Report: LURI 1972-08 Ls*Cs in the REINDEER LICHEN CLADONIA

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Report: LURI 1972-08

ls*Cs IN THE REINDEER LICHEN CLADONIA ALPESTRIS; DEPOSITION, RETENTION AND INTERNAL DISTRIBUTION, 1961 - 1970

L J Sören Mattsson

I August, 1972

Radiation Physics Department, University of Lund, Läsaretttt, S-221 85 Lund, Sweden CONTENTS Page

ABSTRACT 1

1. INTRODUCTION 2

2. COLLECTION AND PREPARATION OF SAMPLES 5

3. METHODS OF MEASUREMENT 10

4. RESULTS AND DISCUSSION 12

4.1 The l37Cs-content in the unfractionated lichen-carpet 12 4.2 Vertical distribution of I37Cs in the lichen-carpet and in underlying layers of gelatinous matter, humus and soil 18 4.3 Mathematical description of the vertical distribution of 137Cs 22 4.4 Factors affecting the vertical distribution of 137Cs 27 4.5 Mechanisms of retention and transport for 1S7Cs in lichen 29 4.6 137Cs/'°Sr-activity ratio in different parts of the lichen-carpet and in underlying material 36 4.7 Estimated effects of reindeer-grazing 37

5. CONCLUSION 39

6. ACKNOWLEDGEMENTS '.. 40

REFERENCES 41 lsTCs IN THE REINDEER LICHEN CLADONIA ALPESTRIS; DEPOSITION> RETENTION AND INTERNAL DISTRIBUTION. 1961 - 1970

L J SÖREN MATTSSON

Radiation Physics Department, University of Lund Lasarettet, S-221 85 Lund, Sweden

ABSTRACT

The accumulation of the fallout radionuclide 137Cs has been studied in a large number of samples of the important reindeer-lichen Cladonia alpestris, collected in the Lake Rogen area (62.3°N, 12.1°E) in central Sweden during the period 1961 - 1970. The maximum area-content of l37 Cs in the lichen-carpet occurred in 196 5 and 1966, and amounted to U6 - 66 nCi m"2, which corresponds to a dry-mass content of 3U - 52 nCi kg"1, due to the special characteristics of the lichen-carpet and of the locality itself. In Septem- ber, 1970, the I37Cs-levels for the whole lichen-carpet were 50 - 80% of the values obtained in 1965. The vertical distribution of 137Cs has been studied during 1966 - 1970, and the results indicate the existence of a short-time internal cycling of the radionuclide in the lichen-carpet, so that in dry periods certain amounts of 137Cs are forced up to the top regions of the carpet, and in periods of high precipitation washed down to its lower layers. The concentration of lf7Cs at various depths recorded in carefully field-fractionated samples of lichen, gelatinous matter, humus and soil collected in the "normal" years 1969 and 1970 follows the empirically found relation: c = constant • exp(-1.6 z°'75), where c denotes the 137Cs-concentration in dry matter (nCi kg"1) at the dry maés depth z (kg m~2). For comparison with the ls?Cs-results, the vertical distribution of two other alkali metals, potassium and sodium have also been studied. According to increasing penetration of the lichen-carpet, the elements can be arranged in the following sequence: l37Csvx» K << Na. Indications of the presence of undissolved particles of radioactive fallout in the lichen-carpet are presented and discussed. The principal ways for elimination of 1S7Cs from undisturbed carpets of Cladonia alpestris are physical decay of the radionuclide and the continuous growth of the lichen plants, the latter combined with a continuous decomposition of older material.

1. INTRODUCTION

During the last ten years increasing attention has been paid to the accumulation and radiological health aspects of long-lived fallout radionuclides in man living in arctic and subarctic regions. The highest concentrations of most fallout radionuclides are here associated with the food-chain: atmospheric deposition - lichen - reindeer - man. Lichens, being duplicate organisms composed of fungi and algae in a kind of symbiosis, effectively accumulate and retain most of the deposited fallout products, and thus act as a reservoir of radioactivity. During th« winter period of about six months, there is very little fresh vegetation, and the reindeer therefore graze on lichen. Reindeer-meat, which is an important component of the Lapps' diet, thus contains a higher level of some fallout-products than normal diet. Future levels of such products in reindeer and man will depend prin- cipally upon the retention-performance of lichen.

