ra activity concentration, soil porosity, grain size,

53 atmospheric pressure and temperature, on 222Rn exhalation. Also, throughout the dry season, all 210Pb deposition is dry deposition. The contrast then occurs for the months of the wet season that gives extreme values of precipitation and soil moisture expected to retard 222Rn exhalation and increase 210Pb deposition. The effect of early or late wet season rains are also of interest to observe the influence these events have on 222Rn exhalation and 210Pb deposition. The study is therefore of intrinsic interest as it will provide results more decisive than those that would be obtained in a temperate region.

54 3

Project location, site selection and measurement schedules

3.1 Overview The aim of this chapter is to introduce the reader in greater detail to the region that the project was performed in, the sampling sites selected within and the measurement schedules used. The tropical region of the Alligator Rivers, which encompasses Kakadu National Park in the Northern Territory of Australia, was selected. It is this region that eriss monitors for any environmental impact of the Ranger Uranium project and performs a large range of research projects over many scientific fields. A brief description of the region has been provided in Chapter 1. Each of the project objectives outlined in Chapter 1 have had measurement sites and schedules determined. First was identification of sites that should enable investigating the effect of various parameters influencing 222Rn exhalation; secondly the selection of sites to perform measurements of 222Rn exhalation rate over one seasonal cycle; thirdly a selection of sites for the collection of soil scrapes and cores to measure excess 210Pb following deposition; and finally the selection of sites to use for the collection of 210Pb wet and dry deposition. These sites, sampling locations, relevant maps, measurement schedules and reasons for selection are covered in the following sections. 3.2 Exhalation from open ground – Investigation of physical parameters [226Ra activity concentration, distribution in grains, grain size and porosity] 3.2.1 Ranger operations The Ranger ore bodies were discovered during an airborne radiometric survey in 1969 and confirmed by drilling in the mid 1970’s. After Government approval, mining at Ranger Uranium Mine commenced on ore body 1 in May 1980. Ore body 1 was mined out in 1994 and the remaining pit is currently used as the primary tailings dam. Government approval for mining ore body 3 was given in 1996 and mining commenced in July 1997; this ore body was still being mined at the time of writing of this thesis in January 2005. Operations for processing commenced in 1981

55 and continue today. Between 1981 and 1997 the processing plant produced approximately 3000 tonnes of U3O8 per annum, improvements completed in 1997 upgraded this to 5000 tones per annum. The total expected U3O8 reserve from both Ranger ore bodies are estimated at 122,000 tonnes (Kendall 1990). Mining at Ranger is performed by the open cut method due to the shallow depth of the ore bodies (ore body 1, 5-70 m; ore body 3, 5-200 m). A large amount of waste rock has been removed from each of the pits and used to create the original tailings dam and the retention pond walls. Ore body 1 had an average uranium concentration of 0.33% while ore body 3 averages 0.27%. Blasting is used to dislodge rock from the pit walls; it is then collected using a heavy mechanical excavator and loaded into open tray dump trucks. At the top of the pit ore trucks stop under a radioactivity discriminator array consisting of four large sodium iodide (NaI(Tl)) detectors. Here the load is graded before being dumped onto the appropriate stockpile relating to its grade. Uranium concentrations used for classification of ore grades at Ranger are listed in Table 3-1. Grade 1 ore is referred to as waste rock; its low uranium concentration makes it uneconomical for milling. A lay out of the Ranger uranium mine operation is provided as Figure 3.1. Table 3-1: Classification of ore grades at Ranger

Ore Grade Number U3O8 (%)

1 0-0.02 2 0.02-0.08 3 0.08-0.12 4 0.12-0.2 5 0.2-0.35 6 0.35-0.5 7 >0.5

56

Figure 3.1: Ranger Uranium Mine, numbers indicate approximate sampling locations used for this project

A flow chart of the processing operation performed at Ranger is provided as

Figure 3.2. Ore is processed on site to produce U3O8 that is shipped to international customers. Processing starts by crushing and grinding the ore to fine particulate sizes using rock crushers and rod mills. Ore is fed into the primary crusher with an average uranium concentration of 0.26%. This average input value is achieved by feeding the crusher from stockpiles of varying grades, based on calculation of the required amounts. A secondary discriminator array, similar to the one located at the top of the pit #3 is located at the entrance of the primary crusher and used to confirm the grade of each load.

57

Figure 3.2: Flow chart of Ranger processing (ERA 2005)

58 After passing through primary, secondary and tertiary crushers the fine ore is stored in a silo before being ground further in a series of rod mills. The finely ground output from the rod mills is added to sulphuric acid in a leach tank where the uranium leaches into a solution over 24 hours. Leaching removes approximately 90% of the uranium from the ore. Sulphuric acid is produced on site in an acid plant from stockpiles of sulphur. The uranium solution is separated from the solid waste in a circuit of wash tanks. The solid waste, or tailings, are neutralized with lime and pumped to the primary tailings dam. Kerosene and ammonia are added to the uranium solution in the clarifier and filter tanks. This removes the uranium from the acidic solution into a kerosene solution. More ammonia is added in the precipitation tanks and uranium is precipitated from the solution as ammonium diuranate, more commonly referred to as yellowcake. Final traces of liquid are removed in a centrifuge and through heating the yellowcake at temperatures of 800 oC in a calciner; this removes the final traces of ammonia. The final product is U3O8, uranium oxide, which is a dark green powder. A large amount of wastewater is produced during milling, and management of wastewater is an important issue for Ranger. From information available at the onset of their operations the company expected lower rainfall and higher evaporation rates for the region than proved to be the case. It proved to be an ongoing operational problem for Ranger. The wet season of 1982-1983 had 20% lower rainfall than average and was classified as a drought year. This resulted in Ranger importing water into the site. By 1985 Ranger was carrying more water than the system could handle and plans were put in place to reduce the excess water load. A number of proposals were made including pumping water directly into Magela Creek. Eventually permission was given to irrigate excess water onto a land filter on the Ranger side of Magela now known as the Magela land application area. Irrigation commenced in 1985 and continues to the present day but only during years where the water loads are in excess. Water irrigated onto this region comes from retention pond 2 and contains trace quantities of uranium series radioactive elements including 226Ra, the direct parent of 222Rn.

59 3.2.2 Ranger site selection Radon-222 is released into the atmosphere during some stages of the uranium mining and milling process. Searching previous literature of work performed at Ranger has identified the major 222Rn sources as Pit #1, ore body 3 (now Pit #3), the ore stockpiles, waste rock dump, tailings dam, land application area and the milling plant (Akber et al. 1993; Kvasnicka 1990). All of these locations are shown in Figure 3.1; the numbered locations indicate sites where 222Rn exhalation measurements were performed during this project. They are identified in Table 3-2 that also provides the site name and Global Positioning System (GPS) coordinates, given in the WGS84 Universal Transverse Mercator (UTM) coordinate system. The UTM coordinate system was used for all positioning during the course of this project. Both pits have been identified as 222Rn sources since it is exhaled from the uranium bearing rocks in the walls and floors. In pit #3 222Rn is likely to be emitted in larger quantities during the blasting process. Atmospheric inversion and calm conditions keep 222Rn in the pit, so most will decay within the pit. Occasionally, conditions arise that transport 222Rn out of the pit; this has been observed through continuous monitoring of 222Rn close to the edge of the pit. During the wet season both pits fill with large amounts of water and exhalation of 222Rn is retarded due to the saturation of rocks and soil with water. Table 3-2: Number, name and position of sites selected at Ranger, refer to Figure 3.1 Site Number Site Name GPS 53L UTM

1 Pit #1 0273430 8596120 2 Pit #3 0273703 8597772 3 Grade 2 Ore Stock Pile 0273135 8596547 4 Waste Rock Dump 0273010 8597072 5 Laterite Stockpile 0273330 8596804 6 Magela Land Application 0275015 8597449 Area 6 Experimental Plot (In MLAA) 0275058 8597406 7 Grade 7 Ore Stock Pile 0274186 8596781

60 All stockpiles at Ranger exhale 222Rn and since they consist of uranium bearing material the exhalation rates from the stockpiles were expected to be a major contributing source. Three ore stockpiles of differing grades and a laterite stockpile were selected for measurement of 222Rn exhalation rates. The three ore stockpiles selected were the waste rock dump, grade 2 and grade 7 stockpiles. It was expected that a correlation between uranium concentration and 222Rn exhalation rate would be observed. Waste rock makes up large surface areas in the operational areas of the Ranger mine. It was used in construction of the tailings dam and retention pond walls. It is an important material on site as it will be used to create the final landforms and as a surface covering for both pits. Sections of the waste rock dump have been ploughed and vegetated as part of the rehabilitation process while other sections are compacted flat by heavy vehicles. A compacted section and a rehabilitated section, located next to each other, were selected for 222Rn exhalation measurements. The aim was to compare results from both sections with each other and other measured stockpiles. Laterite is a fine-grained soil created through weathering of rocks. Laterite soil is found in tropical regions or in regions that were once tropical; it has been leached of soluble minerals but still contains large concentrations of iron oxides and iron hydroxides. It is generally reddish in colour due to the presence of these iron compounds. Large amounts of laterite soil were removed from above the ore bodies at Ranger. The laterite contains elevated concentrations of uranium and it is theorized that the uranium deposit at Ranger has leached through this laterite layer. During the early stages of the Ranger project laterite was processed for its uranium but this became uneconomical when global uranium prices declined. The ore crushing process releases 222Rn that might have otherwise been trapped within the rock fragment. As ore passes through the crushing and grinding phases it is broken into smaller and smaller grain sizes. Smaller grains have greater 222Rn emanation rates due to their larger surface area to volume ratio. Crushing also creates large amounts of dust, and so 222Rn emanating from airborne dust particles passes straight into the atmosphere. With these processes at work the mill has previously been identified as an important 222Rn emission source from the Ranger site (Akber et al. 1993; Kvasnicka 1990). Unfortunately determining the mill source term would have presented several difficulties including measurements problems due

61 to the various engineering control systems in place and associated occupational health and safety concerns with the operations required to obtain samples. It was therefore decided that determining the mill source term was out of the scope of this project. Radium-226 is one of the waste products released into the primary tailings dam in the form of wet slurry. Tails slurry is saturated with water so little 222Rn exhalation is expected from the water saturated tailings. As previously mentioned, Ranger releases some of its excess retention pond 2 water through irrigation onto a land filter called the Magela Land Application Area (MLAA). This area, just to the south of Magela creek, encompasses 33ha of Eucalyptus woodland that has been irrigated with retention pond 2 wastewater since 1985. Wastewater from retention pond 2 contains trace amounts of 226Ra and 238U along with other uranium series radionuclides, salts and sulphates. Deposition of water onto this site makes the land application area a unique site for 222Rn exhalation studies. The water is deposited using irrigation sprinklers attached to pipes laid across the area. Surface deposition results in a higher 226Ra concentration in the surface 5-10 cm of soil at this site compared with natural locations. Irrigated and non-irrigated sections along with a test experimental plot all within the land application area boundary were selected for measurements. The irrigated section selected for study had had not been irrigated in a number of years due to broken pipes in the region. Assessment of future 222Rn exhalation rates from the Magela Land Application Area was obtained from analysis of the results from this section. A small section of land within the boundary of the Magela land application area was used as a test plot to study the effects of surface deposition of salts, sulphates and radionuclides. The plot, known as the Experimental Plot, was set up for a collaboration project between the Office of the Supervising Scientist and the CSIRO (Willett et al. 1993). Over the course of nine months starting in late August 1988 the plot was irrigated with synthetic water designed to match the quality of the retention pond 2 water irrigated onto the land application area. The project’s aim was to estimate long-term effects and observe movement of deposited salts, sulphates and radionuclides. The experimental plot became the focus of intensive study for three years after its irrigation. Results of these studies were presented at a symposium and published in the symposiums proceedings (Akber 1991).

62 3.2.3 Ranger measurement schedule Measurements performed at the stockpiles, pits and waste rock dump were limited due to access restrictions at the Ranger mine site. They were performed on weekends when work at the mine ceased for maintenance, they were performed during the dry seasons of 2002 and 2003. Access to the waste rock dumpsites was easier as it was not used for dumping. Two methods of measuring 222Rn exhalation rates were employed and complete details of equipment can be found in Chapter 4. Along with 222Rn exhalation rates gamma doses and 226Ra activity concentrations were also recorded where possible. Generally, accessibility determined the number and duration of measurements performed. Ease of accessibility to the Magela Land Application Area meant that a more intensive survey could be performed there. Three sections of the land application area were selected; the first was the abandoned irrigated section mentioned previously, while the other two were non-irrigated sections representing two different soil types shown in Figure 3.3. Over 90 charcoal canisters for 222Rn exhalation measurements were deployed across these three sections for periods of 3-5 days. Gamma doses, 226Ra activity concentrations and 222Rn exhalation measurements, using an emanometer, were measured at 11 sites. Soil cores were also taken from 9 of these sites to obtain radionuclide depth profiles. The position of the Experiment Plot may be seen on the bottom right hand side of Figure 3.3. Work there commenced with collection of two soil scrapes followed later by gamma dose rate, 222Rn exhalation rate and soil radionuclide concentration measurements. Twenty-five charcoal canisters were deployed across this site for 222Rn exhalation measurements while gamma doses and 226Ra activity concentrations were measured at each location where a charcoal canister was placed. Unanalysed core samples from a previous study on the area were also prepared for gamma counting (Storm and Martin 1995). The dates of all measurements performed at Ranger are given in Table 3-3. Type and number of measurements performed are seen in Table 3-4.

63

Figure 3.3: Original Magela Land Application Area (MLAA)

Table 3-3: Measurement dates for Ranger Mine sites Site Measurement Dates

Grade 2 Ore Stockpile (OSP2) 08/09/2002 Grade 7 Ore Stockpile (OSP7) 07/07/2002 Laterite Stockpile 05/08/2002 Waste Rock Dump (WRD) 09/07/2002-10/07/2002 Pit #1 03/10/2003 Pit #3 12/10/2003 Magela Land Application Area (MLAA) 06/2002-07/2002 Experimental Plot 20/07/2003-29/07/2003

64 Table 3-4: Number of Ranger measurements Site Emanometer Charcoal Soil Sodium Gamma Canisters Samples Iodide Dose Area Rate

OSP 2 Pad 6 9 1 16 15 Rim - 10 1 - 10 OSP 7 Pad 4 5 1 - 5 Rim 8 5 2 - 3 Laterite Pad 5 15 - 19 20 Rim 4 9 - - 13 Push Zone 1 6 - - 7 WRD Rehabilitated 10 11 1 9 21 Pad 10 10 2 9 20 Pit #1 Bench 6 28 3 - 27 Wall - 2 - - - Pit #3 Pad 4 21 1 - 21 Rubble 2 7 1 - 7 Rock - 2 - - 2 LAA Irrigated 8 33 8 8 8 Non-irrigated 11 60 9 11 11 Experimental - 24 4 24 24 Plot

65 3.3 Seasonal and diurnal radon exhalation [moisture, pressure and temperature] 3.3.1 Site selection Eight sites were selected for seasonal 222Rn exhalation rate measurements. These sites were selected on the basis of the following criteria: − Accessibility; − Proximity to laboratory; − Availability of other relevant data for the site; − Range of natural and disturbed sites; − Acceptance for use of the site from Aboriginal Traditional Owners; − Safety. Accessibility was an essential criterion as during the wet season the rivers and creeks swell making some areas inaccessible. It was extremely important to ensure that most sites were accessible all year round. Proximity to the Jabiru field station was also important; as most equipment used was fragile, so long journeys over unsealed roads were undesirable. One site, Mirray, 40 km from the laboratory was selected as a representative background site for the Kakadu region far enough away from any possible mine related influence. Secondly, Mirray is on the side of a hill and therefore to some extent could provide information of seasonal 222Rn exhalation rate variability from sloped locations. There are three locations where eriss monitors airborne dust and radon progeny concentrations; (i) Mudginberri radon station; (ii) Jabiru East; and (iii) Jabiru Town (Water Tower). Dust concentrations are measured for a period of one week every month while radon progeny concentrations are measured over a day every month. Radon gas monitors constructed by the Australian Nuclear Science and Technology Organization (ANSTO) are used at Mudginberri and Jabiru East to continuously monitor radon concentrations. With the availability of this additional data and as they meet all requirements listed, these three sites were included for seasonal 222Rn exhalation measurements. It was desirable that human disturbed sites and natural sites be studied to determine if disturbed sites have variations in 222Rn exhalation rates when compared with natural sites. For mining influenced seasonal sites the waste rock dump and irrigated section of the Magela Land Application Area were used. The remaining two sites are relatively undisturbed, the first being close to

66 the edge of the Magela Creek at the northern end of the Jabiru East aerodrome. While Magela Creek site didn’t have all year round accessibility it does have a naturally high 222Rn exhalation rate as previously identified (Todd et al. 1998). The final site was on the non-irrigated section of the Magela Land Application Area. The eight sites selected are listed in Table 3-5 providing a reference number, site name and the Global Positioning System (GPS) location of each of them. The map displayed in Figure 3.4 shows the position of these sites around the Jabiru and Ranger project region. Mudginberri radon station lies 10 km from Ranger mine and 2 km from an Aboriginal campsite at Mudginberri billabong. A large area around the site was previously used as a stockyard for an abattoir based where the Mudginberri campsite is now located. A result of its use as a stockyard is that the ground at the site is firmly compacted. It has been cleared of the typical Eucalyptus woodland that dominates the region. The monitoring station here is used to monitor airborne dust, radon and radon progeny concentrations. The results of this monitoring are used to calculate effective doses for people living in the Mudginberri campsite. The station has full year accessibility, a secure compound, an automated weather station and a permanent radon gas monitor. The site was attractive for use because it met all the criteria. Mudginberri radon station is the first of three sites used by eriss for the airborne dust, radon and radon progeny measurements. This site is representative of a disturbed location. The eriss Jabiru field station is located at Jabiru East approximately 3 km from the operational pit at Ranger. The site was once the location of the township of Jabiru East that was demolished in 1990. Common Eucalyptus trees found in the region were planted for rehabilitation. At the rear of the eriss compound is a smaller compound used for meteorological measurements, it was within this compound that seasonal 222Rn exhalation measurements were carried out. Nearby in the eriss compound measurements of airborne dust, radon and radon progeny concentrations are carried out. This site is accessible all year, secure, within 300 m of the weather station based at Jabiru East aerodrome and has a permanent radon gas monitor. This site is representative of a disturbed but rehabilitated location.

67 Table 3-5: Seasonal measurement sites, names and locations Site Number Site Name GPS 53L UTM

1 Mudginberri 0267389 8607610 2 Mirray 0251463 8576966 3 Jabiru East 0271339 8599366 4 Jabiru Water Tower 0264330 8598104 5 Waste Rock Dump 0272326 8596788 6 Irrigated Land Application Area 0274960 8597404 7 Non-irrigated Land Application Area 0275057 8597390 8 Magela Creek 0272441 8600260

The third site used for monitoring airborne dust and radon progeny is the water tower located in the township of Jabiru some 15 km from the mine. This site is accessible all year round, has a secure compound and access to additional data that can be used in conjunction with 222Rn exhalation measurements. This site, similar to large areas of Jabiru Town, is covered with compacted soil excavated from what is now Jabiru Lake. The idea was to ensure that sections of town were above the flooding levels of nearby creeks. This site also represents a disturbed region. Mirray is an eroded hillock some 35 km south south west of the Ranger mine. There is a lookout at the top of the hillock joined by an 800 m walking trail to the car park at the base that is accessible all year. An ambient site 30 m from the car park on the side of the walking trail near the base of the hill was selected for our seasonal 222Rn measurements. This undisturbed site was selected to be representative of ambient 222Rn exhalation rates from natural sites of the region. The site is on sloped ground so analysis of the data, especially from the wet season, was expected to provide information about the importance that soil moisture drainage may have on 222Rn exhalation rates.

68

Figure 3.4: Map of region displaying seasonal sites

Waste rock covers large surface areas of the mine site and has been identified previously as one of the main 222Rn sources from Ranger. The measurements performed on the mine at the sites covered in Section 3.2 only provide 222Rn exhalation rates for dry season conditions. A site at the rear of the waste rock dump was selected to perform seasonal 222Rn exhalation rates measurements. This site, located near the old tailings dam is accessible all year. Previous work performed at Ranger has measured 222Rn exhalation rates from the waste rock dump but never examined it over a seasonal cycle. Results obtained from these measurements were

69 expected to provide insight into the seasonality of 222Rn exhalation rates from ore stockpiles structures. At the end of operations Ranger will be rehabilitated and large areas layered with a covering of waste rock. Studying the seasonality of 222Rn exhalation from this site now provides an idea of expected 222Rn exhalation rates from the rehabilitated site in future. The Magela land application area is accessible all year and close to the eriss field station. Irrigated and non-irrigated sites were selected within zone 6A and the type II soil non-irrigated sections respectively, shown in Figure 3.3. In zone 6A a site beside the access track to Georgetown Billabong near water sample bore #32 was selected. From the type II soil non-irrigated section a site close to the access track to water sample bore #31 was selected. The irrigated site is a natural site that only differs from the surrounding woodland because of its previous irrigation with mine processed water. Both sites represent the common Eucalyptus woodland and are both kept free from fires during the dry season. These sites are within 200 m of each other and both located on type II soil. Comparison of seasonal 222Rn exhalation rates for these sites will enable investigating any variations as a result of the surface deposition of 226Ra on the irrigated section. Measurements on the irrigated section will also provide insight into expected 222Rn exhalation rates from the remainder of the Magela Land Application Area after mining operations cease. The final site used for seasonal measurements was at the edge of Magela Creek 700 m down a track at the northern end of the Jabiru East aerodrome. Todd et al. (1998) measurements at this site identified it as having a high 222Rn exhalation rate. It was easily identifiable from an old car body lying nearby. Proximity to Magela creek however meant the site was covered with water during the wet season once Magela Creek swelled. Radon-222 exhalation rates during these months are assumed to be zero. The site is representative of natural open ground with alluvial sandy soil. 3.3.2 Seasonal site measurement schedule The seasonal sites were measured for 222Rn exhalation rates every month for a year. Readings commenced in August 2002 and ceased in July 2003. The only exception was the Magela creek site where readings commenced in September 2002, also with the swelling of the creek this site was not measured over the period January-March 2003. The two measurement techniques used were charcoal canisters and emanometers; both are described in detail in Chapter 4. Five charcoal cups were

70 deployed for up to several days each month and when equipment was available three measurements were taken over one day each month using the emanometers. The emanometers were not available between October-December 2002 as they were being calibrated. When emanometer measurements were performed soil temperature, atmospheric temperature, humidity and emanometer collection chamber temperature were also recorded. As soil moisture affects 222Rn exhalation, soil moisture depth profiles were measured once a month at four of the seasonal sites. Compact ground, at Mudginberri, waste rock dump and Jabiru water tower plus the likelihood of its submersion at Magela creek meant that the required PVC tubing for the soil moisture probe was install at other sites. The sites Mirray, Jabiru East and both Magela Land Application Area sites were suitable for the required PVC tubing to be installed for the soil moisture probe. Originally it was planned to install the tubing to a depth of 1 m but practicality meant that some tubes only reached half that depth. Soil moisture readings were performed when emanometer measurements were made or during charcoal canister plantation periods. 3.3.3 Diurnal measurement schedule Regional data shows that atmospheric concentrations of 222Rn and its progeny vary diurnally with concentrations peaking between 7-10am (Whittlestone 1992; Akber and Pfitzner 1992; Akber et al. 1994a; Akber et al. 1994b). Few studies, local or internationally, have attempted to observe diurnal variations in 222Rn exhalation rates. Todd et al. (1998) performed two partial diurnal measurements and one complete diurnal measurement from two sites in the Jabiru region. Martin et al. (2002) also reported one diurnal measurement for a site at Nabarlek; this was indicative work leading to this project. Atmospheric temperature, humidity, soil temperature and the emanometer collection chamber temperature were measured over the course of the diurnal measurements.

71 Table 3-6: Sites and dates of diurnal measurements Site Measurement Date

Mudginberri 2-3 April 2003 Mirray 20-21 March 2003 Jabiru water tower 15-16 March 2003 Mudginberri 1-2 September 2003 Jabiru water tower 2-3 August 2003 Magela creek 15-16 August 2003 Jabiru East 20-21 August 2003

Diurnal measurements were performed at the end of the wet season 2003 and again during the dry season 2003. This was aimed to investigate if any diurnal variations were associated with the soil moisture levels. The sites and dates that these measurements took place are listed in Table 3-6. As diurnal measurements required either constantly travelling to or staying at a site for 24 hours time and manpower did not allow for all sites to be measured during both seasons. Measurements were made with an emanometer and readings were taken every hour and a half during the wet season. The dry season regime changed with readings taken every two hours from start till midnight and then at 3 am and 6 am. Since access to the mine site over 24 hours was not possible to arrange therefore neither the waste rock dump nor Magela Land Application Area was included in the diurnal studies. 3.4 Excess 210Pb soil sampling In the Jabiru region during the dry season the wind blows predominantly from an easterly to south easterly direction as shown on a dry season wind rose, Figure 3.5, which was created from data obtained from the Australian Bureau of Meteorology weather station at Jabiru East. It was expected that maximum 222Rn exhalation from Ranger would occur during the dry season. Expecting some correlation between 222Rn exhalation and 210Pb deposition rates a number of samples were collected from the region with the main focus being on samples downwind, to the west of Ranger. In total 8 sites were selected for soil sample collection for analysis of excess 210Pb. Locations are shown in Figure 3.6 while numbers, names and corresponding GPS positions are provided in Table 3-7. Dates of sample collection are provided in Table 3-8.

