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 210 and Pb 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 176 222 7.3.1 Fate of Ranger Rn 177 222 210 7.3.2 Determination of Rn exhalation rates from Pb deposition 210 and excess Pb inventories 177 210 210 7.3.3 Determination of