Very soon after the beginning of the large-scale nuclear- weapons tests in 1956, lichens and mosses were found to accumulate fallout-radionuclides more effectively than vascular plants (BORMAN et al, 1958; GORHAM, 1958; GORHAM, 1959; HVINDEN and LILLEGRAVEN, 1961; LIDEN, 1961; PAAKKOLA and MIETTINEN, 1963; RICKARD et al, 1965; WATSON et al, 1966). The distribution and translocation of radionuclides from fallout in natural populations of plants and animals was also investigated at this early stage (LJUNGGREN, 1959; LJUNGGREN, 1960; MIETTINEN et al, 1961; DAVIS et al, 1963; OSBURN, 1963). After the discovery of the uniquely high body-burdens of 1>7Cs in Swedish Lapps (LIDEN, 1961), more extensive analyses of the radionuclide-content of lichen and other vegetation eaten by reindeer (caribou) were carried out in several arctic and subarctic areas (SALO and MIETTINEN, 1964; SVENSSON änd LIDEN, 1965 b; HÄSÄNEN and MIETTINEN, 1966). The radionuclide accumulation in different species of moss was also studied and served as a comparison for the lichen studies (SVENSSON and LIDEN, 1965 a; SVENSSON, 1967), Extensive reports on the contamination of lichen, reindeer and man, have later been published from Alaska, Scandinavia and Northern USSR (MIETTINEN et al, 1963; HANSON, 1967 a, 1967 b; HANSON et al, 1967; KAURANEN and MIETTINEN, 1967, 1969; LIDEN and GUSTAFSSON, 1967; MIETTINEN, 1967; MIETTINEN and HÄSÄNEN, 1967; NEVSTRUEVA et al, 1967; RAMZAEV et al, 1967, 1969; PERSSON, 1968, 1969, 1970 a, 1970 b, 1972 a; HANSON and EBERHARDT, 1969; JAAKKOLA, 1969; AARKROG and LIPPERT, 1971; MATTSSON, 1972 a; MATTSSON"and PERSSON, 1972).

The aim of the present investigation has been to study the behaviour of 1J7Cs in one important reindeer-lichen, Cladonia alpestris (L.) Rabenh., and in the underlying layers of gelatinous material, humus and soil. The main reason for the study is still the importance of lichen as the first link in a food-chain, which ends in man. It is also of interest to study the lichen-carpet in its possible role as a useful bioindicator for radioactive pollution originating from nuclear weapons and also from nuclear installations such as reactors and fuel-reprocessing plants. Moreover, the lichen-carpet and the material just below the lichen constitute a source of external irradia- tion of man and animals. It is furthermore of interest to know the absorbed dose to the lichen-vegetation itself. Results from studies of the processes of retention, transport and re-circulation of radionuclides can also be a useful contribution to the study of other analogous pollution problems (WOODWELL, 1969). 2. COLLECTION AND PREPARATION OF SAMPLES

Plots for sampling which are suitable for long-term studies of radionuclide-retention must be located in a plane» horizontal area outside the crown projection of trees. The plots must be large enough to enable collections of samples over a period of several years. It is also necessary that they are completely covered with lichen of the species of interest, and not interspersed with other vegetation. The plots must not be affected by reindeer-grazing or other animal or human activities. This work has been concentrated on an extensive investigation in a district with extremely good conditions för long-term studies of lichen. This district consists of the vicinity around, and the islands of Lake Rogen in central Sweden (62.3°N, 12.H°E). The carpets of Cladonia alpestris in this area are extremely plentiful, and the undisturbed carpets have a mean thickness of about 12 cm. The age of the lichen-carpet and the gelatinous layer has been estimated to be more than 100 a by SKUNCKE (SKUNCKE, 1963), who also participated in the initial planning of the lichen-sampling program. In this area we have used four sampling-plots according to the map shown in Figure 1, and the description of the plots in Table 1. We have used the well standardized sampling-technique developed in the beginning of the program for investigation of radionuclide- accumulation in reindeer-lichen and forest-moss (SVENSSON and LIDEN, 1965 a; LIDEN, 1969). Each sample was collected in an area of 0.25 a2, carefully narked with an aluminium frame. All procedure» for collection and separation were N 12,4°E

\\ Rogtn 3 (-710 m) *

% • Rogtn 2 b \ (~7Mm) »2,3°N *

* Rog^n 2a

•

Rog«n 1 (-900 m)