72 Sites for this section were selected mainly on the basis of accessibility, direction and distance from the mine. A site near Georgetown billabong on the east side of the mine is opposite to the prevailing wind direction during the dry season. Jabiru East lies within the second most dominant dry season wind sector while the site on the side of the Arnhem highway lies between both dominant dry season wind sectors. Sampling occurred at various times throughout the project when time and facilities permitted. Soil excess 210Pb results should also aid in determining transport range and residency time of 210Pb in a tropical atmosphere.

Figure 3.5: Dry season (April-October) wind rose for Jabiru East (data courtesy of Australian Bureau of Meteorology) [26 years averaged]

73

Figure 3.6: Map of Jabiru and Ranger, numbers indicate approximate locations of selected sites for soil samples

74 Table 3-7: Sites for excess 210Pb soil samples

Site Number Site Name GPS 53L UTM

1 West of original Tailings Dam 0271501 8596426 2 Eastern Side Barallil Creek 0267244 8596826 3 Western Side Barallil Creek 0266130 8598056 4 Jabiru Township 0265336 8597684 5 Georgetown Billabong 0275587 8597120 6 Jabiru East 0271301 8599844 7 Side of Arnhem Highway 0268579 8599616 8 South of original Tailings Dam 0272171 8595464 9 West of Retention Pond 1 0272136 8598050 10 Magela Land Application Area (including ~0274925 ~8597583 Experimental Plot)

Table 3-8: Soil collection dates and samples taken Site Name Date Collected Sample

West of Old Tailings Dam 20/3/04 Core Eastern Side Barallil Creek 31/1/04 Core & Scrape Western Side Barallil Creek 30/8/03 Core Jabiru Township 30/8/03 Core Georgetown Billabong 28/3/04 Core & Scrape Jabiru East 21/2/02 & Core & Scrape 27/8/03 Side of Arnhem Highway 16/9/03 Core & Scrape South original Tailings Dam 10/10/03 Core & Scrape West of Retention Pond 1 26/9/03 Core Non-Irrigated Magela Land Application Area 22/2/02, 19/7/02 Cores & 26/7/02 Irrigated Magela Land Application Area 20/2/02-1/8/03 Cores & Scrapes (including Experimental Plot)

75 3.5 Pb-210 deposition sampling Only two deposition collectors were constructed so their deployment locations were carefully considered. A previous report examined the levels of various radionuclides, including 210Pb, in rainwater collected from Jabiru East and Jabiru Town (Martin 2003). Keeping in mind the factors such as previous information on deposition values, security, accessibility and a weather station less than 300 m away at the Jabiru East aerodrome, Jabiru East was selected as one of the sites for a deposition collector. Jabiru East was also used for seasonal 222Rn exhalation measurements and soil sampling. Use of additional data collected from this site may allow for the 210Pb deposition measurements to be more holistic. Since the first collector was deployed close to Ranger mine it was decided that the second collector should be installed further away to measure ambient 210Pb deposition for the region away from any possible influence of the mine. Close proximity to a weather station and security for the equipment were other considerations. The second deployment location selected was another Aboriginal community station, Gunbalunya, previously known as Oenpelli missionary station. Gunbalunya is approximately 50 km north north west of Jabiru inside Arnhem Land. With approval of traditional owners the collector was placed in the yard of an employee from the local school. This site is adjacent to a floodplain on the northern edge of the township. Rainfall measurement records in Gunbalunya date back to 1910. In 1957 the weather station was upgraded and in addition to rainfall data now provides information about temperature, humidity and wind speed and direction. Gunbalunya is considered to be far enough away from any mining site that 210Pb depositional rates here should be representative for the ambient values in the region. Sampling using these collectors was performed on a monthly basis over a period of one year starting in May 2003.

76 4

Methodology

4.1 Overview The aim of this chapter is to detail the methodology and analytical techniques used throughout the project. Some equipment, such as the charcoal canisters and radon emanometers were used across various sections. Methods of operation and sampling processes are all described here. Two types of 222Rn exhalation measurement devices were used; (i) passive charcoal canisters; and (ii) an active system designed by ANSTO known as an emanometer. Soil moisture was recorded using a DIVINER soil moisture probe at four seasonal sites. Soil, emanometer and atmospheric temperature values were recorded at all sites when the emanometer was used. A portable thallium doped sodium iodide (NaI(Tl)) detector, Geofizika GS-512, was used to measure equivalent uranium and thorium activity concentrations and percent potassium at all seasonal sites and the majority of sites at Ranger mine where 222Rn exhalation rates were measured. Gamma dose rates were recorded at most sites with a MINI environmental meter type 6-80. Wet and dry deposition collectors for 210Pb deposition were designed with an ion exchange resin column attached to the base. Soil samples were taken with soil corers and scrapers then prepared for gamma spectroscopy analysis. The Australian Bureau of Meteorology supplied meteorological data from their local weather stations. A work plan was created to break up the project into specific tasks as follows: − Establishment of dry season 222Rn exhalation rate from various sources located on the Ranger uranium mine; − Measurement of seasonal variations in 222Rn exhalation rate at several locations around the Jabiru/Ranger region; − Measurement of 222Rn diurnal exhalation rate variations from several sites; − Measurement of excess 210Pb from soil samples taken from the Jabiru/Ranger region;

77 − Measurement of 210Pb fallout in wet and dry deposition for one seasonal cycle from two sites for study of 222Rn transport. 4.2 Available techniques for radon exhalation measurements A variety of techniques are available for 222Rn exhalation rate measurements. Techniques are classified as being either passive or active; active systems are those requiring electrical power and passive are those that do not. Active systems have the advantage of being able to perform prompt measurements of 222Rn exhalation while passive systems have to be left at the measurement location for periods of time ranging from a few days to a few months. The following passive devices have been extensively used in the past: − Charcoal canisters; − Nuclear track detectors; − Electret ion chamber. Active systems generally have air pumps to move air from a collection chamber into a counting chamber. Most active systems utilize scintillation chambers to determine the activity of the air sampled, with a few using silicon surface barrier detectors. It was noted by Rutherford that 222Rn was absorbed onto activated charcoal. In the mid 1970’s Countess (Countess 1976) described a way in which canisters filled with activated charcoal could be used to measure 222Rn exhalation and atmospheric concentration by gamma spectroscopy analysis. Heating activated charcoal will desorb 222Rn and its progeny providing a reusable material for 222Rn measurement. Numerous researchers have used this technique over the years as an easy method to perform 222Rn measurements. For exhalation measurements canisters are deployed upturned over the sample surface, sealed and left for an exposure period between 3-7 days. Upon collection canisters are sealed to trap in 222Rn, and then left for at least four hours to allow for the establishment of secular equilibrium between 222Rn and its short-lived progeny prior to gamma spectroscopic analysis. The amount of 222Rn trapped was determined through measurement of gamma rays emitted following the decay of the 222Rn progeny, 214Bi and/or 214Pb. The amount of 222Rn the canister was exposed to was determined from an integration of recorded gamma counts over exposure time then simple calculation provides the 222Rn exhalation rate.

78 This was the technique selected for this project and is discussed in further detail in section 4.3. Nuclear track detectors are a film-like material that is damaged by alpha particles incident upon them. Etching the film after exposure to alpha particles widens the damage tracks enough so they can be counted manually under a microscope or automatically using electronic equipment. A number of alpha sensitive materials are used as track detectors, some of the more common ones are cellulose nitrate (Kodak’s LR115 and Russian DNC), polycarbonate (Bayer’s Makrofol E), and allyl digycol carbonate (Chinese developed CR-39). Placing a strip of the detector in the bottom of a canister, upturning and sealing it over a sample is a means of measuring 222Rn exhalation rates. A barrier is employed to remove any contribution from 220Rn. When 222Rn enters the canister alpha particles from the decay of its progeny leave tracks on the film that can be etched and counted. Radon- 222 exposure can be determined from an integration of track counts over exposure time and exhalation determined through simple calculation. The final passive technique examined here is the electret ion chamber. This device, developed by Rad Elec Inc., consists of a chamber with an electret disc and a 220Rn barrier. Electret discs are a dielectric material that holds a quasi-permanent charge and they are charged prior to deployment. The negative charge on the electret disc draws alpha particles towards it as they are positively charged. The alpha particles deposit energy onto the disc and it loses some of its charge. The final charge on the electret is read with a capacitance meter. If the chamber is placed over the surface of a sample 222Rn exhalation can be determined as a function of exposure time and the difference between initial and final charges. 4.3 Radon exhalation measurement with charcoal canisters Since Countess (1976) developed the technique for 222Rn measurements using charcoal canisters it has been improved upon and has proven to be a reliable method for the measurement of 222Rn exhalation rates and atmospheric concentration. For this project charcoal canisters were designed and are shown in Figure 4.1. One hundred of these were made and each canister was filled with approximately 25 grams of activated charcoal. Exposure times recommended are 3-5 days but shorter times can be used at sites with elevated 222Rn exhalation rates.

79

Figure 4.1: Charcoal canister

Some authors (Samuelsson 1987; Aldenkamp et al. 1992) have reported that closed cans, such as charcoal canisters, lying over the ground will underestimate the free radon exhalation rate to the atmosphere. Measurement conditions in this study are such that this effect will be small. In a number of situations, charcoal canister and emanometer measurements were performed simultaneously at the same site and the readings obtained by both methods statistically overlap with each other. Charcoal canisters are prepared by heating them in an oven at 110oC for at least 24 hours prior to deployment. Heating desorbs any 222Rn and its progeny currently in the charcoal. Directly prior to deployment canisters are removed from the oven and sealed. They are taken to the sampling site where the lids are removed and the canister upturned, pressed firmly into the ground and sealed with soil if required. At the end of exposure they are collected, resealed with lids and tape then returned to the laboratory for measurement using a Geofizika GS-256 gamma spectrometer. By placing and sealing a canister over the surface of a material all 222Rn exhaling from that surface will enter the canister and be absorbed by the activated charcoal. Previous work (Bollhöfer et al. 2003; Spehr and Johnston 1983) show that 222Rn exhalation rates can be calculated using the following equation:

R.λ2.t .e(λtd ) J = c ε.a.()()1− e−λtc .1− e−λte Equation 4-1

80 where J: is the average 222Rn exhalation rate (Bq.m-2.s-1) R: is the net count rate after background subtraction (s-1) tc: is the counting time (s) λ: is the 222Rn decay constant (s-1) td: is the delay time between collection and counting (s) a: is the surface area covered by a canister (m2) ε: is the counting efficiency of the detector system (s-1.Bq-1) te: is the exposure time (s) The following assumptions are made with Equation 4-1: − There is negligible 222Rn activity on the charcoal at the start of exposure; − The supply of 222Rn is constant over the exposure period; − There is no saturation of 222Rn in charcoal over exposure time; − As the canister is sealed temperature and humidity effects can be ignored; − Back diffusion of 222Rn from the canister into the ground is unlikely due to the high concentration gradient. Derivation of Equation 4-1 starts with a charcoal canister exposed to a source 222 222 of Rn for time te. During exposure Rn activity will grow towards a steady state 222 value, AE, and Rn activity, Ae, at the end of exposure is given by:

−λte Equation 4-2 Ae = AE (1− e )

If t is any time after exposure, 222Rn activity at time t will be: −λt Equation 4-3 At = Aee

222 If the canister is counted for time tc after a delay of time td, average Rn activity will be:

t +t t +t d c d c ⎡ −λtd −λ(td +tc ) ⎤ −λtd 1 Ae −λt Ae e − e Aee −λtc Aave = At dt = e dt = ⎢ ⎥ = []1− e t ∫ t ∫ t λ t λ c td c tc c ⎣ ⎦ c Equation 4-4 Therefore:

λtd λtd Equation 4-5 Aave.λ.t.e R.λ.tc.e AE = = ()()1− e−tc 1− e−te ε ()()1− e−tc 1− e−te

All symbols have the same meaning given for Equation 4-1.

81 When canisters are used as emanometers the 222Rn exhalation rate is given by: A λ J = E a Equation 4-6 Providing us with Equation 4-1. This is the same calculation method used by eriss for their work at the rehabilitated uranium mine Nabarlek (Bollhöfer et al. 2003) and was originally published by Spehr and Johnston (1983). 4.3.1 Charcoal canister counting system, calibration & efficiency Charcoal canisters were counted on a portable gamma ray spectrometer, Geofizika GS-256. This device consists of a NaI(Tl) detector coupled to a photo-multiplier tube connected to a control unit. The detectors NaI(Tl) crystal is 75mm in diameter and height, coupled to the photo-multiplier tube all housed in a water resistant aluminium cylinder. The control unit is a microprocessor system with a liquid crystal display and is able to provide a gamma spectrum over 256 channels. The unit can be controlled by an external computer, on which the spectra can also be recorded. The system’s energy range is 12 keV to 3 MeV, with an energy resolution of approximately 12 keV per channel. Normally a 137Cs source is housed at the end of the detector and used as a reference source by adjusting the gain to bring the 137Cs 662 keV peak to channel 55. For the purpose of charcoal canister measurements the 137Cs source was removed since it interferes with the 222Rn progeny, 214Bi gamma energy at 609 keV. The system has automatic dead time correction so the live time is extended till the effective count time reaches the required value. The detector was housed in a lead castle to reduce background counts from external sources. Originally this was in an environmental radioactivity counting laboratory but moved to another laboratory in August 2002 when the original laboratory was decommissioned and relocated. Further information regarding the operation of the GS-256 is available in the user manual (Geofizika 1990). Throughout the project most canisters were counted for 600 seconds but a few were counted for 300 seconds. A 600 second counting time was used for good counting statistics and easy measurement of many canisters over a couple of days. At the end of each measurement spectra were downloaded from the GS-256 and saved onto a computer in a file format that could be accessed by a spreadsheet program. The file contains the complete spectrum, counting time and sum of counts over four set regions of interest. The GS-256 can store data from the four regions of interest or

82 the entire spectra but its memory can only hold sixty-two complete spectra. The GS-256 had recently been used for counting charcoal canisters from 222Rn exhalation measurements and regions of interest were preset on the energy peaks of the 222Rn progeny isotopes; 214Pb at 242 keV, 295 keV and 353 keV; and 214Bi at 609keV. To cover each of these peaks completely the regions of interest were set over channels 18-22 (216-264 keV), 23-27 (276-324 keV), 28-31 (336-372 keV) and 47-54 (564-648 keV). After correction for background the net counts under these peaks is used as R in Equation 4-1. Analysis of various spectra observed a drift in peak positions, caused by drift in amplification, especially for the 214Bi peak at 609 keV. It was decided that one broad region of interest should be used to determine the net counts, R, in Equation 4-1. A region of interest covering channels 18-60 (216-719 keV) was used as it covers all energy peaks and allows for some drift in higher energy peaks. To determine the counting efficiency of the system a charcoal canister, with 25g of activated charcoal spiked with 327.0 Bq of 226Ra, was used (Bollhöfer et al. 2003). A detection efficiency of (6.96±0.3)% was obtained. Background counts were performed using two different methods. Originally all deployed canisters were measured before deployment and values averaged to obtain a background for that set of readings. Later during the project it was decided that background counts could be made from a random selection of canisters not deployed in the field, these were averaged and used as background for that set of readings. 4.4 Radon emanometers Two radon emanometers, designed and constructed by ANSTO, are in possession of eriss, one of these systems is shown in Figure 4.2. They are active radon sampling systems with the ability to distinguish between the isotopes 222Rn and 220Rn. They sample air from a large sampling saucer that is placed over a surface and are able to provide radon exhalation rates within an hour.

83

Figure 4.2: Radon emanometer

The system consists of an enclosed box, to house the detection equipment, a large metal sampling saucer that is placed over the sampling surface, a computer for system control and tubing to couple the sampling saucer to the detector. To obtain a good seal between the sampling saucer and ground a cutter, shown in Figure 4.3, is used to indent the soil where sampling will take place.

84

Figure 4.3: Cutter used to accurately place emanometer saucer

The detector unit contains two scintillation cells coupled to separate photomultiplier tubes. Air in the sampling saucer is mixed using a small electrical fan at the top of the saucer. Sampled air pumped into the detector unit is filtered to remove 222Rn progeny before entering the first scintillation chamber. In this chamber total alpha counts are recorded which represents the contribution of 222Rn and 220Rn. The air then passes through a delay line and reaches the second scintillation chamber 6 minutes after exiting the first. With a half-life of 55.6 seconds a 6 minute delay is adequate time for the 220Rn concentration to reduce to approximately 2% of the concentration in the first chamber. In the second scintillation chamber the alpha count is measured again but this time the primary contribution is from 222Rn. The air is then pumped back into the sampling saucer. A schematic of the emanometer is shown in Figure 4.4. A laptop computer with the control program installed controls various functions of the detector system. The interface controls sampling, high voltage settings, discriminator voltage settings and displays current sampling information. Complete analysis consists of six 6 minute counting periods where the final measurement draws ambient air into the first chamber while the remaining sampled air is counted in the second chamber. When the control program is active and the system is turned on it samples ambient air that is used as a background reading.

85

Figure 4.4: Schematic of radon/thoron emanometer Data analysis requires a second program supplied with the emanometers. The average of the background measurements are used to correct the readings obtained from sampling. The analysis program outputs 222Rn and 220Rn exhalation rates once supplied with the background and sampling measurement details from the data file recorded during sampling. The following assumptions are made in analysis: − There are no errors due to leaks of 222Rn and 220Rn from sealing, edge effects or back diffusion; − Counting error can be estimated as the square root of counts. The equation for 222Rn exhalation rate determination that the emanometer uses has been previously reported but is also supplied (Zahorowski and Whittlestone 1996): k =5 J R = ∑ D[]()()β2,k +1()C1,k − b1 − ()β1,k C2,k +1 − b2 Equation 4-7 k =1

Where

−1 D = ()α1,k β2,k +1 − β1,kα2,k +1

86 C1,k: Counts in cell 1 at increment k

C2,k±1: Counts in cell 2 at increment k+1

β2,k±1, β1,k, α1,k & α2,k±1: Constants (where efficiency of cells is accounted for) b1 & b2: Background counts for cell 1 and cell 2 4.4.1 Emanometer calibration The emanometers are labelled with their model and serial numbers RTE 2 001 and RTE 2 002 but for this project they were referred to as RTE1 and RTE2. Twice during the project the emanometers were freighted to QUT for calibration. The method used for calibration was the same as that previously reported for the equipment (Todd and Akber 1996). Calibration requires the use of certified 222Rn and 220Rn sources that were available at QUT. Radon is pumped from the source into the sampling saucer through the line that returns air from the detector to the sampling saucer. This set up is shown in Figure 4.5. This input source of 222Rn or 220Rn is representative of a constant exhalation rate. Problems were encountered obtaining consistent 220Rn values from RTE2 so no results of this isotope have been reported from this equipment. Calibration checks were performed in April-May 2002 and October- December 2002. Results from the calibration checks are displayed in Table 4-1. Exhalation results reported in later chapters from emanometer readings have been efficiency corrected. The certified source was not variable so it could not be determined if the discrepancy occurs over a range of 222Rn exhalation rates.

87

Figure 4.5: Set up for emanometer calibration Table 4-1: Emanometer calibration check results Emanometer Source 222Rn 220Rn 222Rn 220Rn exhalation exhalation efficiency efficiency (mBq.m-2.s-1) (mBq.m-2.s-1) (%) (%)

RTE1 222Rn + 127±10 1068±152 72.9±5.8 49.0±6.9 220Rn RTE2 222Rn + 128± - 74.0±2.2 - 220Rn

4.4.2 Associated emanometer measurements When the emanometers were used a number of associated parameters were recorded. It is known that soil and meteorological variables influence 222Rn exhalation. Variables such as soil temperature, atmospheric temperature, rainfall, pressure, humidity, wind speed, wind direction and emanometer temperature may vary throughout the course of a series of measurements. These variables are important to record, as they will likely influence any diurnal fluctuations of 222Rn exhalation. Soil temperature was measured with a Taylor 9878 soil temperature probe while atmospheric temperature, humidity and emanometer temperature was measured with a Vaisala HM34 humidity and temperature meter. The other meteorological parameters, atmospheric pressure, rainfall, wind speed and wind direction were obtained from either Bureau of Meteorology or eriss weather stations. A Bureau weather station is located at Jabiru East aerodrome and

88 individuals at Jabiru Town Council and Cooinda record rainfall. An eriss weather station located at Mudginberri only records wind speed and direction. 4.5 Soil moisture readings It is been shown in Chapter 2 that soil moisture is regarded as the most important factor influencing 222Rn exhalation rates. At four of the seasonal sites PVC access tubes were installed to allow access for a Diviner 2000 soil moisture probe. Initially it was planned for most seasonal sites to have these access tubes installed but difficulties experienced with installation meant that only four sites could be measured. Even then, hardness of the ground at time of installation meant that not all of these sites could have the access tube installed to the preferred depth of 1 m. The sites selected were the irrigated Magela Land Application Area, non-irrigated Magela Land Application Area, Mirray and Jabiru East. A Diviner 2000 soil moisture probe was used to perform the soil moisture measurements. While a number of systems were investigated the Diviner 2000 was the most versatile. It consists of a data logger unit with display and interface along with the probe that can scan to a depth of 1 m and provides results in 10 cm intervals. The sensor head of the probe transmits a high frequency pulse (>100 MHz), it receives the reflected signal from the surrounding soil. Moisture changes the dielectric constant of soil thus the capacitance of soil increases with an increase in moisture. Frequency of response is inversely proportional to capacitance so the frequency of the reflected signal is related to the amount of moisture in the soil. The logger records the raw frequency of response but analysis is performed from a ratio to ensure data quality between units. This scaled frequency (SF) is used to determine the volumetric water content of the soil. Scaled frequency is determined from the raw counts via Equation 4-8.

SF = (FA − FS )/(FA − FW ) Equation 4-8 Where

FA: frequency response in air (Hz)

FW: frequency response in water (Hz)

FS: raw counts or frequency obtained from the soil (Hz)

The values of FA and FW are obtained through a simple calibration procedure for the unit. This calibration is performed with the unit set to calibration mode and the probe placed into PVC housing, readings are then performed with the PVC housing in air (FA), and then immersed in water (FW). These values provide the

89 boundary conditions, air (0% moisture) and water (100% moisture). From them the scaled frequency is obtained from a scan and a preset calibration curve of moisture content versus scaled frequency is used to obtain the volumetric moisture content. This default calibration curve and equation are shown in Figure 4.6 and has been derived from data collected from sand, sandy loam and organic potting mix. A depth profile is created for each site showing the measurement depth of the site and all readings performed there. The logger/display unit can provide information such as the scaled frequency and moisture content in a simple graphic format. The logger can also be downloaded to a computer and imported into spreadsheet programs for further analysis. Further information regarding the Diviner 2000 can be found in the user manual (Technologies 2000).

1.4

1.2

1

0.8

SF=0.27(VWC0.33) 0.6

Scaled Frequency (SF) 0.4

0.2

0 0 20406080100 Volumetric Soil Water Content(%)

Figure 4.6: Default calibration curve for Diviner 2000 soil moisture probe

90 4.6 Soil activity concentration measurements The activity concentration of material in the ground at all sites was performed by one of the following three methods. − Gamma spectroscopic analysis using portable field NaI(Tl) detector (GS-512); − Calculation from recorded gamma dose rates; − Gamma spectroscopic analysis from collected sample; core, scrape or surface. Method of analysis was determined based upon site parameters, and the type of information required. Measurement of 210Pb in soils was only possible by laboratory gamma spectroscopic analysis of collected samples. Cores and scrapes collected from the selected sites were processed for this section. Where 226Ra activity concentration was required a portable NaI(Tl) gamma detector (Geofizika Brno GS- 512) was used. However the GS-512 was not suitable for all locations due to problems recording measurements at locations with very high 238U equivalent activity concentrations, such as a large number of sites found at Ranger. In these cases the gamma dose rates recorded at the sites were used to calculate the 238U activity concentration and hence 226Ra activity concentration. Some surface soil samples were collected by hand for gamma spectroscopic analysis to compare with values obtained through calculation from gamma dose rates.

4.6.1 Geofizika GS-512 portable gamma detector

Similar to the previously described GS-256 unit the GS-512 has a number of different features. Manufactured by Geofizika Brno it contains a 3” diameter, 3” thick thallium doped sodium iodide (NaI(Tl)) crystal coupled to a photomultiplier tube housed in a water resistant aluminium casing and a data/display unit. It is specifically designed for field use and records the gamma spectrum over 512 channels. A photograph of the system is shown in Figure 4.7. This unit is able to directly analyse measured spectra to provide prompt values of equivalent uranium and equivalent thorium activity concentrations and percent potassium. For field measurement the detector is mounted on a tripod frame keeping the crystal at a height of 1m above ground surface. The system is calibrated on test pads with known 238U, 232Th and 40K activity concentrations, prior to use the system had been calibrated in 1998. Stability is checked regularly by eriss at a test site. Like the GS-

91 256 this system also houses a 137Cs source and uses the 662 keV peak as a reference peak for spectrum stabilisation.. Determination of uranium activity concentration is performed by measurement of the 214Bi 1764 keV gamma peak. Analysis then assumes that equilibrium exists between 214Bi, 226Ra and 238U. It is because of this indirect measurement that results are expressed as equivalent uranium. Similarly 232Th is determined from its progeny 208Tl. Percent potassium is obtained directly from the 40K 1461 keV gamma peak after taking into account the natural 40K abundance of potassium. Results are expressed in ppm eU, ppm eTh and %K. Measurement is performed over set regions of interest programmed into the detector. With the 137Cs source the detector establishes the position of these regions of interest setting the 137Cs 662 keV gamma peak onto reference channel 110. Radionuclide activity concentrations are determined through matrix multiplication of count rates and calibration constants. As 214Bi is a progeny of 226Ra, the equivalent uranium activity concentration represents a measure of 226Ra rather than 238U. Further information of the method of operation of this system can be found in the user manual (Geofizika 1998). Given that the GS-512 measures 214Bi for its activity concentration calculations, measurements obtained at the irrigated Magela Land Application Area are more representative of the 226Ra activity concentration than 238U because excess 226Ra was deposited at the site during irrigation. It was realised that the 137Cs source in the detector was not active enough to set the reference channel while measurements were performed at some Ranger ore stockpiles or within pit #3. As a result the GS-512 would only measure for 9 seconds before shutting itself down. While a number of these spectra were analysed the peak drift was substantial and not consistent so this data was of no use. As a result one of the other two methods of determining the radionuclide concentrations was used at these sites.

92

Figure 4.7: Geofizika Brno NaI(Tl) GS-512 gamma spectrometer in use at Rangers waste rock dump

4.6.2 Determination of 226Ra from gamma dose rates The gamma dose at any location is a combination of gamma rays emitted from the natural decay series, 40K and cosmic radiation. UNSCEAR (2000) provides conversion coefficients that can be used to estimate the above ground gamma dose rate at any location if concentrations of these radionuclides are known. Alternatively, at locations where the uranium concentration is high an approximate value of the concentration can be obtained from the gamma dose rate using the following relationship previously reported (Akber et al. 2004b):

93 CU ≈ 0.16 (HTotal – HCosmic) Equation 4-9

Where -1 HTotal: Gamma dose rate (nSvhr ) in air -1 HCosmic: Gamma dose due to cosmic radiation (nSvhr )

CU: Uranium concentration (ppm) For locations where the GS-512 was unable to obtain complete spectra because it could not locate the reference source peak, Equation 4-9 was used to estimate 238U equivalent concentration. It was then assumed that 238U is in equilibrium with all solid progeny and so an estimate of 226Ra activity concentration could be obtained. It is noted that this holds only for locations where 238U concentrations are high. This method was used to determine the majority of 226Ra activity concentrations for ore stockpile grade 7, ore stockpile grade 2, pit #1, pit #3 and laterite rim and push zones. A value of 66 nSv.hr-1 was used as the cosmic component of gamma dose. This value was obtained for the region in a previous study (Marten 1992). 4.6.3 Soil sampling and preparation Soil samples were collected from a number of sites where further analysis was required other than results supplied by the GS-512 or gamma dose rates. In particular analysis of excess 210Pb could only be performed from collection of soil cores and scrapes sectioned by depth, fractionated and analysed by gamma spectroscopy. It also allowed for comparison of results obtained using the GS-512 and calculation from gamma dose rates. Samples were collected using one of the three following methods: − Hand collection; − Soil cores; − Soil scrapes. Hand collection was the means of obtaining samples from locations where readings could not be performed with the GS-512 or gamma meter. They were collected from a number of locations at the waste rock dump, ore stockpile grade 7, ore stockpile grade 2 and the laterite stockpile. Three samples from the general area of each location were taken and the results averaged to obtain 226Ra activity concentrations for sites where no other reading was possible. Samples were collected

94 by hand and placed into a sample bag, labelled and returned to the laboratory to be dried and prepared for gamma spectroscopy. Soil cores and scrapes were used to determine excess 210Pb from the sites listed (Section 3.4). A number of cores were also collected from the Magela land application area and experimental plot to determine the depth profile of irrigated radionuclides. Collected cores were sectioned with the smallest practical sectioning length being about 1 cm. Generally, most cores were sectioned every 5 cm, cores collected at the irrigated Magela Land Application Area were sectioned every 2 cm for the first 10 cm and then every 5 cm thereafter. The corer breaks in half for preparation and sample removal. A locking pin keeps the corer together during sample collection. Preparation requires a clear plastic sheet lined on the inside of the corer for the sample to sit in, the plastic makes removal of the sample in one piece an easy task. The sample is collected by forcibly inserting the corer into the ground with aid of a sand hammer. The corer is then removed, compaction and friction with the plastic lining generally keeps the core in place. In cases when the corer is firmly stuck a levering device can be used to remove it. The soil corer only covers a small surface area (19.6 cm2) but will collect samples to depths of 30+ cm. In order to collect samples from a greater surface area a soil scraper was used. Scrapes were taken to a depth of 10 cm in sections, 0-1 cm, 1-2.5 cm, 2.5-5 cm and 5-10 cm. Scrapes were only collected at sites that also had cores collected. Scrapes provide a better analysis of radionuclides in the top layers of soil, especially excess 210Pb, as scrapes cover a larger surface area with smaller sections. Samples are collected by inserting the surface frame onto the ground, securing it with a tent peg then digging a small trench at the collection end of the frame and lined with a plastic sheet to allow for easy collection of the samples. This trench need only be as deep as the current sampling section and can be enlarged as needed. The handle is then connected to the scraper at the desired first section depth and the scraper drawn across frame. This draws the soil onto the plastic lining in the trench that is collected and placed into an appropriately labelled bag. Once the first scrape is collected the handle can be set to the next depth and the process is repeated until the desired total depth is reached.

95

Figure 4.8: Base of discs used for soil samples

Sampled cores and scrapes were returned to the laboratory where the cores required sectioning. All sections were then weighed before being placed in an oven set at 80oC to dry for at least 24 hours. Upon removal from the oven the samples were weighed again so moisture content could be determined. Then the sections were fractionated using a 2 mm sieve, each fraction was weighed so the amount of pebbles and fine grains was known. Each fractionated section was crushed to powder using either a Labtechnics or Rocklabs grinding mill. The sample was then pressed into a disc, similar to that shown in Figure 4.8, using a manual hydraulic press. Samples were spooned onto the disc, pressed for a minute at 7000 psi, removed, topped up and the process repeated until the sample was flush with the top of the disc. Once full, a greased O-ring was put in place and the lid screwed on to seal the sample. Establishment of equilibrium of 222Rn progeny with 226Ra is achieved by sealing the sample, after 20 days samples were analysed on an HPGe gamma spectroscopy system. Hand grab samples collected from Ranger were not fractionated and were crushed using a separate mill to avoid contamination. They were pressed and sealed as described previously in this section.

96 4.6.4 Excess 210Pb analysis of soil samples Excess 210Pb in surface soils is a result of its atmospheric deposition on the surface followed by transport, mixing and covering in the soil creating a soil profile. Excess 210Pb is determined with two steps; (i) measuring 226Ra activity concentration and using that as the equilibrium component of 210Pb activity concentration; (ii) subtraction of the equilibrium component of 210Pb from measured 210Pb, as displayed in Equation 4-10. This is valid under the assumption that contribution of 210Pb created from 222Rn decay within the soil from 222Rn diffused from a greater depth and leaching of 226Ra from the surface to a deeper position in the soil are both negligible.

Excess 210Pb (Bq.kg-1) = Measured 210Pb (Bq.kg-1) – Equilibrium 210Pb (Bq.kg-1) Equation 4-10

4.7 Pb-210 deposition measurement Collection of wet and dry 210Pb deposition was performed using a large deposition collection system. An ion exchange resin column was connected to the base of the collector. The ion exchange resin extracts metallic ions that pass through it. A photograph of the system is shown in Figure 4.9, this shows the large stainless steel collection tray attached via a tube to the resin column underneath. The collection tray has a diameter of 85 cm covering a total surface area of 0.57 m2. During the dry season only a tube attaches the resin column to the collection tray but during the wet season the tube is replaced by a bucket to store water since the resin retards flow. Two of these were designed and constructed in Brisbane and then forwarded onto Jabiru for deployment at Jabiru East and Oenpelli. This system is designed to collect total 210Pb deposition that is washout, rainout and fallout. For the first four months a filter was placed above the first resin column to remove large particulate matter, it was noted that the filter caused blockages and its use was ceased before the onset of the wet season. The ion exchange resin used was Amberlite IR 120 (H) designed to remove ionic metals passing through it, thus removing atmospheric 210Pb passing through the column. The resin itself must be kept moist in order to retain its properties so a U-tube attached to the bottom of the column filled with water and hung up keeps the resin moist.

97

Figure 4.9: 210Pb deposition collector deployed at Oenpelli A monthly sample collection and resin replacement routine was decided upon. Sample collection required filter removal, for the months it was there, then rinsing the collector with distilled water to remove deposited 210Pb on the collector and pass it through the column. Once the rinse was complete the resin was collected and replaced with new prepared resin. The system is then connected back together with a new filter in place and the U-tube filled with water to cover the resin. Collection routine during the wet season changed slightly, in some cases the column was blocked with matter stopping flow and the collector bucket was partially full. Detaching the U-tube allowed water to flow through but very slowly. It was easier to remove the resin column, unblock it, collect the water and then pass it back through

98 the unblocked column. The system has four resin chambers but only two chambers were used in each collector for faster flow rates. Resin required chemical preparation before use to flush out any metallic ions in it and the resin was reusable by performing the same chemical flushing process. This involved rinsing 30 ml of resin with 200 ml of 6M HCl followed by 200 ml of distilled water, this was performed using a 50 ml glass column with a bottom valve. Flushing it with acid removes metals trapped in the resin and distilled water rinses out any remaining acid so the resin will again hold metals. After preparation it could be stored for prolonged periods providing it remained covered with demineralised water. Collected filters and resins were sent to Darwin for analysis by gamma spectroscopy for the 210Pb 46 keV gamma peak. The filters were placed in the same size disc used for soil samples and a similar efficiency calibration as the soil samples was used. The true efficiency of the filters was never determined as they were abandoned during the project. Analysis of the filter results and observation of dry season 210Pb deposition rates showed that the activity on the filters was negligible. The resin was placed in a similar but larger disc. The results for the resin were corrected for system efficiency that was determined for the resin. When filters were used the 210Pb was determined as a combination of the filter and resin, but it is noted that the value determined for the filter is incorrect, as the efficiency was never determined, but negligible. 4.8 HPGe gamma spectroscopic system Set up and operation of the eriss High Purity Germanium (HPGe) gamma spectroscopy system is covered in an internal report (Marten 1992). Information provided in this section summarises that report to show the operation and maintenance of the system ensuring high quality results are obtained in this project using gamma spectroscopy analysis. The original report should be referenced for precise details and further information of the systems. A number of HPGe detectors are available at the eriss facility originally in Jabiru but now based in Darwin. Samples were analysed on detector N, G or O with the majority analysed on O to avoid discrepancies from using multiple detectors. Detectors N and O are EG&G Ortec ADCAM 100 systems and detector G is a Canberra 7229N, all are mounted in lead shielding constructed from 100mm low-

99 level lead. Shielding minimizes background radiation so the lower limit of detection is improved. The room housing the detectors has been designed to minimize background radiation by using low activity concentration materials in construction. The detectors and their associated electronics are kept at a temperature of 21 oC to avoid electrical noise caused by temperature variations. A photograph of the eriss detector room at the new Darwin laboratory is shown in Figure 4.10. To obtain the best energy resolution, detectors must be cooled to reduce thermally induced leakage currents and electrical noise. To achieve this, detectors are thermally coupled to a 30 L liquid nitrogen Dewar by a copper-cooling rod surrounded by a cryosorption material such as charcoal. Keeping a detector cool is very important, as warming up a detector with high voltage still applied may damage or even destroy it. Dewars are filled with liquid nitrogen weekly.

Figure 4.10: eriss detector room, Darwin (Photograph by Bruce Ryan)

100 HPGe gamma detectors operate on the principle gamma rays passing through the detector crystal will interact with it via one of three processes, photoelectric effect, Compton scattering or pair production. Any interaction deposits energy and generates electrons creating electron-hole pairs in the germanium crystal. The quantity of charge produced is directly proportional to the energy of the incident photon. The high voltage applied to a detector ensures complete collection of the charge, although the exact high voltage varies between makes and models and it is generally of the order of a few thousand volts. Detectors are fitted with charge sensitive preamplifiers acting as an interface between a pulse processor and the analysis electronics. The preamplifier integrates and amplifies the ionised charge produced by incident interacting radiation. From the preamplifier the signal passes to an amplifier to increase signal strength from a few millivolts to a few volts. The amplifier amplifies and shapes the signal before the next stage. Signal to noise ratio can be improved at the amplifier stage. Finally a Multi Channel Analyser (MCA) receives the signal from the amplifier converting it into a number using an analogue to digital converter (ADC). The number produced represents a channel in the spectra and corresponds to the energy of the incident photon. Energy windows can be varied to view high or low energy areas of the gamma spectrum. An electronic gate controls the ADC so some signals may be missed while one is processed and this results in dead time. A counter registers the amount of pulses received by the ADC during processing and is representative of the dead time so the result can be internally corrected by extending the live counting time. Results of counts versus channel number can be displayed on a computer using a program named MAESTRO. This is the user interface providing a visual means to examine spectra. At completion of sample analysis a test source is placed on top of the sample and counted for 600 seconds. This source contains 50 kBq and 30 kBq of 238U and 210Pb, respectively, and is used to ensure that no peak shifts occurred during analysis. The test source also provides data for attenuation correction of the sample. A program named GPEAK is used to extract gamma peak intensities and uncertainties from analysed spectra. Results from use of the test source and background analysis allow GPEAK to output peak intensities and standard deviations corrected for dead time.

101 Determination of activity concentrations of radionuclides from samples is performed from the output of GPEAK by a program called GOUT. This program requires the sample weight to provide activity concentrations (Bq.kg-1) and was the final analysis step used to obtain results reported throughout this thesis. Maintenance of the gamma spectroscopy system, background counts, standards analysis, filling liquid nitrogen Dewars and checking the electronics is performed on a regular basis. Members of eriss staff perform these tasks according to strict quality assurance requirements ensuring the system runs correctly and results are accurate and precise. Currently a report is in preparation covering details of the new Darwin laboratory, associated set up and calibration of the detector system. 4.8.1 Calibration of spectroscopy system for project samples The sample preparation technique of pressing crushed samples into discs was originally developed at the Australian Institute of Marine Science and is a new method employed by eriss. Prior to this soil samples were cast into resins of various geometries for gamma spectroscopic analysis. The press disc method is quicker than casting with an average of four medium sized sample discs being pressed per hour compared with casting of four samples a day. Being a new technique for the laboratory it was required to create calibration standards so the detectors could be calibrated for this geometry. A total of twelve standards were prepared using 238U, 232Th and 226Ra of known activity. A small amount of active sample was mixed with low-level sand used by eriss as a background standard. They were mixed, dried and pressed into the sample discs using the same method described in Section 4.6.3 for soil samples. To create a mix of activities across the samples two batches for each radio nuclide was prepared and two standards created from each batch. For uranium a

BL-5 mixture containing 7% U3O8, in the form of a powder was directly added to the ground sand. The thorium standard was prepared from thorium nitrate salt that was ground with a mortar and pestle and placed in the oven to dry at 80 oC overnight. Thorium nitrate salt is hygroscopic so it was quickly weighed and then mixed with the sand to make the standards. These standards were pressed as quickly as possible to avoid absorption of more water. Radium was in the form of liquid that was added to the sand, and then dried in an oven at 80 oC overnight before making the standards. Weights, activities and standard identifications of these standards are shown in Table 4-2 and the obtained calibration curves are shown in Figure 4.11.

102

Table 4-2: Standards for Pressed Disc Geometry Sample ID Standard Type Weight (g) Activity (Bq)

ZQ0001 14.09 242.97 ZQ0002 16.04 276.68 238U ZQ0003 15.46 266.06 ZQ0004 15.62 268.81 ZQ0005 15.46 135.68 ZQ0006 14.95 131.14 232Th ZQ0007 14.23 162.96 ZQ0008 15.11 173.06 ZQ0009 14.52 203.94 ZQ0010 15.57 218.71 226Ra ZQ0011 14.88 342.81 ZQ0012 15.06 347.00

Table 4-3: Standards for resin Sample ID Standard Type Resin Volume (ml) Activity (Bq)

ZR018 51.7 34.7 ZR019 41.7 33.6 ZR029 26.5 34.6 ZR021 210Pb 25.3 44.5 ZR022 34.9 42.9 ZR023 30.6 48.8 ZR024 15.8 42.3

Similarly the resin used to collect 210Pb deposition also required standards and background samples to determine detector efficiency for this geometry. At the start of the deposition collection regime no specific volume of resin was used so it was necessary to perform efficiency calibration across a range of volumes. Several standards and one background were prepared using varying amounts of resin to determine the detector efficiency over a range of resin volumes. The details of the standards prepared for the resin are shown in Table 4-3.

103 3

det N For ln Energy<5.5 ln eff=-10.94+6.01x-0.67x2 2 For ln Energy>5.5 ln eff=12.40-2.64x+0.13x2 iciency

f f 1 ln e

3

det G For ln Energy<5.5 2 ln eff=-12.68+6.91x-0.77x2 For ln Energy>5.5

iciency 2 f ln eff=16.14-3.79x+0.22x f 1 ln e

3

det O For ln Energy<5.5 2 ln eff=-11.77+6.53x-0.73x2 For ln Energy>5.5 2

iciency ln eff=15.06-3.44x+0.20x f

f 1 ln e

34567 ln Energy Figure 4.11: Calibration curves and equations for pressed disc soil samples, energy unit is keV

104 17.0

15.0

13.0

11.0 Efficiency % 9.0

7.0

5.0 0.0 10.0 20.0 30.0 40.0 50.0 60.0 Resin Volume (ml)

Figure 4.12: Efficiency calibration for resin samples

The analysis of these samples provided an efficiency plot shown in Figure 4.12. There is a clear rise in efficiency above 40 ml of resins. A result from these tests was that a resin volume of 25 ml was used for the remainder of the deposition collection regime to avoid variations in counting efficiency. The average detector efficiency determined for this volume is (11.14±0.37)%. Filter efficiencies were assumed to be the same as the soil samples, it is noted that the filters were insignificant in terms of their contribution to the total 210Pb. The low energy 46 keV gamma emitted by 210Pb is easily self-absorbed even through small samples and it is necessary to correct for this self-absorption that is related to the samples density (Cutshall et al. 1983). Cutshall et al. (1983) provided a means of correcting for this absorption through determination of a correction factor that can be applied to the results. The correction factor is provided by the following equation: A ln(T / I) = O (T / I) −1 Equation 4-11

105 Where A: sample photon emission rate O: attenuated sample output T: attenuated beam intensity I: unattenuated beam intensity The correction factor A/O is multiplied to the 210Pb activity concentrations provided by GPEAK. T is the peak area counts for each sample and for this work I is the test source peak area counts attenuated through the calibration standards, that is the self-absorption corrections are corrected for the calibration standards. After analysis of the self-absorption corrections for more than 50% three categories for different corrections were distinguished. Samples were divided into sand, <2mm fraction and >2mm fraction. The final average correction factors applied to the 210Pb activity concentrations for each detector are shown in Table 4-4.

Table 4-4: Corrections factors applied to activity concentrations of soil samples Sample\Detector G N O

Sand 1.01±0.01 0.98±0.06 1.01±0.02 <2mm Fraction 1.03±0.04 0.95±0.01 1.03±0.02 >2mm Fraction 1.19±0.04 1.10±0.03 1.28±0.07

106 5

Radon sources

5.1 Overview This chapter presents the findings and analysis of 222Rn exhalation and associated measurements carried out during the course of this project. The sites covered represent a large variability in the factors that are known to influence 222Rn exhalation. Grain size, porosity, 226Ra concentration and its distribution within the grain and soil moisture content varied dramatically. Grain size ranged from large rocks to fine grain while the porosity ranged from loose sands to compact soils. Radium-226 concentration varied from ambient values to values typical for mine-grade uranium bearing rocks and its distribution was expected to vary from homogeneous to mainly on the surface of the grain. Moisture content of the ground changed due to seasonal variations from dry to wet. 5.2 Rn-222 exhalation rate and 226Ra activity Radon-222 exhalation rate values given in Table 5-1 are representative of dry conditions. Charcoal cups and emanometers were used to take the readings (Procedural details are given in sections 4.3 and 4.4). Mine sites readings were taken during the dry season of 2002 and 2003; seasonal sites readings are the averages for the dry periods, August-October 2002 and May-July 2003. The location and description of the sampling sites has been supplied in Chapter 3. Three different methods were used to measure 226Ra activity concentration, insitu gamma spectroscopy using a NaI(Tl) system (Section 4.6.1), HPGe gamma spectroscopy of prepared soil samples (Section 4.6.2), and estimates through external equivalent gamma dose rate measurements at 1m above ground (Akber et al. 2004b; UNSCEAR 2000). Values of 226Ra activity concentrations for the corresponding sites listed in Table 5-1 are provided in Table 5-2. The value RE-R in Table 5-2 is the ratio of 222Rn exhalation rate (mBqm-2s-1) to 226Ra activity concentrations (Bqkg-1).

107 Table 5-1: Dry season values of 222Rn exhalation rates from all measurement sites

Site Methods Number of 222Rn Exhalation Rate (mBq.m-2.s-1) * Measurements Area Arithmetic Geometric Mean+ Mean^

Laterite Stockpile Pad C+E 20 (5.2±0.6)*103 4.4*103(1.1)

Push C+E 7 (8.1±1.5)*104 6.6*104(1.4)

Rim C+E 13 (3.8±0.5)*104 3.3*104(1.2) Ore Stock Pile 2 Pad C+E 15 (1.0±0.2)*104 7.2*103(1.3)

Rim C 10 (7.3±2.2)*103 4.3*103(1.5) Waste Rock Dump

C+E Pad 20 (5.3±1.0)*102 4.2*102(1.2)

Rehabilitated C+E 21 (9.4±1.0)*102 8.3*102(1.1)

Overburden E 4 (9.7±1.7)*102 9.2*102(1.2) Ore Stock Pile 7 Pad C+E 9 (3.1±0.7)*103 2.6*103(1.3)

Rim E 8 (9.5±3.5)*102 5.3*102(1.6)

Pile C 5 (1.7±0.7)*103 1.3*103(1.4) Land Application Area C+E 30 70±3 69(1.0) Non-Irrigated Type II Soil Non-Irrigated Type III C+E 30 70±4 67(1.0) Soil

Irrigated C+E 34 (1.1±0.1)*102 1.1*102(1.1)

* C – Charcoal Cups, E – Emanometers + Arithmetic mean with arithmetic standard error ^ Geometric mean with geometric standard error

108 Table 5-1: Continued

Site Methods Number of 222Rn Exhalation Rate (mBq.m-2.s-1) * Measurements Area Arithmetic Geometric Mean+ Mean^

Mine Pit #1 General C+ E 33 (5.0±0.5)*102 4.1*102(1.1)

Wall C 3 (3.0±0.5)*102 3.0*102(1.2) Mine Pit #3 Rocks C 2 (1.0±1.0)*103 2.3*102(8.8)

Pad C+E 25 (2.5±0.6)*103 1.2*103(1.3)

Rubble C+E 9 (1.7±0.7)*103 9.1*102(1.5)

Mudginberri C+E 44 35±2 33(1.1)

Mirray C+E 45 39±2 36(1.1)

Jabiru East C+E 45 43±2 40(1.1)

Jabiru Water Tower C+E 46 18±1 16(1.1)

Magela Creek C+E 31 (2.1±0.2)*102 1.8*102(1.1) Waste Rock Dump Seasonal Site C+E 45 (2.5±0.2)*102 2.0*102(1.1) Land Application Area Non-Irrigated C+E 39 43±5 36(1.1) Seasonal Site

Irrigated Seasonal Site C+E 37 68±4 63(1.1) Experimental Plot Irrigated C 19 (1.4±0.1)*102 1.3*102(1.1)

Non-Irrigated C 5 43±1 43(1.1)

* C – Charcoal Cups, E – Emanometers + Arithmetic mean with arithmetic standard error ^ Geometric mean with geometric standard error

109 226 Table 5-2: Ra activity concentrations and RE-R ratio

226 + Site Methods * Number of Ra RE-R Area Measurements Concentration (Bq.kg-1)+

Laterite Stockpile Pad A+B 22 (1.15±0.05)*104 0.40±0.04

Push C 7 (1.98±0.07)*104 4.06±0.72

Rim C 13 (1.74±0.07)*104 0.60±0.02 Ore Stock Pile 2 Pad A+B 18 (5.15±0.43)*103 1.86±0.30

Rim C 10 (5.79±0.47)*103 1.58±0.59 Waste Rock Dump Pad A, B+C 19 (1.22±0.10)*103 0.47±0.09

Rehabilitated A+B 20 (1.30±0.10)*103 0.77±0.09

Overburden A 4 578±373 2.80±1.28 Ore Stock Pile 7 Pad B+C 6 (2.77±0.23)*104 0.10±0.02

Rim B+C 4 (4.37±0.44)*104 0.02±0.01

Pile# B 1 (3.22±0.50)*104 0.04±0.01 Land Application Area Non-Irrigated Type A 5 74±6 1.21±0.07 II Soil Non-Irrigated Type A 5 73±9 0.85±0.07 III Soil

Irrigated A 8 147±6 0.67±0.04

* A – Insitu gamma spectroscopy GS-512 NaI(Tl) Detector, B – Gamma spectroscopic from soil sample, C – 226Ra Activity estimated from external gamma dose rate + Arithmetic mean and arithmetic standard error # Counting error from gamma spectroscopic analysis supplied

110 Table 5-2: Continued

226 + Site Methods * Number of Ra RE-R Area Measurements Concentration (Bq.kg-1)+

Mine Pit #1 General B+C 30 (1.27±0.08)*103 0.41±0.04

Wall - 0 - - Mine Pit #3 Rocks - 0 - -

Pad B+C 22 (9.93±2.22)*103 0.38±0.12

Rubble B+C 8 (8.50±1.31)*103 0.23±0.11

Mudginberri A 3 182±2 0.19±0.01

Mirray A 3 47±1 0.82±0.04

Jabiru East A 3 52±1 0.83±0.05

Jabiru Water Tower A 3 46±1 0.39±0.02

Magela Creek A 3 43±4 4.94±0.5 Waste Rock Dump Seasonal Site A 3 478±18 0.52±0.05 Land Application Area Non-Irrigated A 9 58±1 0.75±0.08 Seasonal Site Irrigated Seasonal A 6 144±1 0.67±0.04 Site Experimental Plot Irrigated A 19 356±5 0.40±0.01