10 km

FIGURE 1 Map showing the different sampling-places in the Rogen area. carried out when the lichen was damp from natural humidity, because it is very fragile in dry state. At the start of the program all lichen down to the gelatinous layer was collected in one fraction. Minor contributions of leaves, etc.» were not excluded. To obtain a reproducible collecting* procedure in this way is not as easy as it would appear, mainly due to the fact that dead parts of the plants gradually decay and unite with the wet gelatinous layer. TAB:E

Plot- Altitude; Mean Notes identi- m above annual fication sea-level precipita (Name) tion; mm

Rogen 1 900 665xx) Open mountain area, (Gråstöten) above the tree-line. Very small traces of reindeer-grazing in the surroundings of the plots. x) Rogen 2a 760 580 Southern.part of a (Storholmen) small isolated island in Lake Rogen. Soli- tary pines. No visible disturbances. x) Rogen 2b ^760 580 Northern part of the (Storholmen) island. No visible disturbances. x) Rogen 3 780 580 Open pine-forest with lichens. Visible reindeer-grazing, but ob- viously disturbed areas were avoided at sample-collection. x) Measured at the meteorological station Myskelåsen, 11 km ENE from the sampling-plots. xx) Estimated from information on the variation of annual precipitation at the altitude in this area (SWEDISH METEOROLOGICAL AND HYDROLOGICAL INSTITUTE, 1971).

Depending on individual sampling-techniques, varying humidity, etc., different amounts of partly decomposed lichen-material are taken, and this influences both the measured activity per unit of mass and unit of area.

The collections were very carefully carried out in the light of all these difficulties.

In connection with the more penetrating studies of the 8

lichen in the Rogen area, samples of the gelatinous layer, humus and soil were also collected, and the lichen-carpet itself was divided into different fractions. The method used to fractionate the lichen- carpet and underlying material was as follows (compare with Figure 2):

Fraction A: upper 3 cm of the lichen-carpet (0-3 cm), normally eaten by reindeer

11 B: middle 3 cm of the lichen-carpet (3-6 cm)

11 C: lower part of the lichen-carpet, normally

around 6 cm (6-12 cm)

w U: unidentified parts of the lichen-carpet

w L: leaves, etc., from other vegetation " Jelly: gelatinous layer, decomposed lichen- material, varying thickness, normally 1-4 cm

" Humus: zone of fermentation and layers of humus; distinct humus-zones were not observed in all the plots

11 Soil: organic-mineral soil and mineral soil; in some cases, the fraction of soil was separated in different subfractions.

Figure 2, which illustrates the method of fractionation, gives the depth in the lichen-carpet, expressed both in a linear scale and as dry mass per unit area as found for samples fractionated in the field at Rogen 1, 2a and 2b in September, 1969, and September, 1970. Lin—r pth Dry mast d+oth cm — — — —.kgnfg— _*. 0

0.63

humus < so»

FIGURE 2 Profile >f the lichen-carpet and underlying gelatinous matter, humus and soil. The method of fractionation is illustrated. The values for the dry mass depth, which are indicated, refer to mean values for samples from Rogen 1, 2a and 2b, which were field-fractionated in 1969 and 1970.

Some of the lichen-material collected in 1966 was separated in its different components in the laboratory several months after collection. In the meantime, the material was stored in a refrigerator (0-«*°C). In 1967 and 1968, the material 10

was prepared in the laboratory immediately after collection. In 1969, parts of the material were fractionated in the field, and the remainder in the laboratory, with no significant differences in the l37Cs-results. In 1970, all the collected samples were fractionated in their different components in the field during sampling. It was possible to exclude the unidentified parts of the lichen-carpet (the fraction U) at fractionation in the field. The different samples were then, in the laboratory, individually dried to constant weight at 105°C for 18 hours, ground in a mill and mixed to a homogeneous powder.

3. METHODS OF MEASUREMENT

After preparation the samples were put in a cylindrical perspex can with an inner diameter of 110 mm and an inner height of 82, 36, 26 or 13 mm, depending on the amount of material available. The containers were closely fixed to a 12.5 cm (diameter) x 10.0 cm NaKTl)-crystal, which was housed in a lead shield and coupled to a 200 channel pulse- height analyser. An electronic spectrum stabilizer (DUDLEY and SCARPATETTI, 196U) was used to guarantee good long- term stability of the spectrometer. The full-energy-peak- efficiency for the 662 keV line from 137Bam was determined from measurements on samples containing known amounts of lf7Cs, homogeneously mixed with sawdust of the same den- sity as the samples of interest. The !37 Cs-solutions of known activity were obtained from IAEA, Vienna, and The Radiochwnical Centre, Amersham, England and were reported 11 to have an overall uncertainty (ICRU, 1968) of 2 and 3%, respectively, in the given activity concentration value. Analysis of the recorded pulse height distributions was performed by the method of least squares, using automatic data-treatment (SALMON, 1961; SVENSSON, 1967). The pulse height distributions have in this way been analysed in respect to six gamma-emitting radionuclides, normally