Non-Irrigated A 5 125±3 0.35±0.01

* A – Insitu gamma spectroscopy GS-512 NaI(Tl) Detector, B – Gamma spectroscopic from soil sample, C – 226Ra Activity estimated from external gamma dose rate + Arithmetic mean and arithmetic standard error

111 Both the arithmetic and geometric means are provided for the 222Rn exhalation rates in Table 5-1 along with the standard error for each (geometric standard error provided in parenthesis). Radon-222 is a random variable that depends upon a number of other variables that may also behave randomly in nature, such as radioactive disintegration of 226Ra to produce 222Rn, the direction of recoil of 222Rn ion, the local moisture conditions, atomic diffusion etc. A number of studies including the present one have demonstrated that 222Rn exhalation rate distribution is better represented by a log-normal distribution rather than a normal distribution (Akber et al. 2004b; Akber et al. 2002; Holdsworth and Akber 2004). This behaviour indicates that the random variables involved in 222Rn exhalation interact in a multiplicative rather than an additive manner. A total of 626 222Rn exhalation readings were performed in dry season conditions covering 16 different sites both on and off the Ranger mineral lease. Of the total, 317 were performed on identified 222Rn sources directly from the Ranger mine; the remaining 309 at sites that cover natural and man influenced areas. Along with the exhalation measurements a total of 235 measurements of 226Ra concentrations were obtained through various methods described in Chapter 4. In 150 cases 226Ra activity concentration was obtained through insitu measurements using a NaI(Tl) field detector. In order to cross-reference some of these results 15 samples were collected by hand and prepared for analysis on the HPGe system. In cases where insitu gamma spectroscopy results were not available 226Ra activity concentration was derived from an approximation using the gamma dose rate. This approximation is only valid for uranium bearing material above 0.02% U3O8. This approximation has been justified in (Akber et al. 2004b) using a mathematical relationship described by UNSCEAR (2000). Wherever it was possible 226Ra activity concentration measurements were taken at the same spot where 222Rn exhalation measurements were performed. The value of RE-R in Table 5-2 is the ratio of the 222Rn exhalation rate to 226Ra activity concentration (mBq.m-2.s-1/Bq.kg-1). In Table 5-2 it is the average of individual calculations performed for such pairs of measurements. At Ranger, stockpile formation starts by paddock dumping in a designated area to build up a base. Once the base is firm and high enough an access ramp is created to reach the top of the pile, which is then compacted and flattened using graders and other heavy vehicles. From this point the stockpile is a tipping head for

112 loads where the ore is dumped either over or near the edge of the pile to be bulldozed over the edge. Paddock dumping on the pad is also performed in order to grow the stockpile vertically and the process is repeated. The tip head is changed regularly to ensure equal spreading of the material onto the pile. The top of the pile that has been compacted and flattened for the heavy vehicle access is known as the ‘pad’. Around the edge of each stockpile is a raised head of uncompacted material that has been left after dumping, this area is known for the sake of this project as the ‘rim’. The rim is less compacted and generally filled with non-pulverised material. At the periphery of the pad before the rim there is sometimes a zone where bulldozers manipulate the material, this area is referred to as the ‘push zone’. A push zone has less compaction than the pad but is still made from a similar material. As described through the literature review in Chapter 3, soil grain size and soil porosity can affect 222Rn exhalation. For this reason measurements in these different areas have been distinguished from one another. The results across the laterite stockpile show how variations in porosity can influence 222Rn exhalation to a great extent. Laterite is a fine-grained soil created from the weathering of rocks. It has been leached of soluble minerals but still contains large concentrations of iron oxides and iron hydroxides. Laterite at Ranger is a reddish fine-grained uranium bearing material. As such the only variations between the pad, push and rim zones were the degree of soil compaction and hence the soil porosity. As may be seen in Table 5-2 226Ra activity concentrations for these three areas are comparable but the corresponding 222Rn exhalation rates differ greatly. For the pad zone a value of 4.4*103(1.1) mBq.m-2.s-1 is obtained while the less compacted push and rim zones have rates of 6.6*104(1.4) mBq.m-2.s-1 and 3.3*104(1.2) mBq.m-2.s-1 respectively. These are also the largest 222Rn exhalation rates recorded on the mine site and indicate that soil porosity is an important contributing factor to the 222Rn exhalation from this site. Emanometers were used to measure 222Rn exhalation from a pile of overburden material that had been laid a few weeks earlier. This overburden material was also made up of fine grains and appeared similar to a laterite push zone area. Four readings of the 222Rn exhalation rate averaged at 9.2*102(1.2) mBq.m-2.s-1. The 226Ra activity concentration of this soil is 578±373Bq.kg-1 and by comparison the 222Rn exhalation behaviour appears to be similar to that of the push zone of the laterite.

113 The ore stockpiles are mostly made from the schist rock of the upper mine sequence. They have a wide range of rock sizes from boulders to fine grains. The rim zones from the ore stockpiles mainly contain larger rocks; the pad zones contain similar sized rocks but they are compacted with smaller sized gravel in between. The average 222Rn exhalation rate from a grade 7 ore stockpile pad is 2.6*103(1.3) mBq.m-2.s-1 while for the rim and pile areas they are 5.3*102(1.6) mBq.m-2.s-1 and 1.3*103(1.4) mBq.m-2.s-1, respectively. Similarly for the grade 2 ore stockpile the pad area has a higher 222Rn exhalation rate than that for the rim, the values being 7.2*103(1.3) mBq.m-2.s-1 and 4.3*103(1.5) mBq.m-2.s-1, respectively. Rims at these stockpiles contain larger sized rocks 10-20 cm in size while the pad area is compacted and contains a mix of large rocks with small rocks and pulverised gravel down to fine grains. The compaction on the pad results in reduced porosity that will reduce the overall 222Rn exhalation due to constrictions and tortuous path lengths. On the other hand the smaller sized material will provide more surface area for 222Rn to emanate from it. It appears that, in case of Ranger stockpiles the net result of these two competing effects is an increase in the exhalation rate from the pad area. Of particular interest in the case of the ore stockpiles was the variation seen in 222Rn exhalation rate between the grade 7 and grade 2 stockpiles. The stockpiles are graded with regard to the uranium concentration (Table 3-1). Since the grade 7 stockpile has a higher 226Ra concentration than the grade 2 stockpile we expect that the 222Rn exhalation rate would be higher there considering that both stockpiles consist of similar sized material. A possible explanation may lie in the differences in the stockpile heights. The grade 7 stockpile was about 3-4 metres in height above the surface of the natural soil while the grade 2 stockpile was much larger being closer to

25 metres. The waste rock dump pad area was about 10 metres high. The ratio RE-R for these three sites increases with increasing stockpile height. If diffusion is the dominant mechanism of transport of 222Rn from the subsurface then the exhalation rate may be given by the relationship (Holdsworth and Akber 2004): ⎛ H ⎞ Equation 5-1 J = Eexp ρRλ × tanh⎜ ⎟ ⎝ L ⎠

114 Where J: 222Rn exhalation rate (mBq.m-2.s-1) 222 Eexp: Rn emanation coefficient ρ: bulk density of soil (kg.m-3) R: 226Ra activity concentration (Bq.kg-1) λ: 222Rn decay constant (s-1) L: diffusion length (L = D / λ ) D: diffusion coefficient of 222Rn through the material (m2.s-1) H: height of stockpile (m)

This suggests that RE-R for different stockpiles of similar geomorphologic conditions should be proportional to tanh (H/L). Using approximations of the stockpile heights for the three sites calculations were carried out to estimate the 222Rn diffusion lengths through the stockpiles. The results obtained tend to suggest that for stockpile type structures 222Rn diffusion length is likely to be a few tens of metres in magnitude. This estimate is substantially different from that commonly reported in the literature for dry soils (1-2 m) (Graaf et al. 1992) or for sandy type materials (1.9-2.5 m) (Holdsworth and Akber 2004). Other compounding factors not investigated may include differences in uranium distribution for rocks of different ore grades or potential mixing of different grade material between stockpiles. Waste rock is the lowest grade ore at Ranger; it is made up of upper mine sequence schist material. Waste rock is an important material in terms of the post rehabilitation situation at Ranger as it is likely to be the covering surface material for the pits and original tailings dam. Three areas of waste rock were selected for study; a pad area similar to those found on other stockpiles; a rehabilitated area tilled and planted with sparse vegetation maximum 3 metres in height; and the seasonal site that was tilled but contained little vegetation. The pad and rehabilitated sections of the waste rock dump have comparable 226Ra activity concentrations, (1.22±0.10)*103 Bq.kg-1 and (1.30±0.10)*103 Bq.kg-1 respectively, but differing values of 222Rn exhalation rates, 4.2*102(1.2) mBq.m-2.s-1 and 8.3*102(1.1) mBq.m-2.s-1 respectively. Explanation for this difference is the porosity of the ground at each site. The rehabilitated area has been tilled and had trees planted there while the pad area was compacted with heavy vehicles regularly driving over the area. Vegetation found on the rehabilitated area is well developed and will have extensive root structures. Roots

115 break up the ground and maintain moisture thus increasing the soil porosity and soil moisture at this location. The effect of this increase in porosity and moisture is observed with a difference in 222Rn exhalation rates between the two sites. The effects that vegetation can have upon the 222Rn exhalation rates has been previously reported (Morris and Fraley 1989; Morris and Fraley 1994). Radium-226 activity concentration at the seasonal waste rock site was recorded as 478±18 Bq.kg-1, almost one third of the other two waste rock sites. The observed dry season 222Rn exhalation rate from this location was 2.0*102(1.1) mBq.m-2.s-1. Ground at the seasonal site had been tilled for planting but the vegetation here was very sparse with a maximum height of 1.5 m. RE-R values for the three waste rock sites are 0.47±0.09, 0.52±0.05 and 0.77±0.09 (mBq.m-2.s-1/Bq.kg-1) for the pad, seasonal and rehabilitated sites, respectively. The pad and seasonal sites are similar with the only difference being tilling and light vegetation at the seasonal site, this may have resulted in the slightly higher RE-R value. The rehabilitated site is more vegetated and tilled, given all other parameters are similar this is the only explanation for the higher RE-R value obtained here. As waste rock will be the covering surface material for large sections of the project area after rehabilitation the effects of vegetation on 222Rn exhalation rates should be taken into consideration. Ranger’s waste rock dump has been subject to previous 222Rn exhalation rate surveys. One performed by Mason et al. (Mason et al. 1982) derived an exhalation +96 -32 -2 -1 -1 rate per percent of ore grade of 49 mBq.m .s % of U3O8 for waste rock and ore at Ranger, Nabarlek and Rum Jungle. When using this value with known grade uranium concentrations we obtain similar exhalation rates to those observed from ore stockpiles and pits during the course of this project, with the exception of the grade 7 -2 -1 -1 stockpile. The average value reported by Mason et al. of 49 mBq.m .s % of U3O8 was used by Kvasnicka (Kvasnicka 1990) to derive the 222Rn source term for Ranger from the stockpiles, pit #1 and ore body 3. These values were then used for input to an aerial dispersion model. That work determined the emission rates (Bq.s-1) given the area for each source then averaged the exhalation rate over 500 m x 500 m grids. Results presented in this project show that broad approaches cannot accurately determine 222Rn exhalation rates from uranium mines without knowledge of contributing variables. The more holistic approach made in this project should enable better determination of Ranger’s 222Rn source term.

116 Pit #1 has been mined out and is now used as the primary tailings dam. A bench, above the wet season water level was used for the 222Rn exhalation rate survey as being representative of the 222Rn exhalation from this pit. Grasses and other vegetation had taken hold on the bench and were present in most areas covered in the survey. A section of the retaining wall at the rear of this bench was also measured as a representation of the 222Rn exhalation rate from pit walls. The average readings for the bench and pit wall are 4.1*102(1.1) mBq.m-2.s-1 and 3.0*102(1.2) mBq.m-2.s-1 respectively. These levels of 222Rn exhalation rate compare with values obtained at the waste rock pad location. Radium-226 activity concentration is (1.27±0.08)*103 Bq.kg-1 on the bench that is also similar to the value measured at the waste rock pad and rehabilitated sites. The ground structure on the bench is quite different from that observed at the waste rock dump. The base of the bench is solid rock covered in a layer of smaller rocks and fine grains. While the exact depth of cover is unknown on the bench it is likely to be approximately 1 m. While 222Rn from the waste rock dump, with similar 226Ra activity concentration and 222Rn exhalation rates, maybe diffused from much deeper, the smaller grains observed on the pit #1 bench may emanate more 222Rn resulting in an exhalation rate comparable to that at the waste rock dump pad. Measurements were also performed in the operational pit #3. The survey here performed readings from a pad area compacted by heavy machinery, a pile of rubble similar to rim zones on stockpiles as well as two measurements directly on rocks. Results obtained give 222Rn exhalation rates of 1.2*103(1.3) mBq.m-2.s-1, 9.1*102(1.5) mBq.m-2.s-1 and 2.3*102(8.8) mBq.m-2.s-1 respectively. Radium-226 analysis could only be performed through laboratory gamma spectrographic analysis of collected samples or derived from the gamma dose rate. It was expected that ore grades and therefore 226Ra activity concentrations would vary over small distances in the pit. Of particular interest are the results recorded for 222Rn exhalation rates of the two rock samples. Independently they recorded values of 26±7 mBq.m-2.s-1 and (2.0±0.1)*103 mBq.m-2.s-1 indicative of the vast difference that can be observed between two similar samples. Radium-226 activity concentrations for these samples were derived from gamma dose rates and do not accurately reflect the true values of the rocks. Whether they were vastly differing in 226Ra activity concentrations or distribution is not known.

117 Results of measurements performed on the operational mine site show large variations in individual readings taken from stockpiles and pit #3 when compared to the other sites. Some values recorded were unexpectedly low or high but there is no doubt that these variations are real. Localised factors at these sites such as variations in radioactivity content of various rocks and differing path lengths due to the underlying material are the likely cause of these observed variations. With ease of access to the site a thorough 222Rn exhalation survey was performed across a large section of the Magela land application area in July and August 2002. In total ninety-three sites over irrigated and non-irrigated sections of the area were measured. The majority of these measurements were with charcoal cups but at twelve sites additional measurements and sampling was performed including emanometers, soil core, gamma dose rates and NaI(Tl). Sixty of the sites are on non-irrigated areas and the remaining thirty-three from an irrigated region, zone 6A. Non-irrigated sites are further divided between two soil types dominant to the region known as type II and type III soil (Chartres et al. 1988). Results from the non-irrigated sections show natural exhalation from both soil types are comparable with values of 69(1.1) mBq.m-2.s-1 and 67(1.1) mBq.m-2.s-1 recorded for type II and type III soils respectively. Radium-226 activity concentrations for these two locations are also similar with values of 74±6 Bq.kg-1 and 73±9 Bq.kg-1 for type II and type III soils respectively. These values are equivalent to ambient values reported previously from the region (Todd et al. 1998) of 64±25 mBq.m-2.s-1. It is known that the contribution to 222Rn exhalation is greater from the surface layers of the soil decreasing with depth as shown by the tangential hyperbolic relationship of Equation 5-1. Radium-226 deposited by surface irrigation onto the Magela Land Application Area is absorbed into the surface layers of the soils providing a very strong vertical gradient of 226Ra at this location. Additional measurements were carried out by collection of soil cores from the region and are reported in Chapter 6 and show the vertical gradient of 226Ra, this has also been shown by other authors from samples collected from the land application area (Akber and Marten 1991). Radium-226 activity concentration measurements over the area gave a mean and standard error of 147±6 Bq.kg-1. These measurements were performed using the GS-512 that provides an average value of 226Ra based upon measurement of the 214Bi gamma ray. Due to surface irrigation of 226Ra the irrigated area has elevated 222Rn exhalation rate of 1.1*102(1.1) mBq.m-2.s-1.

118 The mean dry season 222Rn exhalation rates from the non-irrigated and irrigated seasonal sites are 36(1.1) mBq.m-2.s-1 and 63(1.1) mBq.m-2.s-1 respectively. While these values are lower than the mean values reported for non-irrigated and irrigated sections they still lie within the range observed. Radium-226 activity concentration at the non-irrigated seasonal site is 58±1 Bq.kg-1 that is less than the average value for type II non-irrigated soil. The non-irrigated seasonal site was also at the edge of a small clearing with little vegetation and compact ground. The combination of low 226Ra activity concentration and soil compaction results in lower 222Rn exhalation from this site when compared to the remainder of type II non- irrigated soil area. Radium-226 activity concentration at the irrigated seasonal site, 144±1 Bq.kg-1 is comparable with the average value reported across the irrigated section. The reduced 222Rn exhalation rate observed at this site must be the result of other influencing factors. Soil at this site appeared more compact than the surrounding area and it was close to a groundwater sampling bore. Specific locations selected for deployment of charcoal cups around this site might not have received a homogenous spread of irrigated water due to blockage from vegetation. Radon-222 exhalation rates of this level were observed across the irrigated site during the survey but they represent the lower level of observed rates. A detailed 222Rn exhalation rate survey was performed at the experimental land application plot in July 2003. The survey included deployment of twenty-four charcoal cups for measurement of 222Rn exhalation rates, collection of soil samples, measurement of gamma dose rates and 226Ra activity concentration. Average 226Ra activity concentration at the site is 356±5 Bq.kg-1, over twice the value recorded from the irrigated section of the Magela land application area, while the mean 222Rn exhalation rate is 1.3*102(1.1) mBq.m-2.s-1. The 222Rn exhalation rate here statistically overlaps with values observed at the irrigated land application area. Differences in 226Ra activity concentrations between the sites are not reflected in a difference in 222Rn exhalation rate. The experimental plot was specifically selected as it was devoid of large amounts of vegetation while the irrigated land application area is representative of the common eucalyptus woodland of the region. We have previously shown the influence that vegetation has upon 222Rn exhalation rates at the waste rock dumps sites the effect is shown again here in the differences between the experimental plot and irrigated land application area.

119 Effort was made to select a non-irrigated area around the experimental plot but results of 226Ra activity concentration measurements show some water must have settled around the edges of the site during irrigation. Similar to the irrigated section of the experimental plot increased 226Ra activity concentration has not resulted in increased 222Rn exhalation rate, measured as 43(1.1) mBq.m-2.s-1. This value is comparable with non-irrigated sections of the land application area. Hence increases in 226Ra activity concentration through land application of water have not resulted in proportional increases in 222Rn exhalation rates. The 222Rn exhalation rate observed at Mudginberri was greater than expected with the mean dry season exhalation rate being 33(1.1) mBq.m-2.s-1, a 226Ra concentration of 182±2 Bq.kg-1 was recorded that was high for natural soils of the region. This site was once used as a holding pen for an abattoir located nearby at the Mudginberri campsite. The area is bare of trees but speargrass (Sorghum intrans) grows abundantly during the wet season. The site that measurements were taken was within and around a radon station compound devoid of vegetation other than small grasses. Ground at the compound is very compacted to an extent that at times during the dry season it was not possible even to insert the soil temperature probe. This compaction is probably a result of the area being devoid of vegetation and its previous use as a cattle holding pen, when cattle and vehicles were regularly driven over the area. While measured 226Ra concentration here is higher for an ambient site, low porosity keeps 222Rn trapped in the soil. The result is a higher exhalation rate compared with other natural sites of the region but not as high as more porous sites observed with similar 226Ra activity concentrations. Jabiru’s water tower is built on soil excavated from the location of Jabiru Lake. In fact earth excavated from the lake was used over large areas of Jabiru to build it up above the surrounding floodplain. It is typical that ground built up in this fashion is compacted with heavy machinery resulting in solidly compacted ground at the site. The site is in an open paddock devoid of vegetation other than grass. Radium-226 activity concentration here is 46±1 Bq.kg-1, similar to other natural soils in the region. The compaction and lack of vegetation has a noticeable effect upon the rate of 222Rn exhalation here with the recorded dry season value of 16(1.1) mBq.m-2.s-1. Mirray is a natural undisturbed location representative of the region. Dry season levels of 222Rn exhalation rates here have a mean value of 36(1.1) mBq.m-2.s-1

120 comparable to sites like the non-irrigated land application and the value for common woodland in the region reported by Todd et al (1998). Radium-226 activity concentration here is 47±1 Bq.kg-1, also a typical level comparable with that of the natural soils in the region. Vegetation consisted of varieties of trees, shrubs and grasses; the soil at the site was relatively porous. Jabiru East is a rehabilitated area once the location of a township that serviced Ranger mine. The town was demolished in 1990 and the site rehabilitated through clearing and planting of local vegetation species, soil at the measurement site is relatively porous. In the decade since rehabilitation the area has recovered well but is noticeably different from surrounding woodland as it has younger vegetation. The mean dry season 222Rn exhalation at this site is 40(1.1) mBq.m-2.s-1 with the 226Ra concentration being 52±1 Bq.kg-1. The 222Rn exhalation rate here is comparable with other natural sites measured. The site alongside Magela creek consists of porous sandy soil. Todd et al. (1998) previously measured 222Rn exhalation rates here and recorded high readings. During this work it was observed that 226Ra activity concentration here is similar to other natural sites with a value of 43±4 Bq.kg-1 but the mean 222Rn exhalation rate is much greater being 1.8*102(1.1) mBq.m-2.s-1. The soil here is alluvial with no rocks and a sandy consistency with high porosity. High 222Rn exhalation rate observed here is due to high soil porosity and small grain size. Vegetation consisted of grasses and a few small trees. Careful selection of the sampling sites enabled us to select sites that covered a broad range of 226Ra activity concentrations over four orders of magnitude. This has in turn provided a large range of 222Rn exhalation rates also over four orders of magnitude. Plotting the results of 222Rn exhalation rates against 226Ra activity concentrations in Figure 5.1 shows that a weak positive correlation between the variables exists.

121 100000 ) -1 10000 .s -2 y = (0.452 ± 0.384)x R2 = 0.176

1000

Rn Exhalation Rate (mBq.m Rate Exhalation Rn 100 222

10 10 100 1000 10000 100000 226Ra Activity Concentration (Bq.kg-1)

Figure 5.1: Plot of 222Rn exhalation rates vs. 226Ra activity concentrations for all sampling sites

This data is presented in a different format in Figure 5.2 where a plot of the 222 226 ratio, Rn exhalation rate/ Ra activity concentration (RE-R) for all the dry season results is shown organised in ascending order of RE-R. From this figure we observe the broad trends discussed referring to comparisons or variations in soil parameters from various sites. Sites have been categorised into one of four geomorphic clusters, non-compacted boulders, barren, vegetated and non-compacted fine grains. Despite a number of orders of magnitude variation in 226Ra activity concentration and 222Rn exhalation rates sites belonging to similar geomorphic structures have clustered together. The greatest RE-R values are for sites of non-compacted fine-grained material such as Magela, laterite push zone and over burden. A result of identifying these clusters enables a reanalysis of the correlation between 222Rn exhalation rates and 226Ra activity concentration similar to that performed in Figure 5.1. Locations such as the grade 7 ore stockpile pad, grade 2 ore stockpile rim and grade 2 ore stockpile pad, as displayed in Figure 5.2, are outliers from their clusters due to the variation in 222Rn diffusion length discussed previously. With these outliers removed a clearer pattern and better relationships are established

122 for three of the four geomorphic categories. The results of reanalysis are displayed in Figure 5.3. The results from Figure 5.3 are summarised in Table 5-3 with the outliers removed.

123 )

-1 10.00 Vegetated Non-compacted boulders Non-compacted fine grains Barren

1.00 Ra Concentration (Bq.kg 226 )/ -1

.s 0.10 -2

0.01 Rn Flux (mBq.m k e l l y t I le d rri T Irr m s II d 222 i a ad W nal a en P P roc p J era Pad ona o Ea R Rim P one 7 7 3 e pad n s te ri R 2 2 z nbe it it D i LAA Irr as Mirra h it3 Rubbl pplot r ge r Rehab NI rburd Magela SP 7 Rim P P x te Se D biru OSP OSP te WR A LAANIROSP II OSP e O it 3 E it 1 La Ja pus Mudgi P Expplot Non La P WR LA Ov Non rite WRD Sea e LAA Irr Seasonal A LA Lat Location

222 226 Figure 5.2: Ratio of Rn exhalation rate to Ra activity concentration (RE-R) for locations during dry conditions

124 For barren sites such as Jabiru water tower, all stockpile pads, Mudginberri, -2 -1 -1 pit bases and the experimental plot an average RE-R of 0.49±0.14 mBq.m .s /Bq.kg is obtained. With the exception of the experimental plot they are all disturbed sites where compaction of the soil has taken place as a result of human influence. In most cases this compaction has resulted in a decrease in the 222Rn exhalation rate compared with natural undisturbed sites. The grade 2 and grade 7 ore stockpile pads are clear outliers and their removal from the analysis provides a lower, but -2 -1 -1 statistically better RE-R of 0.36±0.04 mBq.m .s /Bq.kg . Vegetated woodland and rehabilitated sites such as Jabiru East, rehabilitated waste rock, Magela land application area and Mirray have an RE-R of 0.82±0.06 mBq.m-2.s-1/Bq.kg-1. This group includes all common woodland sites investigated and represents the ratio for relatively porous vegetated soils of the region.

The greatest RE-R values are obtained from non-compacted fine grains such as

Magela, laterite push zone and overburden locations. The average RE-R for these regions is 3.1±1.0 mBq.m-2.s-1/Bq.kg-1 indicating that the porosity of the soil at these locations plays a dominant role in controlling the 222Rn exhalation rates.

The cluster of non-compacted boulders has the lowest RE-R value of 0.12±0.05 mBq.m-2.s-1/Bq.kg-1. This covered the stockpile rims, piles and rocks studied in pit #3. This result is expected, as it is known 222Rn has difficulty emanating from within enclosed crystalline lattices found in rocks. Again the grade 2 ore stockpile rim provides an outlier in this set and is likely the result of 222Rn diffusion from greater depths as mentioned previously, this was removed from calculation of the average. The results from the regression analysis of the four geomorphic clusters are provided in Table 5-3. An important outcome of this work is the 222Rn exhalation rate to 226Ra activity concentration relationship derived as a result of this regression analysis. This relationship kappa (κ), in Table 5-3, is an RE-R derived from regression analysis that can be applied to similar geomorphic structures at dry tropical locations. All geomorphic clusters, with the exception of non-compacted boulders returned positive correlations with good determination coefficients. Due to large air gaps and random 222Rn diffusion paths within non-compacted boulder structures the typical positive relationship between 222Rn exhalation rate and 226Ra activity concentration does not hold. The results presented in Table 5-3 will be used in Chapter 7 to determine the dry season 222Rn source term for the Jabiru and Kakadu regions.

125 Table 5-3: Analysis result of geomorphic clusters

-2 -1 -1 2 Geomorphic Average RE-R κ (mBq.m .s /Bq.kg ) R group (mBq.m-2.s-1/Bq.kg-1)

Barren 0.36±0.04 0.27±0.05 0.82 Vegetated 0.82±0.06 0.61±0.03 0.98 Non-compacted 3.1±1.0 2.70±0.42 0.93 fine grains Non-compacted 0.12±0.05 - - boulders

126

Barren Vegetated 10000 1000 ) -1 ) Φ Rn-222 = (0.61 ± 0.03)CRa-226 -1 .s 2

-2 .s R = 0.98 -2 p < 0.01

1000

100 100

ΦRn-222 = (0.27 ± 0.05)CRa-226 R2 = 0.82 Rn Exhalation Rate (mBq.m p < 0.01 (mBq.m Rate Exhalation Rn 222 222 10 10 10 100 1000 10000 100000 10 100 1000 10000 226 -1 226 -1 Ra Activity Concentration (Bq.kg ) Ra Activity Concentration (Bq.kg ) Non-compacted Boulders Non-compacted fine grains 100000 10000

) ΦRn-222 = (2.70 ± 0.42)CRa-226 -1

2 ) .s R = 0.93 -1 -2 .s

p < 0.01 -2 10000

1000 1000

100 Rn Exhalation Rate (mBq.m

(mBq.m Rate Rn Exhalation 222 222 10 100 10 100 1000 10000 100000 1000 10000 100000 226Ra Activity Concentration (Bq.kg-1) 226Ra Activity Concentration (Bq.kg-1) Figure 5.3: Plot of 222Rn exhalation rate vs. 226Ra activity concentration for all sites categorised by geomorphic groups

127 5.3 Diurnal measurements of radon exhalation Diurnal measurements of 222Rn and 220Rn exhalation rates were recorded seven times from five of the seasonal sites. Towards the end of the wet season 2002 readings were performed at Mudginberri, Jabiru Water Tower and Mirray sites. Later that year during the dry season measurements were performed at Mudginberri, Jabiru Water Tower, Jabiru East and Magela Creek. The aim was to observe any possible correlation of 222Rn or 220Rn exhalation rates with meteorological parameters. On the basis of information from the reported literature parameters of primary interest were soil temperature and atmospheric pressure. Literature shows that radon emanation is directly proportional to soil temperature (oC) and that the radon exhalation rate is directly proportion to the square root of the soil temperature (oC). The effect of temperature variations on diffusion is limited to the top few decimetres of soil. Temperature should therefore influence 220Rn exhalation more than 222Rn exhalation as 220Rn is exhaled only from the top few decimetres of soil. Other studies have shown 222Rn exhalation is inversely proportional to atmospheric pressure. The inverse relationship between 222Rn exhalation rate and atmospheric pressure has been used previously to explain observed diurnal variations in 222Rn exhalation rates. These potentially influencing parameters have been discussed in detail in Chapter 2 of the thesis. In the Alligator Rivers Region the atmospheric pressure, atmospheric temperature and soil temperature vary diurnally. To demonstrate this, measurements for a selected period of time are shown in Figure 5.4 and Figure 5.5. Atmospheric pressure records low values in the mid afternoon and high values in the morning. Temperature variations are as expected with minimums recorded before dawn and maximums in the mid afternoon. In the region, on a diurnal basis, typical atmospheric temperature and pressure variations are 12-14 oC and 5-7 hPa respectively. It was noted that variations in the collection chamber temperature measured during these tests followed the same pattern as air temperature variations. As variations in air temperature have not been reported to influence radon exhalation no analysis using this variable has been performed.

128 1016

1014

1012

1010 Atmospheric Pressure (hPa)

1008 20/08/03 20/08/03 20/08/03 20/08/03 21/08/03 21/08/03 21/08/03 21/08/03 0:00 6:00 12:00 18:00 0:00 6:00 12:00 18:00 Date Time

Figure 5.4: Diurnal variations of atmospheric pressure observed at Jabiru East (Data courtesy of Australian Bureau of Meteorology) 40 Site Air Temp. Site Soil Temp. Weather Station Air Temp.

30 C) o

Temperature ( 20

10 20/08/03 20/08/03 20/08/03 20/08/03 21/08/03 21/08/03 21/08/03 21/08/03 0:00 6:00 12:00 18:00 0:00 6:00 12:00 18:00 Date Time

Figure 5.5: Diurnal variations in atmospheric and soil temperatures at Jabiru East (Data courtesy of Australian Bureau of Meteorology)

129 Combining the information supplied in Figure 5.4 and Figure 5.5 with previous studies it is expected that peak 222Rn exhalation should occur during the mid or late afternoon when atmospheric pressure is dropping and temperature is at a maximum. For the region of this study a small set of advantageous diurnal measurements was reported (Todd et al. 1998). They reported poor correlation coefficients (R-values) between 222Rn, 220Rn exhalation rates and soil temperature, -0.536 and 0.368 respectively, and atmospheric pressure, 0.428 and -0.482 respectively. Variation between positive and negative correlations is further display of the poor relationship between the variables In Figure 5.6 and Figure 5.7 the combination of the seven diurnal measurements has been reported. For each site the average daily 222Rn and 220Rn exhalation rates have been used to normalise the data. The observations suggest that for both radon isotopes, diurnal trends, if any, are masked by the random variation of the signal. Standard deviations of all the measurements are 0.46 and 0.15 for 222Rn and 220Rn respectively. This displays the general scatter in the results with no distinct pattern. To further investigate any variation with the influencing parameters that change diurnally, the data are plotted as a function of soil temperature in Figure 5.8 and Figure 5.9 and the change in exhalation rate against the change in atmospheric pressure in Figure 5.10 and Figure 5.11. Change in atmospheric pressure and change in exhalation rates are obtained through subtracting the previous readings from the current one. No trends appear in the data suggesting that we are observing systematic variations with these parameters. To confirm this regression analysis was performed on the data displayed in Figure 5.6 to Figure 5.11 similar to that performed by Todd et al. (1998), the results returned poor correlations varying between positive and negative relationships for the variables. These statistical results are displayed in Table 5-4 Diurnal measurements were performed during the wet season and again during the dry season to see if diurnal variations were dependent upon the moisture content of the soil. Separate analysis of these results indicated no diurnal variations or relationship with soil temperature and atmospheric pressure during either season.

130 5 Jabiru Wet Mirray Wet Mudginberri Wet 4.5 Jabiru Dry Magela Dry Jabiru East Dry Mudginberri Dry 4

3.5

3

2.5 Rn Exhalation Rate

222 2

1.5

Normalised 1

0.5

0 6:00 10:00 14:00 18:00 22:00 2:00 6:00 10:00 14:00 Time of Day

Figure 5.6: Normalised 222Rn exhalation rate for all sites vs. time of day of measurement 1.6 Jabiru Wet Mirray Wet Mudginberri Wet Jabiru Dry Magela Dry Jabiru East Dry Mudginberri Dry 1.4

1.2

1 Rn Exhalation Rate 220

0.8 Normalised 0.6

0.4 6:00 10:00 14:00 18:00 22:00 2:00 6:00 10:00 14:00 Time of Day

Figure 5.7: Normalised 220Rn exhalation rate for all sites vs. time of day of measurement

131 5 Jabiru Wet Mirray Wet 4.5 Mudginberri Wet Jabiru Dry Magela Dry Jabiru East Dry Mudginberri Dry 4

3.5

3

2.5 Rn Exhalation Rate

222 2

1.5

Normalised 1

0.5

0 25 27 29 31 33 35 37 39 Soil Temperature (oC)

Figure 5.8: 222Rn exhalation rate vs. soil temperature 1.6 Jabiru Wet Mirray Wet Mudginberri Wet Jabiru Dry Magela Dry Jabiru East Dry Mudginberri Dry 1.4

1.2

1 Rn Exhalation Rate 220

0.8 Normalised 0.6

0.4 25 27 29 31 33 35 37 39 Soil Temperature (oC)

Figure 5.9: 220Rn exhalation rate vs. soil temperature

132 Jabiru Wet Mirray Wet Mudginberri Wet Jabiru Dry 100 ) Magela Dry Jabiru East Dry Mudginberri Dry -1 .s

-2 80

60

40

20

0

-20 Rn Exhalation Rate (mBq.m 222 -40

-60 Change in -80 -3 -2 -1 0 1 2 3 Change in Atmospheric Pressure (hPa)

Figure 5.10: 222Rn exhalation rate vs. change in atmospheric pressure

800 Jabiru Wet Mirray Wet Mudginberri Wet Jabiru Dry Magela Dry Jabiru East Dry Mudginberri Dry

) 600 -1 .s -2 400

200

0

-200

Rn Exhalation Rate (mBq.m -400 220

-600

Change in -800

-1000 -3-2-10123 Change in Atmospheric Pressure (hPa)

Figure 5.11: 220Rn exhalation rate vs. change in atmospheric pressure

133 Table 5-4: Correlation coefficients for diurnal measurements Radon-222 Exhalation Rate correlation against Site Atmospheric Humidity Soil Pressure Temperature Temperature Change

Mudginberri Dry - 0.03 0.27 -0.08 Jabiru East Dry -0.16 0.11 -0.20 0.64 Magela Dry -0.22 0.23 0.15 -0.01 Jabiru Dry 0.38 -0.20 0.49 0.03 Mudginberri Wet -0.56 0.59 -0.56 0.11 Mirray Wet -0.49 0.64 -0.59 0.27 Jabiru Wet -0.12 -0.12 -0.04 0.42

An important finding of this experiment relates to the input to models used for radiological impact assessment of Ranger. These results show that daytime measurements of 222Rn exhalation rates from Ranger, as were performed in Section 5.2, are valid for dose assessment calculations. The methodology used for the charcoal cups is also justified. If diurnal variations exist then charcoal cups should be collected at the same time of day that they were deployed so complete diurnal cycles are measured. As no diurnal cycles in 222Rn exist in the region the results obtained from charcoal cup measurements deployed and collected at different times of day are valid.

134

5.4 Seasonal measurements of radon exhalation Expectations from theory and previous studies indicated that wet season rainfall should retard 222Rn exhalation rates. Sites selected for seasonal measurements have been described previously in Chapter 3. Charcoal cups and emanometers were used to measure 222Rn exhalation rates. The equipment used, the measurement procedures and schedules have been described previously in Chapters 3 and 4. Many authors have reported the effects of precipitation and soil moisture on 222Rn emanation and exhalation rates as covered in detail in Chapter 2. It is widely accepted that when water penetrates soil it fills interspatial spaces and increases 222Rn emanation as recoiling atoms are slowed more quickly in water than air. Above certain soil moisture content 222Rn exhalation rates decrease because 222Rn is soluble and becomes trapped in wet soils. However in the large collection of work investigating the effects of moisture on 222Rn emanation and exhalation little work has been performed for a systematic coverage over a seasonal cycle, particularly within tropical regions. At most sites it was observed that 222Rn exhalation rates decreased throughout the wet season. Radon-222 exhalation rates were recorded over a 4 to 10 day period on a monthly basis between August 2002 and July 2003 for eight sites. Results of this work are graphically shown in Figure 5.12 and Figure 5.13. Cumulative rainfall recorded from the nearest meteorological station at Jabiru East, Jabiru Town or Cooinda is also displayed.

135 Magela Land Application Area Irrigated Zone Jabiru East Cumulative Rainfall Rn-222 Exhalation Cumulative Rainfall Rn-222 Exhalation

1400 140 1400 140

1200 120 1200 120

1000 100 1000 100 ) ) -1 -1 .s .s

800 80 -2 -2 800 80

600 60 600 60 (mBq.m (mBq.m Rainfall (mm) Rainfall (mm) 400 40 400 40 Radon Exhalation Rate Radon Exhalation Rate 200 20 200 20

0 0 0 0 Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul 2002-2003 2002-2003

Magela Creek Magela Land Application Area Non-Irrigated Cumlative Rainfall Rn-222 Exhalation Cumulative Rainfall Rn-222 Exhalation

1400 420 1400 140

1200 360 1200 120

1000 300 1000 100 ) ) -1 -1 .s 800 240 .s -2 800 80 -2

600 180 600 60 (mBq.m Ranifall (mm) (mBq.m Rainfall (mm) 400 120 400 40 Radon Exhalation Rate Radon Exhalation Rate 200 60 200 20

0 0 0 0 Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul 2002-2003 2002-2003

Figure 5.12: Seasonal variations of 222Rn exhalation rates and cumulative rainfall 136 Waste Rock Dump Jabiru Water Tower Cumlative Rainfall Rn-222 Exhalation Cumlative Rainfall Rn-222 Exhalation

1400 560 1600 32

1200 480 1400 28 1200 24 1000 400 ) ) -1 1000 20 -1 .s 800 320 .s -2 -2 800 16 600 240 600 12 (mBq.m (mBq.m Rainfall (mm) Rainfall (mm) 400 160 400 8 Radon Exhalation Rate Radon Exhalation Rate 200 80 200 4

0 0 0 0 Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul 2002-2003 2002-2003

Mirray Mudginberri Cumulative Rain Rn-222 Exhalation Cumulative Rainfall Rn-222 Exhalation

1200 180 1400 140

1000 150 1200 120

1000 100 800 120 ) ) -1 -1 .s

.s 800 80 -2 600 90 -2 600 60 (mBq.m Rainfall (mm) 400 60 Rainfall (mm) (mBq.m 400 40 Radon Exhalation Rate

200 30 Radon Exhalation Rate 200 20

0 0 0 0 Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul 2002-2003 2002-2003

Figure 5.13: Seasonal variations of 222Rn exhalation rates and cumulative rainfall continued 137 Elevated 222Rn exhalation rates were observed during January 2003 at Jabiru East, irrigated Magela land application area, Mirray and Mudginberri. Conditions at these sites were similar to other sites so explanation of this outlier comes from analysis of precipitation events and its influence on 222Rn emanation. The day prior to charcoal cup deployment recorded 124 mm of rain over 24 hours till 9 am, this was the largest rain event that wet season. During charcoal cups exposure only light rain events occurred. Precipitation occurring before cup deployment has increased 222Rn emanation. Also 222Rn exhaled from layers underneath became trapped in the water. It is possible that evaporation of water during charcoal cup exposure has released trapped 222Rn and resulted in the observed increased 222Rn exhalation rate. A similar effect has previously been observed and reported (Todd and Akber 1996). In that study 220Rn exhalation rates, three days after addition of water to a monazite sample increased by 20% compared to dry exhalation rates. As 222Rn has a much longer half-life compared to 220Rn it can remain trapped in water for many days. There are large errors associated with the January measurements that are an effect of the large variations in individual measurements. It indicates that transport of 222Rn in water, rather than normal diffusion, is dominant. Movement of 222Rn with water through soil and evaporation through capillary channels has resulted in some cups recording higher 222Rn exhalation rates. Elevated levels were observed at Mirray during February and March along with the January peak. This site is on sloping ground and most water deposited their runs off quickly. This may be an indication that enough water is retained at the site to increase emanation followed by an increase in exhalation due to evaporation explained previously. There is a slight increase in 222Rn exhalation at the non- irrigated Magela land application site in January and February but it is not as great as that observed at other sites. This site is similar to the irrigated Magela land application area with more soil compaction and without as much vegetation. The slight increase observed there may be a result of minimal water absorption, placement of charcoal canisters away from major evaporative sites or a combination of these.

138 20 160 Rn-222 Atmospheric Concentration Rn-222 Exhalation

) 18

-3 140 )

16 -1 .s

120 -2 14 100 12

10 80

8 60 6 40 4 Rn Exhalation Rate (mBq.m 222 Rn Atmospheric Concentration (Bq.m 20 222 2

0 0 9-12 9-12 7-9 8-11 6-9 8-10 4-7 4-10 4-10 3-8 3-10 3-8 Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Deployment Date 2002-2003

Figure 5.14: Averaged 222Rn exhalation rates and atmospheric concentrations for sampling periods at Mudginberri

Monitoring of 222Rn atmospheric concentrations has been conducted in the region for many years. Data collected from this monitoring program shows seasonal variations of 222Rn concentrations with a low throughout the wet season (Akber et al. 1993). Lower 222Rn exhalation rates in the wet season would be expected to be a major contributor to low 222Rn atmospheric concentrations. The Mudginberri sampling site is adjacent to an atmospheric radon monitoring station maintained by eriss. Data from this station was used to obtain average 222Rn concentrations for the days when the exhalation measurements were performed. The aim is to see any direct correlation between the local 222Rn exhalation rate and atmospheric concentration. The general trend is a decrease in both 222Rn atmospheric concentrations and exhalation rates during the wet season as shown in Figure 5.14, however correlation between the two variables is not clear. Particularly the exhalation peak in January 2003 does correspond to an increase in atmospheric concentration. It is known that a number of meteorological factors that change on a daily basis, such as inversion layers and wind speed, substantially change 222Rn atmospheric concentration in the area. Hence, these meteorological

139 factors override the day-to-day variations that 222Rn exhalation rates have on atmospheric 222Rn concentrations. Peak 222Rn exhalation rates were not observed at the waste rock dump, Jabiru water tower or non-irrigated Magela land application area during January 2003. The waste rock dump is very porous with a good capacity to absorb large amounts of water. Water deposited onto the waste rock dump will quickly travel to depth. Jabiru water tower however is compact with low capacity to absorb water and the majority of water deposited at this site runs off. Therefore there is little absorption and so no increase in emanation or exhalation as a result. Mudginberri has also been noted as being compact but compaction here is not as great as at the Jabiru water tower, this is confirmed by considering formation methods. Mudginberri was cleared and used as a cattle holding pen while Jabiru water tower was constructed on excavated soil compacted with heavy machinery. Therefore it is likely that larger amounts of water are absorbed at Mudginberri increasing emanation and releasing 222Rn during evaporation. Effects of soil moisture on 222Rn exhalation are complex and controlled by more than one implication of a moisture profile. Previous studies (Hart 1986; Todd and Akber 1996) have demonstrated the effect that increasing moisture content have on 222Rn and 220Rn exhalation rates. Those works examined total moisture content of a sample. Field observations in this study show that a diversity of moisture content exists in the soil depths that change throughout a wet season. Observations at Jabiru East are displayed in Figure 5.15. These measurements are discrete, however it is expected that the moisture changes due to the intensity, duration and intervals of rain events. In turn this will have an effect on 222Rn exhalation rates and during a wet season 222Rn exhalation rates from localised areas may vary greatly over short periods of time. As the dry season progresses, soil moisture levels drop rapidly compared with wet season values and remain relatively constant (Figure 5.16). Wet season variations in soil moisture profiles similar to that seen in Figure 5.15 were observed from all four sites selected for measurement. Some differences in 222Rn exhalation rates observed throughout the wet season can be explained through examination of soil moisture profiles. After precipitation, moisture penetrates the ground and seeps downward. The rate and depth of penetration depends on factors such as amount of deposited water, current soil moisture and soil porosity. For normal soils, recent rain will increase soil moisture in top layers and this will have a

140 15.00 October November January February March

10.00

5.00 % Moisture by Volume

0.00 10 20 30 40 50 60 70 80 90 Depth (cm)

Figure 5.15: 2002-2003 wet season moisture profiles for Jabiru East

4.00 May June July

3.00

2.00 % Moisture by Volume 1.00

0.00 10 20 30 40 50 60 70 80 90 Depth (cm)

Figure 5.16: 2003 dry season soil moisture profiles for Jabiru East

141 capping effect on 222Rn exhaled from depth. If a long enough time passes between precipitation events, moisture penetrates deeper and 222Rn exhalation will occur from the top layers of soil but will be capped at a deeper level. Soil moisture measurements at Jabiru East for the month of January 2003 were performed on the 10th. This was three days after the heavy rain event previously mentioned and soil moisture was less than 5% at all depths. Water deposited has either runoff, evaporated or penetrated to depths beyond measurement or a combination of these. Considering the compact nature of the ground at the site runoff is the most likely explanation. Similar soil moisture values were observed at three of the four measurement sites where no January peak in 222Rn exhalation was observed, the exception being Mirray where a peak was observed. At Mirray 30-50 cm readings of soil moisture were high. All soil moisture readings from Mirray are displayed in Figure 5.17. The soil moisture reading for January indicate that either water absorbed uphill is travelling through the soil at depth or water deposited has penetrated quickly to depth, the latter being more plausible given the porous nature of soil at the site. Compared to November’s results where 222Rn exhalation is low due to capping in the soils surface layers January’s soil moisture profile is almost opposite. February and March moisture levels are low compared with other wet season levels and correspond with elevated 222Rn exhalation rates. January’s peak in 222Rn exhalation is still likely to be a result of increased emanation followed by release during evaporation. February and March results are due to increased emanation with increased soil moisture but the moisture levels are not high enough to retard exhalation. The soil moisture access tube at non-irrigated Magela land application area went to a depth of 50 cm. Unexpected elevated levels of soil moisture were recorded there at the 50 cm depth throughout the dry season. This site is only 20 m from land that is irrigated almost daily over the course of the dry season. The observed elevation in soil moisture may be indicative of water movement from the irrigated area.

142 14 Aug-02 Sep-02 Nov-02 Jan-03 Feb-03 Mar-03 Apr-03 May-03 Jun-03 Jul-03

12

10

8

6 % Moisture by Volume by Moisture % 4

2

0 10 20 30 40 50 Depth (cm)

Figure 5.17: Mirray soil moisture profiles, all readings

5.5 Chapter summary Investigations of the parameters known to influence 222Rn exhalation have been performed within a tropical region and are presented. It has been shown that while 226Ra activity concentration is an important contributing factor of 222Rn exhalation, it can be dominated by soil variables such as porosity and grain size. Dry season results obtained from waste rock dumps, ore stockpiles, overburden, land application area (irrigated and non-irrigated) and five ambient sites were used to determine our conclusions, as the influence of soil moisture could be neglected. Diffusion length of 222Rn through rocky stockpile formations may be of the order of a few tens of metres that differ greatly from standard soils. Compaction of the ground and its resultant reduction of soil porosity decrease 222Rn exhalation rates. Vegetation with established root structures lead to higher 222Rn exhalation rates. Differences in 222Rn exhalation rates for sites based upon their geomorphologic structure were displayed with the 222Rn exhalation rate to 226Ra activity concentration ratio (RE-R). It was observed that sites of similar geomorphic structure had comparable RE-R values and that four broad groups could be formulated. Regression analysis of results from the four geomorphic groups provided

143 linear relationships with very good correlations for three of the four groups; (i) vegetated; (ii) barren; and (iii) non-compacted fine grain sites. No relationship was determined for non-compacted boulders due to large variations in 222Rn diffusion paths. These results are used in Chapter 7 to determine the 222Rn source term for the Kakadu region. Several diurnal measurements were performed to observe possible variations of 222Rn or 220Rn exhalation rates with changes in soil temperature or atmospheric pressure. Diurnal variations of soil temperature and atmospheric pressure occur but have no observable effect, if any, on either 222Rn or 220Rn exhalation rates. Distinct wet and dry seasons enabled observation of the effect of precipitation on 222Rn exhalation. In general, 222Rn exhalation rate reduces during the wet season. Site to site and day to day behaviours could be complex, even localised variations could occur due to uneven soil moisture profiles and possible evaporation as the ground heats up after a precipitation event. Large variations between individual wet season measurements result in large uncertainties in site averages and such variations indicate that transport in water is dominant over diffusion. Porous sites and very compacted sites displayed expected seasonal trends with reduced 222Rn exhalation throughout the wet season. Soil moisture was measured with a probe that provides a moisture depth profile at 10 cm intervals. Soil moisture profiles at all sites varied throughout the wet season due to water transport and strengths, times and duration of precipitation events. This explains the large variations of 222Rn exhalation rates observed throughout the wet season. Radon-222 exhalation can be expected to change over short periods of time throughout the wet season as soil moisture profiles vary.