155 lt>% l37 I25 l06 5% Eu, Cef Cs, Sb, Ru and Mn. As an alter- native method, the *37Cs-content was determined by manual calculation of the net counting-rate in the energy region 610-710 keV, corrected for minor contributions from the interfering radionuclides.

In our laboratory, the first experiments with Ge(Li)- detectors for low activity measurements wer* performed in 1967 (MATTSSON, 1967). Later on, more and more of the measurements were performed with Ge(Li)-spectrometers, and the NaKTD-spectrometers are now only used for a rapid control-analysis of the 137Cs-content cf the lichen- samples. The introduction of Ge(Li)-spectrometers has re- sulted in a considerable improvement of the possibilities of identifying the yemitting radionuclides and of getting proper determinations of the amounts of other gamma- emitting radionuclides than 1 7Cs in environmental samples (MATTSSON, 1972 b). The determination of 1S7Cs in samples of soil by means of NaKTD-spectrometers is difficult, mainly because of the presence of the 22fRa decay product 21*Bi, which emits S09 keV photons, disturbing the 662 keV- peak from II7Cs. By means of a Ge(Li)-spectrometer, these two peaks are well separated. All results concerning humus- 12

and soil-fractions given in this paper have been measured by a 50 cm3 Ge(Li)-spectrometer.

Results for gamma-emitting radionuclides other than l37Cs, studied by the Ge(Li)-spectrometer, will be published si- multaneously with this paper (MATTSSON, 197 2 b).

Determination of the contents of stable potassium and stable sodium in the samples was carried out by means of flame-spectrophotometry after the ashed (H00-U50°C)

samples had been dissolved in HC1 and HN03.

RESULTS AND DISCUSSION

U.1 The !37Cs-content in the unfractionated lichen- carpet

Published results on the ll7Cs-content in lichen normally refer to the entire lichen-carpet and are often reported as activity per dry mass unit (nCi kg"1), and only occa- sionally as activity per unit area (nCi m"2). The 137Cs- activity concentrations in dry lichen from different countries in the arctic and subarctic regions have been calculated from published data and are given in Figure 3. The vertical lines in this figure indicate the range of mean values of the *i7Cs-content in lichen (Cladonia alpestris, Gladonia silvatica and Stereocaulon paschale) from twelve sampling-plots uniformly distributed all over 13

7Cs contont in dry Ifchon nCi kg"1 I I I I IN N N P^Rango off moan-valuos 70 1 for 1122 Swedish local»*» N

4«

ALASKA (Ogoturuk Crook) (Anaktuvuk Pass) o FINLAND • GREENLAND If - NORWAY A U.S.S.R. (Murmansk, Komi) • SWEDEN (Rogofi 1) \ o ( » 2a) >Mhiswork High yiotd explosions • »i ( »i 3) J Low »• i« I I I B B BB 1151 »SI 1 11H# IN I 1N4I 1 INf INfl Wfl Yoar

FIGURE 3 Previously published data on the l37Cs- content in dry lichen (nCi kg"1) from different countries (AARKROG and LIPPERT, 1971; HANSON, 1972; HANSON et al, 1967; HVINDEN and LILLEGRAVEN, 1961; LIDEN and GUSTAFSSON, 1967; MIETTINEN and HÄSÄNEN, 1967; RAMZAEV et al, 1967, 1969; RICKARD et al, 1965; WATSON et al, 1966), together with the Swedish data from this work. The uncertainty limits indica-fre 1 S.E. Nuclear weapons-tests, which have been performed in the atmosphere of the Northern Hemi- sphere are indicated on the abscissa.

Sweden (LIDEN and GUSTAFSSON, 1967; SVENSSON and'LIDEN, 1971). The lS7Cs-content in Cladonia alpestris-carpets, as measured in the present work, are shown in the same 1»*

figure for comparison. These three curves will later on

be discussed in more detail. As can be seen from the

figure, there is a large spread of the concentration-

values reported from different regions. It is evident

that information on radioactivity-concentrations in

lichen is of little general value if no detailed in-

formation on species, the mass of lichen per unit area,

annual precipitation and exact geographical location

are given. Only the Alaskan and the Swedish data fulfil

most of these requirements.