144 6

Lead-210 deposition and excess

6.1 Overview This chapter provides results of the seasonal 210Pb depositional rate measurements and excess 210Pb inventories of the soil samples collected from the region. Distinct seasonality in depositional rates is observed at Jabiru East and Oenpelli with the majority of deposition occurring in the wet season. Jabiru East had a larger annual depositional rate than Oenpelli, which may be a result of its proximity to Ranger mine. Dry season deposition rates are low and indicative of long 210Pb atmospheric residency times, the order of a few months. Excess 210Pb inventories from 14 soil cores collected across the region are included. These results have been compared with an excess 210Pb inventory derived from the deposition rate measurements. Penetration half depth of 210Pb for the region was determined to be 2.5±0.7 cm, which compares well with values reported for Queensland, Australia (Akber et al. 2004a). Discrepancies in excess 210Pb inventories observed with some samples have been investigated and explained. One unique site, the Magela Land Application Area, was studied in detail for retention of spray irrigated radionuclides found in Ranger wastewater. The soil’s retention of 226Ra and 210Pb was within the top few centimetres with a distribution on the fine grain fraction. Uranium-238 retention differed with results showing horizontal and vertical transport more than expected at the onset of irrigation program. 6.2 The 210Pb story After entering the atmosphere 222Rn spreads vertically and horizontally and eventually decays, via its short-lived progeny, into 210Pb. With a half-life of 3.82 days for 222Rn, and a reported average residency time of between 5-9 days for 210Pb in the atmosphere (Beks et al. 1998; Pourchet et al. 2000; Koch et al. 1996; Moore et al. 1977), deposition of 210Pb occur at large distances from its point of origin. Previous studies have shown that the vertical distribution of 222Rn and its progeny decreases rapidly with increasing altitude (Cuculeanu and Lupu 1996; Moore et al. 1977). Both model and experimental studies of the horizontal distribution of 222Rn

145 from Ranger have shown that mine related concentrations decrease to negligible levels within a few kilometres of the source (Urban et al. 1992; Davey 1994). This means the majority of 222Rn concentration at environmental sites is mostly due to the local contribution. As 222Rn is soluble in water, the exhalation rate from water bodies is extremely low. This causes some complexity to the budgeting estimates for the Kakadu region, as there are large floodplains, creeks and billabongs that expand with wet season rains and shrink throughout the dry season. The size to which they expand and shrink is dependent on the rainfall of the wet season. Generally floodplains fill and are submerged throughout the wet season but the size of creeks and billabongs can vary dramatically with short-term changes in rainfall patterns. As 222Rn moves through the atmosphere it eventually decays and its transport characteristics change to that of a solid. The majority of progeny rapidly attach themselves to aerosol particles in the atmosphere and become subject to the same transport and deposition patterns as the aerosol particles they are attached to. This has been detailed in Chapter 2. Typically, surface soil is finer and more compact than the soil below it and as a result it has greater moisture retention and a smaller diffusion coefficient that causes torturous 222Rn path lengths. Radon-222 migrating through soil that reaches this layer can become trapped beneath it and will decay there. This is part of the natural process that redistributes 210Pb in the environment and results in a higher 210Pb concentration relative to 226Ra in the surface layers of soil. This process combined with deposition contributes to excess 210Pb found in surface soils. For this work excess 210Pb resultant from 222Rn being trapped in the surface soil is considered to be negligible. Another consideration is the high solubility of 226Ra in water. Some 226Ra normally found in surface soils will leach down after precipitation events. The amount of 226Ra leached out of the surface soil depends primarily on its distribution in the soil grain as surface distributed 226Ra is more accessible to water than homogenously distributed 226Ra. Lead-210 is much less mobile so 226Ra leaching results in excess 210Pb with respect to 226Ra in surface soils. This effect has been investigated in this chapter.

146 6.3 Pb-210 deposition 6.3.1 Seasonal 210Pb results As described in Chapter 3 Jabiru East and Oenpelli were selected for 210Pb deposition measurement over one seasonal cycle. Oenpelli lays approximately 50 km north north west of the Ranger operation and Jabiru East lays 3 km west of the operational pit #3. Lead-210 was collected in an ion exchange resin column attached to the base of a wet and dry deposition collector as described in Chapter 4. The results obtained from these measurements are shown in Figure 6.1 and Figure 6.2 and tabulated in Table 6-1 and

147 Table 6-2. During the wet season the East Alligator River becomes impassable so a light aircraft was chartered to collect the Oenpelli samples. Due to weather conditions and flight availability the monthly collection routine was disrupted. Collection periods were between five and six weeks for Oenpelli and it was decided that Jabiru East sample collections should match those of Oenpelli. Due to operational problems the Jabiru East collector was overlooked during April 2004 and the sample was not collected until May 2005.

Rainfall Pb-210 Deposition

2000 400 1800 )

350 -1 1600 .day -2 1400 300

1200 250

1000 200 800 Rainfall (mm) 150 600 100

400 Pb Deposition Flux (mBq.m 210 200 50

0 0 May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Month (2003-2004)

Figure 6.1: Jabiru East 210Pb deposition and cumulative rainfall

148 Rainfall Pb-210 Deposition

2000 180

1800 160 )

1600 -1 140

1400 .day 120 -2 1200 100 1000 80 800 Rainfall (mm) 60 600

40 Pb Deposition (mBq.m 400 210

200 20

0 0 May Jun Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Month (2003-2004)

Figure 6.2: Oenpelli 210Pb deposition and cumulative rainfall Table 6-1: Results from Jabiru East 210Pb deposition collector Sampling dates Rainfall (mm) Average 210Pb deposition (mBq.m-2.day-1)

1/5/03-31/5/03 5.4 32.4 1/6/03-30/6/03 0.2 0 1/7/03-31/7/03 0 16.1 1/8/03-31/8/03 0 24.1 1/9/03-30/9/03 0.8 23.3 1/10/03-31/10/03 31.2 369.2 1/11/03-2/12/03 105 260.8 3/12/03-9/1/04 482.8 242.8 10/1/04-6/2/04 458.2 174.0 7/2/04-12/3/04 538.6 165.1 13/3/04-18/5/04 149.8 89.7 19/5/04-23/6/04 53 34.7

149 Table 6-2: Results from Oenpelli 210Pb deposition collector Sampling dates Rainfall (mm) Average 210Pb deposition (mBq.m-2.day-1)

1/5/03-31/5/03 1.2 2.9 1/6/03-30/6/03 0 15.8 1/7/03-31/7/03 0 2.0 1/8/03-31/8/03 0 0.8 1/9/03-30/9/03 5.4 10.2 1/10/03-31/10/03 0 5.3 1/11/03-3/12/03 79.5 58.7 4/12/03-9/1/04 572.2 (64.0 calculated) 10/1/04-13/2/04 549.4 61.5 14/2/04-13/3/04 308.2 57.9 14/3/04-19/4/04 242.8 114.7 20/4/04-17/5/04 4 49.3 18/5/04-23/6/04 124.4 47.2

There were problems with sample collection at Oenpelli for October and December 2003, a result of the resin column being blocked with organic matter. This does not represent a large problem for October as meteorological data shows that there were no rain events during that month. An average value obtained from all other dry season months has been used to represent the October deposition. Loss of the December sample is problematic and in order to obtain data for modelling deposition is obtained from normalising January’s deposition against rainfall and calculating December’s deposition from this. January was used over instead of all wet season months, as the rainfall amounts for December and January are similar. This problem was overcome during later collection when a bucket was used to collect any spilt water. After blockages were removed from the column, excess water was placed back into the top of the collector and allowed to pass through the resin before the sample was collected. The results displayed in Figure 6.1 and Figure 6.2 show distinct seasonality for 210Pb depositional rates at Jabiru East and Oenpelli. As expected 210Pb depositional rates are greater during the wet season. In some cases dry season

150 deposition rates are lower than the detection limits. This is evidence that precipitation scavenging of 210Pb is the dominant removal mechanism of 210Pb from the atmosphere. At Jabiru East maximum 210Pb deposition is observed at the onset of the wet season and it decays exponentially throughout the wet season. At the start of the wet season 210Pb atmospheric concentrations are at a peak and the rain scavenges this. Exponential decrease is a result of a similar reduction in the local 210Pb source term. In Chapter 5 the waste rock dump seasonal site had a delay in retardation of 222Rn exhalation rate at the onset of the wet season. This was because stockpile porosity meant that early wet season rains penetrated the stockpile with minimal effect on 222Rn exhalation. Providing this occurs for all stockpiles then there is an exponential decrease in 222Rn source term from Ranger as the wet season progresses. This may be reflected in the 210Pb deposition rates observed at Jabiru East as a component of this deposition is a result of the Ranger operation. It may also be possible as 222Rn exhalation rate measurements were discrete that the local area source term follows a similar pattern to the stockpiles. This would most likely occur in porous soils and can also explain the wet season exponential decrease in 210Pb deposition rates at Jabiru East. As the 210Pb source term reduces, atmospheric concentrations and 210Pb deposition reduce in a similar pattern. Lead- 210 deposition rates in May 2003, April and May 2004 are higher than dry season averages and are a result of light rain events that occurred during these months. Similar results were observed at Oenpelli when rain events occurred during dry season months. Oenpelli displays a relatively steady state 210Pb depositional rate throughout the wet season with the exception of a peak in April 2004. The local 210Pb term is likely to suffer a similar decrease but precipitation events occur at frequencies that maintain deposition rates at steady state values. The peak in April 2004 is a result of reduced precipitation events and a likely increase in 222Rn exhalation rates throughout this month. Normally convective storms occur in the late afternoon or early evening every few days throughout March and April. The time between precipitation events allows for an increase in 222Rn exhalation rates as the soil dries and the moisture aids emanation, this in turn results in higher 210Pb atmospheric concentration and deposition rates. Readings for April 2004 at Jabiru East can not be used for comparison as they are averaged over two months.

151 The overall low 210Pb deposition rates at Oenpelli may be attributed to a regional lack of 210Pb during the wet season. Oenpelli is surrounded by floodplains and inaccessible other than air throughout the wet season, the township is an island that rises out of the surrounding floodplain. The area will likely have very low 210Pb atmospheric concentrations during the wet season resulting in low 210Pb deposition. At Oenpelli there is also relatively increased 210Pb deposition in May and June 2004 compared with normal dry season values. This is due to uncharacteristic rain events occurring during these months. In May there were four rain events totally 4 mm, while June displayed heavier rains, 6 events totally 124 mm. The 210Pb deposition rates are similar for these months being evidence that light rain events are as efficient in scavenging 210Pb from the atmosphere as heavier events, this observation supports previous studies (Baskaran 1995; Baskaran et al. 1993) and the observations at Jabiru East. As 210Pb deposition rates for these months are similar to averaged wet season values for Oenpelli it is speculated that the increase in surface area due to reduction in inundated floodplains results in greater 222Rn emission in the area and increases 210Pb atmospheric concentrations. The scavenging events during these months, although only a few, have higher 210Pb atmospheric concentrations available to extract. Therefore a few rain events result in averaged 210Pb deposition rates similar to wet season values. The results obtained here only provide 210Pb deposition rate monthly averages. Previous work performed in the area provides information on 210Pb deposition rates from individual rain events, including sequential sampling for six rainstorms (Martin 2003). Three rainstorms sequentially sampled displayed large amounts of 210Pb washout in the first few millimetres while the other storms sequentially sampled did not. The former supports the conclusion discussed for Oenpelli deposition rates in May and June 2004 that light rains can scavenge a lot of 210Pb, most likely true if the majority of 210Pb deposits in the first few millimetres of rain. Of note from the study by Martin (2003) are the results obtained from light rainfall at Jabiru East on the 20/9/85 with high 210Pb deposition, (1.7±0.1 kBq.l-1). September rain, while not uncommon, is unseasonable and usually represents the first rain for many months. A conclusion drawn from this study and Martin (2003) is that the large deposition rates observed during October 2003 at Jabiru East are a result of scavenging the large amount of available 210Pb. The first few rain events are

152 likely responsible for the majority of the 210Pb deposition observed during this month. 6.3.2 Annual depositional rate, average values and residency time Values for annual 210Pb depositional rates as well as wet and dry season depositional rate averages are listed in Table 6-3. It has been reported that 210Pb deposition rates can vary annually by a factor of three (Bonnyman and Molina-Ramos 1974; Beks et al. 1998; Baskaran 1995). This project has only observed results from one seasonal cycle and whether these results are indicative of an average year cannot be determined. From the global database an average annual southern hemisphere 210Pb depositional rate is determined as 53±4 Bq.m-2.y-1 (Preiss et al. 2003). The observed annual 210Pb depositional rate at Jabiru East of 49±14 Bq.m-2.y-1 is within the bound of the southern hemisphere average. Oenpelli’s annual deposition rate of 15±4 Bq.m-2.y-1 is low but still comparable with the southern hemisphere average and other values reported in the database. Annual 210Pb deposition at Jabiru East also compares well and is within bounds of the Australian depositional rates reported by Bonnyman et al. (1972) from which an average 56±4 Bq.m-2.y-1 is derived. Annual 210Pb at Jabiru East also compare with Darwin where an average 210Pb deposition rate of 90±9 Bq.m-2.y-1 was reported (Bonnyman et al. 1972). Over the six-year study period annual 210Pb deposition at Darwin varied between 60 Bq.m-2.y-1 to 195 Bq.m-2.y-1. The 2003-2004 wet season was higher than average with a total of 1765 mm of rainfall and 126 rainy days compared to the annual average of 1485.3 mm and 104 rainy days. While 210Pb is scavenged by precipitation the additional inundation in the region reduces 222Rn exhalation rates and the local source of 210Pb. This may explain the lower value of 210Pb deposition rates observed over the course of this study. For both locations approximately 90% of the annual 210Pb deposition occurs during the wet season. The duration and intensity of the wet season will be the major influencing factor in annual variations of 210Pb deposition in this region.

153 Table 6-3: Seasonal and annual 210Pb depositional rates and rainfall

Rate\Location Jabiru East+ Oenpelli#

Pb-210 Rainfall Pb-210 Rainfall deposition (mm) deposition (mm)

Wet Season 217±43 1765.4 65±10 1880.5 (mBq.m-2.day-1)

Dry Season 22±5 59.4 6±3 6.6 (mBq.m-2.day-1)

Annual Rate 49±14 1824.8 15±4 1887.1 (Bq.m-2.y-1) +Jabiru East wet season 1/10/03-18/5/04 #Oenpelli wet season 1/11/03-23/6/04 (included early dry season rains)

The low 210Pb deposition rates observed at Oenpelli has been attributed to the fact that Oenpelli is surrounded by floodplains during the wet season. Most deposition occurs from 222Rn exhaled during the wet season and at this site there is a lack of a local source term as billabongs swell and floodplains are inundated with water. A previous study in the region used sticky vinyl aligned horizontally and vertically to measure dry deposition rates of a number of uranium series radionuclides in the vicinity of Ranger (Pettersson and Koperski 1991). That work produced a model for determination of dry deposition rates at various distances from Ranger. Using the model for Jabiru East results in very large dry depositional rates (~500 Bq.m-2.y-1) leading to the conclusion that they may have encountered problems with the horizontal measurement technique. From his thesis, Pettersson provides an average dry deposition of 10 Bq.m-2.y-1 for all long-lived uranium series radionuclides at Jabiru East (Pettersson 1990) The 210Pb dry deposition rate at Jabiru East was determined to be 27 Bq.m-2.y-1 which was reported as being likely to be the upper limit (Pettersson and Koperski 1991). From the work presented here a 210Pb dry deposition rate of 8.0±1.8 Bq.m-2.y-1 and 2.2±1.1 Bq.m-2.y-1 are obtained for Jabiru East and Oenpelli respectively, the value for Jabiru East is comparable to that determined by Pettersson and Koperski (1991)

154 From the results presented in Figure 6.1 and Figure 6.2 the low 210Pb dry deposition rates indicate that dry season 210Pb atmospheric residency times are higher than values reported at other locations. Martin (2003) reported residency times between 0-70 days averaging 18 days. He continued by determining early and late wet season residency times with averages of 36 days and 8 days respectively. From Pettersson and Koperski (1991) a dry season 210Pb atmospheric residency time of 140 days is derived. Both these results imply that 210Pb does not settle easily during the dry season and that it remains suspended for many months or settles and resuspends. Pettersson and Koperski (1991) also derived 210Pb depositional velocities and showed that beyond 2 km from Ranger’s operational pit depositional velocities of 210Pb reduce to 0.25 cm.s-1, at Jabiru East 210Pb depositional velocity is 0.28 cm.s-1. Some dry deposition observed at Jabiru East is dust transported from Ranger but the techniques used in this study could not distinguish this from 210Pb originating from exhaled 222Rn decay in the atmosphere. Differences observed between the 210Pb deposition rates at Jabiru East and Oenpelli can be attributed to a number of factors. Firstly the proximity of Jabiru East to the Ranger operation means that a proportion of the 210Pb deposition there is a result of the mine operation. Oenpelli is closer to the coast than Jabiru so there is ocean air mixing, especially during the wet season when dominant winds come from the northwest. This explanation raises the question however why the reported value of 210Pb deposition at Darwin, of 90±9 Bq.m-2.yr-1 (Bonnyman et al. 1972), is higher than Jabiru East or Oenpelli. A more plausible explanation is that during the wet season the 210Pb source term for Jabiru East and Oenpelli is reduced as creeks, billabongs, floodplains and the soil fill with water retarding 222Rn exhalation. Throughout the wet season Oenpelli is only accessible by air as the East Alligator River swells, becoming impassable, the township itself is surrounded by floodplains. Jabiru East is further inland and there is less inundation of water in this region compared to Oenpelli. The ratio of dry to wet season 210Pb depositional rates for the two locations is approximately the same indicating that 222Rn and its progeny is evenly mixed over the region. Low annual 210Pb deposition rates in the region are most likely attributed to prevailing meteorological conditions. Complete budgeting of 210Pb and explanation of the dry season loss shall be examined in further detail in Chapter 7.

155 6.3.3 Pb-210 deposition summary The distinctive seasonality of 210Pb deposition is a result of the precipitation that occurs during the wet season. The exponential decrease in deposition rates throughout the wet season at Jabiru East is likely caused by a gradual reduction in the localised 210Pb source A similar reduction in source term is expected at Oenpelli but frequency of precipitation events retains 210Pb deposition rates at steady state values. Unseasonal rain events that occurred in May and June 2004 at Oenpelli kept 210Pb deposition rates at wet season values due to increased 210Pb atmospheric concentrations. Analysis shows that the dry season 210Pb atmospheric residency time is of the order of a few months. Compared to globally reported 210Pb deposition rates, Oenpelli is on the lower bound and Jabiru East is comparable. The 210Pb deposition rate at Jabiru East compares well to Australian and Darwin averages obtained from previous research. Air mixing, retarded wet season 222Rn exhalation rates, long 210Pb atmospheric residency times causing low dry deposition rates explain the low annual 210Pb deposition rates. Oenpelli’s location being surrounded by floodplains, results in low wet season 210Pb atmospheric concentrations and deposition rates. 6.4 Pb-210 excess in soil samples This section details the results from analysis of excess 210Pb from the soil cores and scrapes collected in the Jabiru region. The sampling locations have been specifically dealt with previously in Chapter 3 and the sampling techniques are covered in Chapter 4. All samples were measured on HPGe detectors and analysed using the GPEAK program. 6.4.1 Pb-210 inventories Results of excess 210Pb inventories from the sampling sites provided in Chapter 3 are provided in Table 6-4. This table provides excess 210Pb over 226Ra and 238U inventories to a depth of 10cm for all ambient sites. The exception is the non- irrigated Magela Land Application Area, where the inventory of excess 210Pb over 238U is not presented due to increased levels of 238U observed in the upper layers of soil that is speculated to be a result of horizontal transport of 238U from the irrigated zones. From the results presented in Table 6-4 a discrepancy is observed at the site west of the original tailings dam. This site is 200 m from the Ranger ore stockpiles

156 and has high levels of 238U, 226Ra and 210Pb in the surface layers of soil indicative of dust transport from the stockpiles. This dust deposition masks the excess 210Pb deposited as a result of 222Rn decay in the atmosphere. This site has not been included in determination of the regional average excess 210Pb inventory. To support this conclusion it is noted that the site to the south of the original tailings dam, which is also close to the ore stockpiles but not within any dominant wind sector, has 238U, 226Ra and 210Pb values similar to ambient sites. With the exception of the four sites; (i) western side of Barallil Creek; (ii) eastern side of Barallil Creek; (iii) west side of original tailings dam; and (iv) Georgetown billabong the remaining results closely match.

157 Table 6-4: Total inventories of excess 210Pb

Site Excess 210Pb over 226Ra Excess 210Pb over 238U (kBq.m-2) to 10 cm depth (kBq.m-2) to 10 cm depth

West of Old Tailings Dam -0.5±1.0 -7.4±1.3

Eastern Side Barallil 0.88±0.30 0.47±0.65 Creek

Western Side Barallil 7.9±0.9 3.2±1.8 Creek

Jabiru Township 1.8±0.3 1.8±0.9

Georgetown Billabong 4.5±0.6 1.0±1.1

Jabiru East 3.4±0.5 2.3±0.8

Side of Arnhem Highway 2.4±0.4 1.8±0.9

South original Tailings 2.4±0.5 1.1±0.8 Dam

West of Retention Pond 1 2.3±1.0 2.9±2.0

Non-Irrigated Magela

Land Application Area

Core 10 4.1±0.5 -

Core 13 2.0±0.4 -

Core TM2 1.9±0.8 -

Average (minus West 3.0±0.6 1.8±0.3 Tailings Dam

Average above (minus

Eastern Barallil Creek & 2.5±0.3 2.0±0.3 Georgetown)

158 The variations from average observed at the western side of Barallil Creek, eastern side of Barallil Creek and Georgetown billabong is explained due to the clay content of the soil. Potassium and its natural isotope 40K is an indicator of clay content of soil (Ivanovich and Harmon 1992). Clay has good absorption properties for fine particles and this influences 222Rn exhalation and excess 210Pb. Clay will trap 222Rn exhaled below it creating an of excess 210Pb and also deposited excess 210Pb moving through the soil will become trapped in clay soils. Georgetown billabong and the western side of Barallil Creek have high 40K inventories explaining the excess 210Pb inventories observed. The eastern side of Barallil Creek has a low 40K inventory indicating it is a sandy soil. It is speculated at this site the retention of fine particles is poor explaining the low excess 210Pb inventory here. A demonstration of the relationship between excess 210Pb and 40K is shown in Figure 6.3 where a positive correlation is observed. The average excess 210Pb inventories for the region, over 226Ra and 238U, are 3.0±0.6 kBq.m-2 and 1.8±0.3 kBq.m-2 respectively. The western side of Barallil Creek, eastern side of Barallil Creek and Georgetown billabong sites are noted as being outliers due to their clay content. However an alternative explanation for the excess 210Pb inventories differing at these sites may be the potential influence of fluvial processes. These three sites are all located near natural water bodies and fluvial movements may result in erosion or deposition of fine particles providing unreliable excess 210Pb inventories. Removal of these outliers changes the 210Pb inventories to 2.5±0.3 kBq.m-2 and 2.0±0.3 kBq.m-2 over 226Ra and 238U respectively Differences between sites and the difference between the excess 210Pb over 226Ra compared to excess 210Pb over 238U can be explained due to other influencing factors that will be examined in Chapter 7.

159 Eastern Side Barallil Creek Western Side Barallil Creek Jabiru Township Georgetown Billabong Jabiru East Side of Arnhem Highway South of original Tailings Dam West of Retention Pond 1 Core 10 Core 13

10 9

) 8 -2 7 6 2

Ra (kBq.m R = 0.59 5 226 4 3 Pb over

210 2 1 Excess Excess 0 0 5 10 15 20 40K (kBq.m-2)

Figure 6.3: Relationship between excess 210Pb and 40K

No relationship between excess 210Pb inventory and distance from the mine was observed, indicating that 222Rn exhaled from the mine is readily mixed in the atmosphere and distributed evenly across the region. As no samples were collected from locations further away from Ranger than Jabiru the data presented cannot be used to determine if any relationship exists between excess 210Pb inventories and distance from Ranger on a larger scale. No relationship between excess 210Pb inventory and the dominant dry season wind direction was observed. This project focused on sites that are downwind of Ranger during the dry season as these sites were readily accessible. Georgetown billabong, the non-irrigated Magela Land Application Area and south of the original tailings dam were all outside of the dominant dry season wind sectors from Ranger with Georgetown billabong and the Magela Land Application Area in the dominant wet season wind sector. None of these sites show large deviations from regional average excess 210Pb inventory with the exception of Georgetown billabong that has been discussed. While there are no previously reported values of excess 210Pb inventories for this region a comparison with work performed in Queensland, Australia and the

160 global database can be made (Akber et al. 2004a; Preiss et al. 2003; Pfitzner et al. 1993). Akber et al. (2004) presented an average excess 210Pb inventory of 2.4±0.5 kBq.m-2 for eight sites studied over Southeast Queensland while Pfitzner et al. (2004) reported an excess 210Pb inventory of 1.5±0.2 kBq.m-2 for sites around Townsville, North Queensland. Results reported here are in good agreement with both these studies. The global database contains results of 155 measurements from five continents, excess 210Pb inventories in soils range from 0.228±0.007 kBq.m-2 measured in Portugal to 14±0.04 kBq.m-2 recorded in Japan. The average global excess 210Pb inventory determined from the database is 3.7±0.2 kBq.m-2, also in close agreement with the results presented here. 6.4.2 Penetration half depth It is expected for undisturbed soils that the majority of excess 210Pb will lay within the top layers of the soil. Lead-210 will redistribute in soils due to diffusion, convection and bioturbation creating a depth profile. Depth profiles for the relative cumulative excess 210Pb inventory for the scrapes are presented in Figure 6.4 and for the cores in Figure 6.5. The samples from Jabiru East, eastern side of Barallil Creek, side of Arnhem highway and west of retention pond 1 all had large uncertainties associated with them and were excluded from analysis. This was due to a reduction in 210Pb inventories compared to 226Ra resulting in increased errors and reduced cumulative excess 210Pb inventories. This may have been due to the small cross sectional area of the corer and the collection of an unrepresentative portion of the larger than 2mm fraction. Georgetown billabong was also removed from analysis because of the large amounts of excess 210Pb in this sample. Due to cluttering of the graphs, as the depth points are the same for most samples in Figure 6.4 and Figure 6.5, error bars have only been displayed for one data series. They are representative of the errors associated with the other data series.

161 Eastern Side Barallil Creek Georgetown Billabong Jabiru East Side of Arnhem Highway South of original Tailings Dam 1.4

1.2

1.0 Pb Inventory

210 0.8

0.6

0.4

0.2

Relative Cumulative Excess Cumulative Excess Relative 0.0 024681012 Depth (cm)

Figure 6.4: Relative cumulative excess 210Pb versus depth for soil scrapes

Western Side Barallil Creek Jabiru Township South of original Tailings Dam Core 10 Core 13 TM2 Core

1.6

1.4

1.2

Pb Inventory 1.0 210

0.8

0.6

0.4

0.2

0.0 Relative Cumulative Excess Cumulative Excess Relative 0 5 10 15 20 Depth (cm)

Figure 6.5: Relative cumulative excess 210Pb versus depth for soil cores

162 The trendlines displayed in Figure 6.4 and Figure 6.5 are derived from a penetration half depth equation. The penetration half depth value was determined using the data-fit program Datafit v8.0, by Oakdale Engineering, from all the samples displayed. Penetration half depth is the depth at which the inventory is half the total inventory and has been previously reported for 7Be studies (Wallbrink and Murray 1996). It is provided by a solution of Equation 6-1 for τ:

⎡ ⎛ ln(2) × d ⎞⎤ Equation 6-1 I E = IT ⎢1− e⎜ ⎟⎥ ⎣ ⎝ τ ⎠⎦ Where d: is the depth (cm) 210 IE: is the relative cumulative excess Pb inventory at depth d 210 IT: is the total relative cumulative excess Pb inventory for the sample τ : is the penetration half depth (cm). From these samples a regional penetration half depth of 2.5±0.7 cm has been derived. There are currently no reported values for penetration half depth of excess 210Pb. However Akber et al. (2004) presented a value for excess 210Pb penetration half depth in Southeast Queensland of 3.6±0.3 cm at the SPERA 2004 conference. The results presented here are comparable with this value and differences are likely attributable to variations in soil types and methods of 210Pb redistribution in the soils. The regional penetration half depth of 2.5±0.7 cm means that 210Pb can be used for soil redistribution studies in this region. 6.4.3 Excess 210Pb summary With the removal of outliers due to the high clay content or fluvial processes the average excess 210Pb inventory is 2.5±0.3 kBq.m-2 and 2.0±0.3 kBq.m-2 over 226Ra and 238U respectively. The relationship between excess 210Pb and 40K, representative of clay content, justifies the removal of the outliers. The site to the west of the original tailings dam displayed high deposition of dust particles from the nearby stockpiles making excess 210Pb measurements here undeterminable. Other than the dust deposition at this one site there was no clear relationship between excess 210Pb inventories and distance from the mine site indicating that 222Rn transported from the mine is evenly mixed over the sampled area. The excess 210Pb inventories observed in this study compared well with results reported in Queensland, Australia and globally. Using a previously established calculation the penetration half depth for excess 210Pb in the region was determined as 2.5±0.7 cm, comparable to a value of 3.6±0.3 cm reported for Southeast Queensland.

163 6.5 Magela Land Application Area 6.5.1 Introduction It has been previously mentioned that excess water from retention pond 2 at Ranger is irrigated onto an area of land between the processing plant and Magela Creek, commonly known as the Magela Land Application Area. A section of this site, known as the experimental plot, was selected in 1988 for a study of this process. An irrigated section of the application area and the experimental plot were selected for an extensive 222Rn exhalation rate survey, the results of which have been presented in Chapter 5. A number of soil samples were taken from both areas to investigate the distribution of the 226Ra deposited and its influence on the 222Rn exhalation rate. It was shown in Chapter 5, that for both locations the effect of surface deposition has had little influence upon the 222Rn exhalation rate. While the 222Rn exhalation rates are enhanced at these locations the 222Rn exhalation rate to 226Ra activity concentration ratios are similar to ambient sites of the same geomorphic structure. 6.5.2 Uranium-238, 226Ra and 210Pb depth profile inventories Measurements of 226Ra activity concentration, using insitu gamma spectrometry at the irrigated Magela Land Application Area and the experimental plot was presented in Table 5.2. It was mentioned that these values were not representative for the area due to assumptions that the GS-512 makes in its calculations. Soil samples collected from the area provide a much better analysis of the 226Ra activity concentration, depth profile and distribution in the soil. The 226Ra and 210Pb inventory depth profiles for these samples are displayed in Figure 6.6 and Figure 6.7 while the 238U inventory depth profiles are displayed in Figure 6.8 and Figure 6.9. Comparisons to non-irrigated areas have been included for 226Ra and 210Pb.

164 2.0 2.0

Ra-226 Core2 (Irrigated MLAA) Ra-226 Average (Non-irrigated MLAA) Pb-210 Core2 (Irrigated MLAA) Pb-210 Average (Non-irrigated MLAA)

1.5 1.5 ) ) -1 -1 ).cm ).cm -2 -2

1.0 1.0 Inventory ((kBq.m Inventory ((kBq.m

0.5 0.5

0.0 0.0 0-5 5-10 10-15 15-20 0-5 5-10 10-15 15-20 Section (cm) Section (cm) Figure 6.6: Inventory depth profile for 2cm sectioned cores from irrigated TM1 and non-irrigated TM2

4.5 4.5

Ra-226 TM (Irrigated MLAA)1 Ra-226 TM2 (Non-irrigated MLAA) 4.0 4.0 Pb-210 TM1 (Irrigated MLAA) Pb-210 TM2 (Non-irrigated MLAA)

3.5 3.5

3.0 3.0

2.5 2.5

2.0 2.0

1.5 1.5 Inventory ((kBq.m-2).cm-1) Inventory ((kBq.m-2).cm-1)

1.0 1.0

0.5 0.5

0.0 0.0 0-2 2-4 4-6 6-8 8-10 10-15 15-20 0-2 2-4 4-6 6-8 8-10 10-15 15-20 Section (cm) Section (cm) Figure 6.7: Inventory depth profile for 5cm sectioned cores, irrigated (core 2) and averaged non-irrigated cores

165 80 U-238 Core TM1 (Irrigated MLAA) 70 )

-1 60 ).cm -2 50

40

30

U Inventory ((kBq.m 20 238

10

0 0-2 2-4 4-6 6-8 8-10 10-15 15-20 Section (cm)

Figure 6.8: U-238 inventory depth profile for irrigated core TM1 100 U-238 Core1 (Irrigated MLAA) U-238 Core2 (Irrigated MLAA) 90

80 ) -1 70 ).cm -2 60

50

40

30 U Inventory ((kBq.m Inventory U 238 20

10

0 0-5 5-10 10-15 15-20 Section (cm)

Figure 6.9: U-238 inventory depth profile for irrigated core 1 and core 2

166 Surface irrigation is evident in Figure 6.6 and Figure 6.7 in the 226Ra and 210Pb inventories compared with the non-irrigated counterparts. Deposited 226Ra and 210Pb is held in the top few centimetres of the soil, with Figure 6.6 indicating that it is held within the top 2 cm of the soil. As the irrigated water contains 226Ra greatly in excess over 210Pb a substantial fraction of the 210Pb seen in the top 2 cm is from ingrowth from 226Ra in the soil. This explains why the 210Pb is elevated in the surface soil but is not as high as 226Ra. The distribution of irrigated water across the area is not even, as seen by the large variations in radionuclide inventories in the top layers of the soil for the irrigated samples. This is due to the type of impulse sprinklers used for water application and obstruction of spray patterns by trees and shrubs (Akber and Marten 1991). While an exponential decrease is observed in Figure 6.7 and the inventories are higher than the non-irrigated area to a depth of 15 cm, below the first section inventories become comparable with other ambient sites. These results agree with findings from an earlier study into spray irrigation application of radionuclides at the area (Willett and Bond 1991). Willett and Bond (1991) noted that for all soil types 226Ra and 210Pb absorption should be within the top few centimetres of soil and that they would not easily remobilise after initial bonding. They also realised that remobilisation would most likely occur with 238U as seen from the depth profiles of Figure 6.8 and Figure 6.9. Willett and Bond (1991) showed that 238U absorption was lowest on soil classified as Unit III followed by Unit I, with Unit II soils retaining the greatest amount in the upper layers. Soil classification on the Magela Land Application Area was part of an earlier study (Chartres et al. 1988). Core 1 was taken from Unit II soil and core 2 was collected from Unit III soil. Core TM1 was collected from an area close to the boundary of these two soil types. While no accurate determination can be made for 238U penetration due to the small sample set it is estimated as being 6-8 cm for core TM1, 15-20 cm for core 1 and 5- 10 cm for core 2. Below these levels the 238U inventory is comparable to that of other ambient sites in the area. Willett and Bond (1991) estimated that the top 50 cm of soil should retain applied radionuclides for up to 100 seasons of application. Using an assumption of 1800mm.yr-1 of irrigation Willett and Bond (1991) also estimated that there should be no downward movement of 238U beyond 4 cm until the surface soils are brought to saturation after approximately 22 years of irrigation. From 1986-2000 there has been

167 an average of 964±221 mm.yr-1 of irrigated water applied to this area above normal rainfall averaging 1485.3 mm.yr-1. The results indicate that 238U mobilization has occurred beyond the estimated value. The retention of 226Ra and 210Pb in the top 2 cm demonstrates that the vertical movement of 238U is not primarily due to physical movement of the soil particles. It is also noted that two of the three non-irrigated sites had high 238U inventories in the surface layers of the soil indicating there has been horizontal mobilisation of 238U from the irrigated sections. The 6.5.3 Experimental plot inventories The experimental plot was the site of previous study for absorption of radionuclides and movement of ions (Bond and Willett 1991; Akber and Marten 1991). After the original intensive sampling program this area has been left relatively undisturbed other than the decommissioning of a nearby shed and a gamma dose rate survey (Storm and Martin 1995). During the course of this project the experimental plot was selected for an extensive 222Rn exhalation rate survey and two scrapes were collected from undisturbed locations on the plot. A soil core from previous work collected by J. Storm in 1994 was unprocessed and available at eriss, this core was prepared and analysed during the course of this project. The inventory depth profiles for 238U, 226Ra and 210Pb of the scrapes and the activity concentration depth profile of the core are displayed in Figure 6.10, Figure 6.11 and Figure 6.12 respectively. From the depth profiles of Figure 6.10 and Figure 6.11 it is seen that peak radionuclide activity occurs within the first two sections down to 2.5 cm. These figures show that there is an increase in activity compared to ambient sites down to the extent of the scrapes 5-10 cm. From the analysis of the core it is noted that increased activity is extends to the 6.7-11.4 cm section beyond which activities level out to ambient values. It is noted that 226Ra and 210Pb inventories for the experimental plot are more than an order of magnitude greater than those observed from the irrigated Magela Land Application Area while 238U inventories are comparable. This is because the concentrations of 210Pb and 226Ra applied to the experimental plot were one and two orders of magnitude greater, respectively, and 238U concentrations were twice as great compared to retention pond 2 water, (Akber and Marten 1991). The observed inventories presented in Figure 6.10 and Figure 6.11 is consistent with loads applied to the experimental plot and reported by Akber and Marten (1991).

168 80 U-238 Ra-226 Pb-210

70

60 ) -1

).cm 50 -2

40

30 Inventory ((kBq.m 20

10

0 0-1 1-2.5 2.5-5 5-10 Section (cm)

Figure 6.10: Inventory depth profile for scrape 1 from experimental plot 70 U-238 Ra-226 Pb-210 60

) 50 -1 ).cm -2 40

30

Inventory ((kBq.m 20

10

0 0-1 1-2.5 2.5-5 5-10 Section (cm)

Figure 6.11: Inventory depth profile for scrape 2 from the experimental plot

169 100000

U-238 Ra-226 Pb-210 10000 ) -1

1000

100 Activity Concentration (Bq.kg

10 0-1.9 1.9-6.7 6.7-11.4 11.4-16.2 16.2-20.9 20.9-25.7 25.7-30.4 30.4-35.2 35.2-38.9 Section (cm)

Figure 6.12: Activity concentration depth profile for core collected by J. Storm 1994 (fine grains, <2mm only)

The increase in 226Ra and 210Pb inventories observed to 10 cm at the experimental plot is a result of the saturation of absorption sites resulting in a downward movement of 226Ra and 210Pb. No more bonding in the upper layers of the soil occurs and these radionuclides, transported with soil moisture, penetrate deeper until they are absorbed. The 238U depth profile at the experimental plot is comparable with that observed at other irrigated sections. 6.5.4 Radium-226 and 210Pb distribution Previous work has reported that when irrigated the radionuclides 238U, 226Ra and 210Pb absorb to the fine grain fraction of soil (Akber and Marten 1991). This is expected as fine grains have better absorption properties due to their larger surface area to volume ratios compared with larger grains. This makes areas with coarse grains unsuitable locations for the surface deposition radionuclides. To demonstrate this Figure 6.13 displays the 226Ra <2mm inventory plotted against the 226Ra >2mm inventory for irrigated and non-irrigated sampling sites.

170 1000.0 Irrigated Soil Non-irrigated Soil

100.0 ) -2

10.0 Ra <2mm fraction (kBq.m Ra <2mm 226

1.0 1.0 10.0 100.0 226Ra >2mm fraction (kBq.m-2)

Figure 6.13: Distribution of 226Ra in soil fractions for 0-10cm

The continuous line shown in Figure 6.13 is the trendline for the non-irrigated samples which was the same as a line of equality indicating that 226Ra distribution in normal soils is even across the <2mm and >2mm fractions while all irrigated sites lie on the upper side of the line of equality. For the irrigated sites both fractions have higher 226Ra inventories compared with the non-irrigated sites. This is due to absorption of 226Ra onto the larger fraction but also shows that dry sieving does not remove the entire finer fraction from the larger grains. The two outliers seen in the upper right are the experimental plot scrapes where deposited 226Ra concentrations were much higher. The results agree with those previously reported, that 226Ra deposited from spray irrigation adheres to the finer grains of soil. Lead-210 also absorbs on finer grains and this is an attractive feature for a radionuclide to be used in soil transport studies, as finer grains are subject to erosion. The distribution plot for 210Pb is shown in Figure 6.14 where it is noted that it is primarily attached to the finer grains at most sites, the continuous line shown in this figure is a line of equality. This effect is observed at irrigated and non-irrigated sites due to the atmospheric deposition of excess 210Pb.

171 12.0 Irrigated Non-irrigated

10.0 ) -2

8.0

6.0

4.0 Pb <2mm Inventory (kBq.m 210 2.0

0.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 210Pb >2mm Inventory (kBq.m-2)

Figure 6.14: Distribution of 210Pb in soil fractions for 0-10 cm

6.5.5 Magela Land Application Area summary The irrigation of retention pond 2 water has occurred here since 1986 and extended to surrounding areas. It was introduced to provide Ranger a means of dealing with excess water loads. From the analysis presented it can be seen that the irrigation of water has an impact by increasing the 238U, 226Ra and 210Pb inventories compared to ambient soils. For 226Ra and 210Pb the increase is within the top few centimetres of the soil due to the soils capacity to absorb these radionuclides. Absorption of 238U was known to be different and original studies expected that 238U would penetrate deeper which has been shown in this analysis. Increased inventories and greater depth penetration of 226Ra and 210Pb at the experimental plot is a result of the high concentrations used at this site. This resulted in saturation of the upper soil layers and these radionuclides penetrated deeper into the soil before being absorbed. In agreement with other reports it was shown the absorption of 226Ra and 210Pb is primarily on fine grains. For 226Ra this was evident only at irrigated sites but for 210Pb it was evident at all locations due to the atmospheric deposition of excess 210Pb.

172 6.6 Chapter summary The results presented in this chapter are essential for determination of the regional 210Pb budget performed in Chapter 7. Measurements of annual 210Pb deposition rates and excess 210Pb in the soils around the Jabiru region also provides a picture of the natural cycle of the 238U series from 222Rn exhalation to 210Pb deposition and its transport through the soil. It has been shown that 210Pb deposition rates measured between May 2003 and May 2004 for Jabiru East and Oenpelli are low compared to globally reported values. Partially this is due to low dry season 210Pb deposition when 222Rn exhalation is at a maximum resulting in a net loss of 210Pb from the region. At the onset of the wet season at Jabiru East initially high deposition rates are observed that exponentially decrease as rain slowly retards the local 210Pb source. Oenpelli displays a more steady state 210Pb depositional rate over the wet season with the exception of the April 2004 peak. This is due to a balance between the source term and scavenging events that result in an even distribution. Overall Oenpelli’s low annual deposition rate is due to dilution of 210Pb from air mixing and a lack of 222Rn exhalation during the wet season as the area is surrounded by inundated floodplains. From a previous study by Pettersson and Koperski (1991) and observations dry season 210Pb atmospheric residency times are determined to be of the order of a few months (140 days). This agrees with the upper value of residency times reported by Martin (2003) of 70 days determined for an early wet season rain event. Excess 210Pb inventory was measurable at most sampled sites. At some locations high clay content, fluvial processes or dust deposition from Ranger stockpiles resulted in high and undeterminable values respectively. Average excess 210Pb inventories for the region agree well with values reported for Queensland, Australia and globally. The penetration half depth was found to be 2.5±0.7 cm which agrees well with a result reported for Southeast Queensland. The Magela Land Application Area was subject to an intensive 222Rn exhalation rate survey and soil collection to analyse radionuclide deposition resultant from spray irrigation. It was observed that 226Ra and 210Pb was retained within the top few centimetres of the soil while 238U penetrated deeper. Absorption predominantly occurs on the finer grain soil fraction for 226Ra at the irrigated region and for 210Pb at all locations. Horizontal transport of 238U from the irrigated section

173 to a non-irrigated section is evident and vertical transport of 238U is greater than originally expected.

174 7

Lead-210 budget

7.1 Introduction Radon-222 exhalation gives birth to 210Pb that attaches to aerosols. Its behaviour in the environment through transport and deposition provides an insight into the behaviour of the aerosols it is attached to. The objective here is to extend the knowledge of 222Rn exhalation, 210Pb deposition and excess 210Pb inventories to examine this behaviour. The aim of this chapter is to make comparisons between estimated and observed values of 222Rn exhalation rates, 210Pb deposition rates and excess 210Pb inventories and also to examine the relationship between them. Explanation for the annual loss of 210Pb from the region is also provided. Budgeting 210Pb is performed firstly on local scale of the Jabiru region then on a broader scale over Kakadu. In order to perform this some justifiable assumptions have been made and reasoning behind these is provided. However given the half-life of 210Pb, long atmospheric residency time determined from the previous chapter and large annual variations in meteorological conditions in the region it must be noted that inventory calculations determined from deposition measurements over 1 year are less useful. 7.2 Hadley circulation It is speculated on the basis of the results that the reason for the net loss of 210Pb during the dry season is a result of it being transported away from the region by the dominant dry season winds. These winds are created by an atmospheric circulation pattern known as the Hadley circulation pattern. Explanation of this circulation pattern is presented here and a model is shown in Figure 7.1. This circulation pattern was proposed by a British physicist George Hadley (1685-1768) in 1735 to explain global air circulation patterns. Hot, moist air from equatorial regions rises into the tropopause losing moisture as it rises thus creating convection rain events. The now dry air is cycled towards the pole but descends at the mid-latitudes where the global desert bands lies creating high pressure cells. The descending air then moves back towards the equator, which is at low pressure. Rotation of the earth creates the Coriolis effect that turns the southerly wind into

175 south easterly and easterly wind. Hadley circulation is further complicated by the tilting of the earth on its axis and by landmasses. 7.3 Local area 210Pb budget A large number of measurements were performed around the Jabiru area. These include; 222Rn exhalation rates from ambient sites of Jabiru East, Jabiru water tower and the non-irrigated Magela Land Application Area; 210Pb deposition rates from Jabiru East; and excess 210Pb inventory measurements from 11 ambient sites. For this local area it is possible to perform a budget of 210Pb. Lead-210 depositional rate measurements were performed over one seasonal cycle from May 2003 to May 2004 at Oenpelli however no additional data, such as measurements of 222Rn exhalation rates or excess 210Pb inventories were taken so it is not possible to perform 210Pb budgeting for the Oenpelli region. It has been reported that 210Pb deposition rates can vary annually by a factor of three (Bonnyman and Molina-Ramos 1974; Beks et al. 1998; Baskaran 1995). This complicates budgeting estimates and comparisons, as a long-term 210Pb depositional analysis for the region is not available.

Figure 7.1: Global Hadley circulation model (curtesy Australian Bureau of Meteorology)

176 7.3.1 Fate of Ranger 222Rn Measured 222Rn and 222Rn progeny mine related contributions at Jabiru East are 14% and 10% respectively and at Jabiru 5% and 15% respectively (Whittlestone 1992). The results for Jabiru East compare well with another measurement study that reported the mine related contribution of 222Rn atmospheric concentrations there to be 20% (Akber and Pfitzner 1994). Model based estimates for Jabiru East and Jabiru reported annual mine contributions of 222Rn atmospheric concentration are 17% and 14% respectively (Akber et al. 1993). This compares well with an earlier model that estimated the mine related component of exposure to 222Rn progeny at Jabiru to be approximately 16% of the annual average (Kvasnicka 1990). The information provided shows that despite being a strong point 222Rn source term, 222Rn from Ranger is readily dispersed in the local region and makes a small contribution to the local 222Rn atmospheric concentrations, 210Pb deposition and excess 210Pb inventories. The incremental additional 210Pb contribution to the local area is likely to be within the experimental uncertainties reported through this thesis. 7.3.2 Determination of 222Rn exhalation rates from 210Pb deposition and excess 210Pb inventories Assuming that 222Rn atomic exhalation to the environment is equal to the 210Pb atomic deposition in the Jabiru area, the data presented in Table 6-1 and Table 6-2 can be used to estimate 222Rn exhalation rates. Measured and estimated seasonal and annual 222Rn exhalation rates for Jabiru East are shown in. Measured seasonal 222Rn exhalation rates in Table 7-1 for Jabiru East exclude the transitional months of October and April. The 222Rn exhalation rates for these months are non-indicative of the seasonal value due to variations in soil moisture and the influence it has on 222Rn exhalation. However the transitional months October and April are included in determination of the wet season 222Rn exhalation rates from the 210Pb depositional measurements as the scavenging events that occur in these months cannot be ignored. Measured 222Rn exhalation rates are higher than those predicted on the basis of 210Pb deposition. The discrepancy, of a factor of two during the wet season, increases to a factor of twenty in the dry season. On an annual basis it results in a factor of seven higher for the measured exhalation rates over the estimated value. Lack of scavenging events throughout the dry season results in atmospheric

177 residency times of the order of months and it is speculated that locally produced 210Pb transports large distances away from the region resulting in a net loss of 210Pb. The annual 222Rn exhalation rate estimated from the 210Pb inventories agrees well with the annual value derived from the 210Pb deposition rates but is slightly higher. This is explained due to the natural redistribution of 210Pb in the soil that will be discussed and also as the 2003-2004 wet season was above average, with 1760 mm recorded for a year to the start of May 2004 compared with the annual average of 1485.3 mm. There were 126 rainy days in the year up to the start of May 2004 compared to an annual average of 104. It is speculated that this increased precipitation over this wet season has reduced the regional 222Rn exhalation and 210Pb source and resulted in a lower estimate of 222Rn exhalation rates compared to that derived from excess 210Pb inventories. An explanation for the regional net loss during the dry season comes from 210Pb attached to aerosols being transported from the region. During the dry season regional surface winds are easterly to south easterly, known as trade winds, created by the Hadley circulation cell and Coriolis effect Kakadu is located between 12-14oS and receives the full influence of the easterly to south easterly trade winds. Lead-210 attached to aerosols from the Kakadu region is transported by this wind to the ocean, where it is likely scavenged by more frequent precipitation events and mixed with 210Pb deficient air. This air is lifted into the upper atmosphere where it is further diluted before being swept back towards the surface. The wet season value estimated for 222Rn exhalation rates from the 210Pb deposition rate at Jabiru East is comparable and within the statistical bounds of the observed value. The uncertainties can be attributed a number of phenomena. The measurements were performed over different wet seasons so variations in precipitation and soil moisture and the effect that this has upon 222Rn exhalation rates explains some uncertainty. Also 210Pb is deposited evenly across the region while 222Rn only exhales from land not inundated with water. Uncertainty also lies with the fact that 222Rn exhalation rate measurements were discrete, between one and several days a month, while the 210Pb deposition values are determined as an average of a continuous monthly measurement. There was also a small number of 222Rn exhalation measurement sites compared with the size of the region.

178 Table 7-1: Measured and estimated seasonal and annual 222Rn exhalation rates for Jabiru East

222Rn exhalation rate (mBq.m-2.s-1)

Measured (Jul Estimated from Estimated from 02-Jun 03) 210Pb deposition excess 210Pb (May 03-May 04) inventory

Wet season 23 5.9±0.4 - 126 * average

Dry season average 48 1.8±0.1 - 3830 *

Annual average 32 3.7±0.4 5.3±0.6 2723

* Excludes October and April as transitional months

7.3.3 Determination of excess 210Pb inventories from 210Pb deposition From the observed 210Pb deposition rates estimates of expected excess 210Pb inventories can be made. The excess component of 210Pb in the surface soil is obtained by averaging deposition over the mean lifetime (τ) of 32 years. Using the annual deposition rate reported in Table 6-1 to represent averaged annual loadings of 210Pb to the surface soils for Jabiru East and Oenpelli measured and estimated inventories are shown in Table 7-2. For Jabiru East and the Jabiru region the values observed compare well with the estimations. Differences can be attributed to the physical factors affecting 226Ra, 222Rn and 210Pb transport through the soil. A model of the natural process of 210Pb redistribution is shown in Figure 7.2. It has been previously mentioned that 226Ra can be leached down or removed with water infiltration. To demonstrate this effect occurring in the region a plot of 226Ra against 238U is provided in Figure 7.3 where the continuous line represents a state of equilibrium. The majority of values lie below this line of equilibrium. Removal of 226Ra is insufficient to explain the total excess 210Pb inventory observed but it in part explains why the excess 210Pb over 226Ra inventory derived from the soil samples is higher than those predicted from 210Pb deposition.