Figure 4 gives the measured area-content of I37Cs in the

entire lichen-carpet (down to the layer of gelatinous

matter ("jelly")) from three different places in the

Rogen area. Maximum values were observed in 196 5 and

1966. Clear differences in the rate of decrease of the

137Cs-content after 1966 are recorded for the different

lichen-carpets, so that retention is most effective at

Rogen 2a, somewhat less effective at Rogen 1 and con-

siderably lower in the comparatively thin lichen-carpet

at Rogen 3. It has not been possible to make direct

measurements of the deposition-rate of 137Cs in the

Rogen area due to its remote location. Therefore, the

total accumulated deposition at various times has been

calculated from reported data on the mean deposition-

rate of l37Cs for Sweden (LINDBLOM, 1969; BERNSTRÖM, if' 1969, 1971). The relative mean deposition-values calcu-

lated in this way at different times have been normalized

to the total deposition measured at the different sampling-

plots by summing-up the area-content of l37Cs in the dif- 15

Area content of WC» nCi nr* •t 71 Accumulated ft mCs-deposition: St Ropen i^--^ 4t OTC» in the Cladonia alpestris carpet: Rogen 1

is i i ft 71 Accumulated ft M

4f Ropen 2a

3t Ropen 3 mC* in the Oadonia alpestris carpet: 2t

IS 1t«1 I 1M2 I 1W3 I 1H4 I INS! If M I Iff7 I 1H§ I If ft I 1t7l

FIGURE »• The ll7Cs area-content of the Cladonia alpestris-carpet at the sampling-places Rogen 1 (undisturbed lichen, around 2.1 kg dry weight per m2), Rogen 2a (undisturbed lichen, around 1.9 kg dry weight per m2) and Rogen 3 (partly reindeer-grazed lichen, around 0.8 kg dry weight per m2). The estimated total accumulated ll7Cs-depositions at the different sampling-places are also given in the figure* The figures labelling the points give the number of samples studied. The uncertainty indicated is 1 S.E. of the mean. ferent layers of lichen, Hjelly", humus and soil down to 10 ca below the "jelly11 -layer. The values for accu- 16

mulated l37Cs-depositions thus obtained for the different plots at various times are also given in the figure. The measured ratio between total deposition of IS7Cs at Rogen 1 and at Rogen 2a (Rogen 3) is 1.18. According to Table 1, the estimated ratio between the mean annual precipitation at the same plots is 1.15, which would indicate that the differences in deposition most probably depend on the differences in the amount of precipitation, assuming a linear relationship between l37Cs-deposition and the amount of precipitation.

A comparison between the estimated accumulated deposition and measured area-content of l37Cs in the lichen- carpet clearly illustrates the ability of the lichen- carpet to accumulate 137Cs from wet and dry deposition.

In earlier studies concerning radionuclides in lichen, the lichen-carpet has been considered as one compartment from which elimination is proportional to the amount present in the compartment. When using such a first order transport model for the entire lichen-carpet, various half-times for ls7Cs in the lichen-carpet (Cladonia species) have been reported by different authors, according to Table 2. The reported values for the effective half-time differ highly, from around 3 a to around 11 a. The explanation for these great variations may lie in differences in lichen- biomass, amount of precipitation, type of species studied, growing-rate, etc. TABLE 2

Estimated half-time for 137Cs in carpets of lichen species from measurements under field-conditions

Species Part of Locality Time-period Biological Effective Reference lichen- for the study half-time half-time carpet a studied

Cladonia Sweden +1 LIDEN and GUS- whole 1961-65 17±4 11 silvatica (Gällivare) -2 TAFSSON, 1967 MIETTINEN and whole Finland 1964-65 7-14 6-10 HÄSÄNEN, 1967 Cladonia Alaska alpestris, 0-4 a after X) x) HANSON and EBER- Cladonia whole (Anaktuvuk application 6.7 5.5 HARDT, 1971 rangiferina Pass) top part USSR xx) XX ) NIZHNIKOV et al, eaten by 2.7 2.5 reindeer (far North) 1971