179

Table 7-2: Estimated and observed excess 210Pb inventories

Jabiru East Jabiru Region Oenpelli

Measured Inventory 3.4±0.5 2.5±0.3 - over 226Ra (kBq.m-2)

Measured Inventory 2.3±0.8 2.0±0.3 - over 238U (kBq.m-2)

Estimated Inventory 1.6±0.5 - 0.48±0.13 (kBq.m-2)

Radium-226 loss can account for the difference between the excess 210Pb inventory over 226Ra and that over 238U. Another contributing factor lies with the fact that not all 222Rn diffusing through the soil exhales into the atmosphere. Some becomes trapped in the upper layers of the soil and in the water above it where it decays. This is most likely to be predominant during the wet season when precipitation has a capping effect upon 222Rn exhalation. More compact surface soil than the soil underneath can lead to a similar effect. This results in more excess 210Pb in the surface layers of the soil than can be attributed to atmospheric deposition and this is also shown in Table 7-2. Two outliers were removed for the purpose of Figure 7.3. The sites lay between the irrigated section and the Magela Creek boundary fence. The data points lay far to the right of the plot and the high 238U observed in the surface layers is indicative of horizontal transport of 238U from the irrigated section of the Magela Land Application Area.

180

Figure 7.2: Natural redistribution of 210Pb

Eastern Side Barallil Creek Western Side Barallil Creek Jabiru Township

Georgetown Billabong Jabiru East Side of Arnhem Highway

South of original Tailings Dam West of Retention Pond 1 Core 10 (Non-Irrigated MLAA)

8.0 )

-2 6.0

4.0

Ra Inventroy (kBq.m Ra 2.0 226

0.0 0.0 2.0 4.0 6.0 8.0 10.0 12.0 238U Inventory (kBq.m-2)

Figure 7.3: Radium-226 versus 238U to 10 cm to demonstrate 226Ra/238U disequilibrium

181 7.3.4 Determination of 210Pb deposition and inventories from 222Rn exhalation rates With the net loss of 210Pb from the region estimation of annual 210Pb deposition rates from the 222Rn exhalation rate is difficult. It is possible to provide an estimation of the wet season 210Pb deposition rates from the observed 222Rn exhalation rates. This estimation is based upon wet season 222Rn exhalation between November and March. The January 222Rn exhalation anomaly, if existing for a site, has been removed from the analysis. To determine the contribution of dry season deposition to the excess 210Pb inventory, given the dry season loss of 222Rn, the measured dry season deposition is used. For determination of the excess 210Pb inventory wet season deposition rates extend from October to March. Contribution of the transitional month of April is considered to be comparable to the dry season. The results for estimated wet season 210Pb deposition rate and total excess 210Pb inventories are provided in Table 7-3 Approximately 28% of Kakadu is inundated with water during the wet season (Santos-Gonzalez et al. 2002). Water inundation is greater near the coast at the mouth of the rivers where the floodplain coverage is greater. For the Jabiru region an approximation of 20% inundation is used to determine the values in the final column of Table 7-3. Using this approximation a more accurate determination of 210Pb deposition rates and excess 210Pb inventories from 222Rn exhalation rates is obtained. While excess 210Pb inventories are similar to those observed they are higher because of the assumptions made and do not take into account the observed removal of 226Ra or capping of 222Rn that both result in additional excess 210Pb in the surface soil. Other uncertainties are again attributed to the fact that measurements were performed over different wet seasons and 222Rn exhalation rate measurements are not truly indicative of the real 222Rn exhalation rates, as they were discrete and limited in area.

182 Table 7-3: Estimated 210Pb deposition rate and excess 210Pb inventory

Jabiru East Jabiru Jabiru region region (inc. inundation)#

Wet season 210Pb Estimated* 487±167 390±83 312±66 deposition rate + (mBq.m-2.day-1) Measured 212±14 - -

Estimated from 3.0±0.2 2.4±0.2 1.9±0.2 exhalation* 210 Pb inventory (kBq.m-2) Estimated from 1.6±0.5 - - deposition+

Measured 3.4±0.5 2.5±0.3 2.5±0.3

* November 2002 to March 2003 + October 2003 to March 2004 # 20% inundation assumed

7.3.5 Local area 210Pb budget summary The results presented in this section show there is a good comparison between 222Rn exhalation rates, 210Pb deposition rates and excess 210Pb inventories for the wet season on a local scale. The fact that 222Rn exhalation measurements were performed in the wet season previous to the 210Pb deposition measurements explains some of the discrepancies between the results. At the onset of the project it was planned that 222Rn exhalation rate and 210Pb deposition rate measurements would be performed simultaneously. A delay in the construction of the 210Pb deposition collectors resulted in this not being possible. Local water inundation is a possible explanation for why estimated 222Rn exhalation rates are lower and estimated 210Pb deposition rates and excess 210Pb inventories are higher than the observed values. Some mixing with 210Pb deficient air is also likely to occur during the wet season with predominant winds coming from the ocean across the floodplains. Discrete measurements of 222Rn are also not truly indicative of the monthly average when compared to averaged continuous measurements of 210Pb deposition. Small areas of measurement compared to the relatively large area that the model is applied to will also create discrepancies.

183 A variation between excess 210Pb inventories obtained from measured soil samples and those derived from depositional measurements occurs because of two phenomena. First water acts as a cap on 222Rn exhalation trapping it in the upper layers of the soil where it decays. Secondly 222Ra can be removed the soil with water infiltration. These phenomena create additional excess 210Pb, more than can be accounted for by deposition. Overall it is observed that there is a net loss of 210Pb from the region and it has been speculated that this is due to a much broader effect caused by the trade winds created by a Hadley Circulation cell. Removed 210Pb is likely deposited over the equatorial regions to the north west of Kakadu. From the data presented it is estimated that for Jabiru East there is a net loss of 210Pb of 350±75 Bq.m-2.yr-1 while 49±14 Bq.m-2.yr-1 was deposited from May 2003 to April 2004. Approximately 78% of this loss occurs during the dry season while 90% of the deposition occurs during the wet season. 7.4 Regional 210Pb budget Using the results presented in this thesis with other information available for the region it is possible to make estimates of Kakadu’s 222Rn exhalation rates. The value κ presented in Chapter 5, for determination of 222Rn exhalation rates from 226Ra activity concentrations, is used to estimate dry season 222Rn exhalation rates across Kakadu. Deriving a wet season κ from a limited dataset the wet season 222Rn exhalation rate for Kakadu can also be derived. This is used to estimate the 210Pb depositional rate and excess 210Pb inventory for Kakadu as a whole. 7.4.1 Kakadu dry season 222Rn emission Kakadu National Park covers 19,800 km2 and has been classified into its geomorphic landscapes in a previous study (Lowry and Knox 2002). A map adapted from Lowry and Knox (2002) is shown as Figure 7.4. Chapter 5 distinguished four geomorphic landscapes and a value (κ) for three of them that could be used to estimate 222Rn exhalation rates from 226Ra activity concentrations. Ambient values of 226Ra activity concentrations from measurements performed in this project are combined with ambient values provided from other previous work performed in the region and are used as an estimate for total averaged ambient 226Ra activity concentrations (Todd et

184 al. 1998). The following assumptions have also been made to determine the dry season exhalation rates that are presented in Table 7-4: − Eroded Koolpinyah, Koolpinyah and dissected foothill surfaces are considered to be vegetated; − Coastal and alluvial floodplains are considered to be fine grained material; − Plateau is modelled as 50% the value of non-compacted boulders, this accounts for the solid matrix; − The 226Ra activity concentration of the plateau is the average of the 226Ra activity concentration from all sites (the surface soils in the region originate from erosion of the plateau over millions of years); − The amount of water coverage in the dry season is considered negligible (perhaps up to 10% but within the bounds of uncertainties); − The area of barren sites is considered to be negligible.

185 132oE 133oE 12oS

13oS

14oS

Koolpinyah Surface – 17% Dissected Foothills – 7%

Alluvial Floodplains – 7% Plateau – 17%

Coastal Floodplains – 11% Eroded Koolpinyah Surface– 41% Kakadu Boundary

Figure 7.4: Geomorphic landscapes of the Kakadu region (Adapted from Lowry and Knox (2002))

186 Table 7-4: Dry season 222Rn emission from Kakadu National Park

Geomorphic landscape Area (km2) Dry Season 222Rn Emission (Bq.s-1)

Koolpinyah Surface 3,366 (1.2±0.2)*108

Dissected Foothills 1,386 (4.7±0.6)*107

Eroded Koolpinyah Surface 8,118 (2.8±0.4)*108

Alluvial Floodplains 1,386 (1.1±0.2)*108

Coastal Floodplains 2,178 (1.8±0.4)*108

Plateau 3,366 (9.9±5.0)*106

Kakadu Total 19,800 (7.4±0.6)*108

Hadley Circulation cells, previously discussed, create easterly and south easterly trade winds during the winter months, dry season, through Kakadu. It was observed that the dry season 210Pb deposition rate at Jabiru East was twenty times lower than could be accounted for from the source term. The dry season 222Rn source from Kakadu is assumed to suffer the same fate that is observed on the local scale in the Jabiru region. It is speculated that this will be similar at all tropical locations that experience dry seasons with dominant winds created by Hadley Circulation. It has been shown in section 7.2.1 that Ranger operations represented approximately (0.8±0.1)% of the 222Rn emission from the Kakadu region. From the estimate provided in Table 7-4 again using and the averaged value of 7.04 MBq.s-1 for Ranger’s dry season 222Rn emission it is redetermined that Ranger’s 222Rn emission represents (0.95±0.08)% of the total emission from Kakadu. The two estimates are comparable and discrepancies attributed to the assumptions made in determination of the 222Rn emission for Kakadu.

187 7.4.2 Kakadu wet season 210Pb budget From a smaller dataset covering only several locations a wet season κ is determined for the geomorphic landscapes shown in Table 7-5. The small data set meant that regression analysis was not possible and the value κ presented is the average of the readings for each geomorphic landscape. An additional geomorphic landscape is included in Table 7-5 as observations at the seasonal site of Mirray hillock noted it had a different wet season 222Rn exhalation rate compared to other vegetated areas. This is likely a result of water transport downhill from the site reducing the influence of moisture on 222Rn exhalation rates. Water inundation during the wet season varies the 222Rn exhalation rate as observed in the seasonal results of Chapter 5. Currently the exact amount of wetlands within Kakadu is under debate because of the imagery tools used. To date the best estimate is that approximately 28% of the region is either swamp or land subject to inundation (Santos-Gonzalez et al. 2002). This compares well to the 18% classified as coastal or alluvial floodplains in Figure 7.4 and the additional 10% is attributed to the addition of swamps in the latter report.

Table 7-5: Wet season kappa for various geomorphic landscapes

Geomorphic Landscape Average (κ) (mBq.m-2.s-1/Bq.kg-1)

Barren 0.13±0.09

Vegetated 0.20±0.11

Foothills 0.57±0.20

188

Figure 7.5: Wetland area of Kakadu (Santos-Gonzalez et al. 2002)

189 The wet season 222Rn emission from Kakadu is determined using the information in Table 7-5, a wet season inundation of 28%, and the following assumptions: − Floodplains have no 222Rn exhalation during the wet season; − Eroded Koolpinyah and Koolpinyah surfaces are considered to be vegetated; − The additional 10% water inundation is over eroded Koolpinyah and Koolpinyah surfaces; − The area of barren sites is considered to be negligible; − Plateau is modelled as 25% the value of dry season non-compacted boulders, this accounts for the solid matrix and decrease in 222Rn exhalation due to water coverage; − The 226Ra activity concentration of the plateau is the average of the 226Ra activity concentration from all sites (the surface soils in the region originate from erosion of the plateau over millions of years). The wet season 222Rn emission source term from Kakadu is presented in Table 7-6.

Table 7-6: Wet season 222Rn emission from Kakadu National Park

Geomorphic landscape Area (km2) Wet Season 222Rn Emission (Bq.s-1)

Koolpinyah Surface 3,366 (3.5±0.8)*107

Dissected Foothills 1,386 (3.7±1.4)*107

Eroded Koolpinyah 8,118 (8.4±2.0)*107 Surface

Alluvial Floodplains 1,386 0

Coastal Floodplains 2,178 0

Plateau 3,366 (5.0±2.5)*106

Kakadu Total 19,800 (1.6±0.3)*108

190 Table 7-7: Estimated Kakadu region 222Rn exhalation rate, wet season 210Pb deposition rate and total excess 210Pb inventory

Estimated average wet Estimated wet season Estimated total excess season 222Rn exhalation 210Pb deposition rate 210Pb inventory (kBq.m-2) rate (mBq.m-2.s-1) (mBq.m-2.day-1)

8.1±1.3 334±53 2.1±0.2

From the results presented in Table 7-6 this it was possible to determine the Kakadu regional wet season 210Pb deposition rate and excess 210Pb inventory using a similar approach as used in section 7.2.4, these results are presented in Table 7-7.The results presented in Table 7-7 agree well with the estimates determined for the Jabiru region presented previously in this chapter and the measured results presented in Chapter 5 and Chapter 6. 7.5 Chapter Summary This chapter focused on the relationship between 222Rn exhalation, 210Pb deposition and excess 210Pb inventories. Using the data collected throughout the project it is observed that there is a net loss of 210Pb from the region. The majority of this loss occurs during the dry season. The lack of scavenging events and trade winds created by a Hadley Circulation cell transports 210Pb attached to aerosols away from the region where it is likely deposited over the ocean and land masses to the north west of the Northern Territory. The results of Chapter 5 and Chapter 6 allowed for determination of estimates of 222Rn exhalation rates, 210Pb deposition rates and excess 210Pb inventories to be compared with the observed values. While there is substantial loss of 210Pb, the majority during the dry season, 90% of 210Pb deposition occurs during the wet season. When 210Pb redistribution, regional water inundation, measurement times and methodology are considered the estimations compare very well to the measured values. There is a loss of 350±75 Bq.m-2.yr-1 of 210Pb from the Jabiru region, 78% of which occurs in the dry season. For Kakadu a dry season 222Rn emission of (7.4±0.6)*108 Bq.s-1 is derived from estimations using justifiable assumptions. This shows that the Ranger source term, although a strong point source is negligible on a broader scale. Kakadu’s 222Rn emission during the wet season is reduced to (1.6±0.3)*106 Bq.s-1 when water

191 inundation and reduction in 222Rn exhalation rates due to soil moisture are accounted for. Regional estimated 210Pb deposition rates for the wet season and total excess 210Pb inventories for Kakadu compare well with the values measured in the Jabiru region.

192 8

Conclusions and future directions

8.1 Project outcomes From a study of the literature soil moisture is the most dominant variable that influences 222Rn exhalation rates. The location of the project made it possible to observe the influence of other variables, such as 226Ra activity concentrations, grain size and soil porosity for various geomorphic landscapes, by performing dry season measurements. These measurements showed that similar geomorphic landscapes 222 226 have similar Rn exhalation rate to Ra activity concentration ratios (RE-R). Regression analysis on the dataset of dry season measurements was performed to obtain a value (κ) for three of the four geomorphic landscapes identified. These coefficients (κ) can be applied to determine dry season 222Rn exhalation rates for tropical locations from a measurement of the 226Ra activity concentration. In general compacted ground had decreased 222Rn exhalation rates while root structures, associated with vegetation, creates more porous soil resulting in higher exhalation rates. Several diurnal measurements of 222Rn and 220Rn exhalation rates were performed, some towards the end of the wet season and others during the dry season. Even though there are diurnal variations in soil temperature and atmospheric pressure, which both influence 222Rn and 220Rn exhalation rates and there are diurnal variations in 222Rn atmospheric concentrations no diurnal variations in the exhalation rates were observed. Diurnal variations, if any, are most likely masked by normal fluctuations in exhalation of these isotopes. Results presented here confirmed and agree with a previous study in the region that also reported no diurnal variations. Distinctive seasonality of 222Rn exhalation rates was observed at the eight seasonal sites. With the onset of the wet season increased soil moisture retarded 222Rn exhalation rates. Variations in the magnitude of retardation between sites was due to the porosity of the local ground where more porous sites allow moisture to be drawn away so after the first few weeks of rain 222Rn exhalation was not greatly affected. Localised variations in 222Rn exhalation at a site occur during the wet season due to uneven soil moisture distributions. A peak in 222Rn exhalation rates

193 observed at some sites during January 2003 is most likely due to an uneven moisture profiles and evaporation that results in a release of trapped 222Rn. While the 222Rn exhalation rate measurements were discrete they are representative of the seasonal variations for the region. Variations in soil moisture were observed through measurement of soil moisture profiles. Dry season soil moisture profiles varied minimally as small amounts of moisture were removed but large variations throughout the wet season were a result of the intensity, intervals and duration of precipitation events. It is expected there are large variations in 222Rn exhalation rates over short periods of time due to these variations in soil moisture. Distinct seasonality in 210Pb deposition rates is observed with 90% of the deposition occurring during the wet season. Jabiru East displayed an exponential decrease in the 210Pb deposition rates throughout the wet season while Oenpelli maintained a relative steady state value. Observations at Jabiru East are explained that there is a gradual reduction in the localised source term throughout the wet season. At Oenpelli the effect on the 210Pb source term is likely to differ but scavenging events are occurring at frequencies that result in a steady level of 210Pb deposition. The lower wet season 210Pb deposition rates at Oenpelli compared to Jabiru East are because of its location, surrounded by water inundated floodplains that significantly reduce the local 210Pb source term. Compared to globally reported values Jabiru East and Oenpelli have low annual 210Pb depositional rates. In part this is due to the net loss of 210Pb from the region that occurs during the dry season. Lead-210 attached to aerosols is drawn away with the winter easterly to south easterly trade winds that result from a Hadley Circulation cell. Lead-210 is transported from the region towards equatorial zones where it is most likely deposited over the ocean and landmasses to the north west of the Kakadu region. However the 210Pb deposition rates at both sites are within the bounds of globally reported values, particularly southern hemisphere values. Jabiru East is within bounds of the Australian and southern hemisphere averages and compares well with values previously reported for Darwin. Excess 210Pb inventories were measured in most of the soil samples collected at 14 sites around Jabiru and Ranger. With the exception a few samples that returned anomalous values due to dust deposition from Ranger stockpiles or high clay content in the soils, excess 210Pb inventories over the region were relatively even. This indicates that 222Rn and its progeny is dispersed evenly and well mixed over the

194 region within a few kilometres of a strong point source. Also as the majority of 210Pb deposition occurs during the wet season the retardation of 222Rn exhalation reduces the contribution from Ranger and makes it difficult to distinguish from the ambient signal. As such there was no clear relationship between excess 210Pb inventories and distance from the Ranger operation for the sampling locations. Vertical transport of 210Pb in the soil was observed through analysis to determine the penetration half depth of 2.5±0.7 cm. This was similar to a value reported for Southeast Queensland. Using a disused section of the Magela Land Application Area an intensive study of 222Rn exhalation, 226Ra, 210Pb and 238U deposition was performed. From previous studies it was expected that retention of 226Ra and 210Pb would be within the top few centimetres of the soil and 238U penetration should be deeper. Radon-222 exhalation rates over the area, while enhanced, reflected the expectation that 226Ra is absorbed in the top few centimetres. Analysis of soil samples collected in the area confirmed this. Absorption of irrigated 226Ra over at the Magela Land Application Area was predominantly on the finer grain material as was absorption of the naturally deposited excess 210Pb at all locations. Adherence to finer grains for surface deposited radionuclides is due to better ionic bonding and larger surface area to volume ratios of small grains compared to larger grains. Uranium-238 transport at the site differed from what was estimated at the onset of the irrigation program. Analysis of non-irrigated samples close to the irrigated areas indicates there has been horizontal transport of 238U from the irrigated area. By combining the data presented in Chapter 5 and Chapter 6 the relationship between 222Rn exhalation rates, 210Pb deposition rates and excess 210Pb inventories is observed. Lead-210 budget was first performed on a local scale for the Jabiru region, where wet season estimations agreed well with the measured values when factors such as 210Pb redistribution, water inundation, measurement times and methodology was considered. Dry season estimates of 222Rn exhalation rates were twenty times lower than the observed values due to the net loss of 210Pb from the region. The defined geomorphic landscapes of Kakadu were categorised to match the geomorphic landscapes identified in this project. Hills were added as an additional geomorphic landscape for the wet season. Using the coefficient (κ) it was possible to obtain wet and dry season estimates of the 222Rn emission source term for the Kakadu region of (7.4±0.6)*108 Bq.s-1 and (1.6±0.3)*106 Bq.s-1 respectively. From

195 the wet season emission it was possible to estimate the average 210Pb deposition rate and excess 210Pb inventory for Kakadu, these estimations agreed very well with the values observed on a local scale around Jabiru. The lack of 210Pb scavenging events during the dry season results in long atmospheric residency times calculated from previous work to be of the order of a few months (140 days). This results in a net loss of 210Pb from the region that is a result of it being transported to equatorial regions with the south east trade winds caused by a Hadley Circulation cell. This loss has made complete 210Pb budgeting for the region beyond the scope of this project. The quantity of the loss observed, 350±75 Bq.m-2.yr-1, was an unexpected result. The objectives outlined in Chapter 1 have been met and a number of additional objectives were achieved. Primarily the major project outcomes were: − Investigating the principal contributing meteorological, geographical & geological factors that affect the exhalation of 222Rn and deposition of 210Pb; − Measurement of the seasonal variations in 222Rn exhalation rates from several sites; − Measurement of the seasonal variations in 210Pb deposition rates in wet and dry deposition; − Study the transport of 210Pb through the surface layers of the soil through the measurement of radionuclides in soil samples; − Modelling the 210Pb budget in the Kakadu region. Additionally the following outcomes were also achieved: − Measurement of the dry season 222Rn exhalation rates from numerous geomorphic landscapes at Ranger Uranium Mine; − Derivation of a value (κ) to determine 222Rn exhalation rates from measured 226Ra activity concentration for four geomorphic landscapes applicable to dry tropical locations; − Measurement of diurnal variations of 222Rn and 220Rn exhalation rates for the region; − Determination of the dry season removal mechanism of 210Pb from the region;

196 − Investigation of 238U, 226Ra and 210Pb retention at the Magela Land Application Area; − Modelling of the 210Pb budget on a local scale and determination of the net loss of 210Pb. 8.2 Future directions This study has drawn to a close but has given rise to a number of potential future research projects. No relationship between ore grade and 222Rn exhalation rates for the measurements performed at the Waste Rock Dump, grade 2 and grade 7 ore stockpiles was observed. On the basis of diffusion theory it was stated that diffusion lengths of 222Rn in stockpile structures differed substantially from normal ground and are of the order of tens of metres compared to 1-2 m in normal soils. Only three ore stockpiles at Ranger were measured and other than observation and personal communications no direct measurement of the stockpile heights were recorded. For rehabilitation purposes and the determination of radiological impact future studies should investigate this matter further to determine whether 222Rn diffusion lengths are greater in stockpiles structure or whether the results presented here are due to other variables. While it was possible to determine the 222Rn exhalation rate to 226Ra activity concentration ratios (RE-R) for four geomorphic structures results for non-compacted boulders were derived from a small dataset and are relatively inconclusive. The lack of information for 222Rn exhalation rates and 226Ra activity concentrations for the plateau region meant that some justifiable assumptions were required to determine the 210Pb budget for Kakadu. Future studies should add to the dataset for better determination of RE-R ratios and κ values as well as expanding on the classification of geomorphic landscapes. Measurements of soil moisture during this project were discrete, performed once a month when 222Rn exhalation measurements were performed. Comprehensive analysis of a single site throughout the wet season with frequent measurement of 222Rn exhalation rates and soil moisture would provide a clearer view of the effect of soil moisture. Furthermore experimental laboratory procedures could be developed to investigate the speculated evaporative effect that is believed to have given rise to the 222Rn exhalation anomaly in January 2003.

197 Averaged monthly 210Pb deposition rates provide a clear picture for seasonal measurements but lack the determination of individual rain event contributions. A previous study investigated a number of precipitation events and provided good analysis of the 210Pb contribution of these events. A more holistic approach would be to perform measurements of 210Pb deposition more frequently over a wet season. Either a daily or weekly measurement schedule throughout a wet season would allow for better determination of the individual contribution of precipitation events. Observations should include the intensity, duration and type of precipitation events for complete analysis. If performed in conjunction with a strict 222Rn exhalation rate program for a site as previously discussed a much clearer determination of the wet season 210Pb budget could be obtained. The net loss of 210Pb from the region will most likely be observed on a broader scale. If it were possible to perform measurements of 210Pb deposition rates from sites such as Melville or Tiwi Island or as far as East Timor and Indonesia an answer to the 210Pb loss might be obtained. Soil measurement analysis of samples taken from the Magela Land Application Area non-irrigated area returned elevated levels of 238U in the upper levels of the soil. This indicates that there has been some horizontal transport of 238U from the irrigated sections. Transport of 238U at the Magela Land Application Area has been noted by eriss who have marked it as a priority research project. When complete the results of this project will provide a more detailed analysis of the transport of 238U from the irrigated sections than what has been provided here. 8.3 Conclusions At the onset of this project it was noted that there was a lack of information regarding 222Rn exhalation and 210Pb deposition from tropical locations. The results of this project have increased the body of knowledge for the natural cycle of 222Rn exhalation, 210Pb deposition and its transport through the soil. This information was obtained from the tropical location of Kakadu National Park, Australia. The dry season allowed for the influence that soil moisture has on 222Rn exhalation to be removed as a variable so a better estimate of the other influencing factors could be determined Only a small number of studies have attempted to combine and analyse 222Rn exhalation rates and 210Pb deposition rates at a location. Analysis has found there is a

198 net loss of 210Pb from the region and confirmed that precipitation is the major scavenging process of 210Pb from the atmosphere. With determination of the atmospheric residency times this has further increased knowledge of the behaviour of 210Pb, and the aerosols it attaches to, in the atmosphere. The measured excess 210Pb inventory in the surface soils and a penetration half depth of 2.5±0.7 cm is good for use as a tracer in erosion studies providing a good baseline can be established. The project has generated sufficient information for future studies to be developed from the findings.

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