Alaska MARTIN and KORANDA, Cladonia whole (alpine 1967-70 7.5-9 6-7 areas) 1971

Alaska MARTIN and KORANDA, Cladonia whole (coastal 1967-70 3-4 2.7-3.5 areas) 1971 x) Tracer-experiment with 137Cs applied in single drops to individual podetia. xx) Value based on measurements of reindeer-flesh. 18

In order to describe the experimental data in Figure 4 by a first order transport model it is necessary to apply a varying effective half-time from around 30 a during the period 1962-1964 to 8 a for Rogen 1, 1»» a for Rogen 2a and k.5 a for Rogen 3 during the time period 1968-1970. The higher elimination-rate of 137Cs

from the lichen-carpet at Rogen 1 (Teff = 8 a) than for

the very similar "carpet at Rogen 2a (Teff = 14 a) may be a result cf the higher precipitation at Rogen 1 by about 15%. A much more rapid elimination-rate at Rogen 3 than at Rogen 2a can be expected due to the considerably smaller biomass of the lichen-carpet at Rogen 3 (0.8 kg m"2) than at Rogen 2a (1.9 kg m"2).

The results reported above show that the elimination of 1S7Cs from a lichen-carpet is too complex to be exactly described by means of a simple first order transport model if the time period for the study and the biomass of the lichen-carpet are not given. This has also been confirmed by data on the elimination-rate of I37Cs from an artificially contaminated Alaskan lichen-community (HANSON and EBERHARDT, 1971). The measured effective half-time of I37Cs became shorter and shorter the longer the measurements were made after the application of *'7 Cs to the carpet.

i*.2 Vertical distribution of 1>7Cs in the lichen- carpet and in underlying layers of gelatinous matter, humus and soil

Investigations of the vertical distribution of ll7Cs in 19 lichen (mainly Cladonia species) have on a lesser scale been carried out in Scandinavia (PAAKKOLA and MIETTINEN, 1963; LIDEN, 1969) Northern USSR (NEVSTRUEVA et al, 1967 i NIZHNIKOV et al, 1969) and recently in Central Europe (KREUZER and SCHAUER, 1972) and USA (PLUMMER, 1969; HANSON and EBERHARDT, 1971; RITCHIE et al, 1971). The results of these investigations are very scanty and also difficult to compare, mainly because of the different methods used to fractionate the lichen-carpets and the lichen-plants, and the different manners of presenta- tion of results. However, all the field-studies report decreasing concentrations of deposited 137Cs in the deeper levels of the lichen-carpet. The vertical distributions of other fallout-radionuclides, such as 90Sr, 2l0Pb, etc, in the lichen-carpet have also been reported (PAAKKOLA and MIETTINEN, 1963; PERSSON, 1970 b, 1971, 1972; HANSON and EBERHARDT, 19 71).

Figure 5 shows the mean 13 7Cs-concentration in the different layers A, B and C of the lichen-carpets at Rogen 1 and Rogen 2a during the period 1966-1970. The figure shows the maximal concentration in the top layer of the carpet, and a decreasing concentration with increasing depth during the entire period of time studied. Figure 5 also indicates a correlation between vertical distribution and amount of precipitation during the months preceeding collection. Low precipitation gives more concentration in the top layers than high precipitation; high precipitation gives more concentration in the bottom layers than low precipitation. This phenomenon may indi- 20

mC»-confnt In dry lichti nCiktf' 1M

FIGURE 5 Dry mass-concentration (nCi kg"1) of l37Cs in the different layers A (0-3 cm), B (3-6 cm) and C (6-12 cm) of the Cladonia alpestris-carpet at the sampling places Rogen 1 (filled symbols) and Rogen 2a (open symbols) during the period 1966-197 0. The histogram at the bottom of the figure shows the amount of quarterly precipitation at Rogen 2a. The precipitation-rate at Rogen 1 is higher by a factor of around 1.15

cate that some l37Cs follows the upward net transport of water during periods when precipitation is low and evaporation is high and that this l37Cs is washed down during periods of high precipitation.

In order to study the vertical distribution of 137Cs more carefully, the carpet of Cladonia alpestris and its underlying layers of gelatinous matter, humus and soil were carefully field-fractionated in immediate connection with the collection procedure in 1969 and 1970, so that the fraction of unidentified parts of the plants could be 21

"TC>-contirt in dry material nCi ho1 m r r 11 1 1—1 1 1 1 11 1—i—i—| i 111 1 1—i i i irf T— a

so - — •Mt itfv Rogtti 1 o • 2t - » 2a D • •• 2b * 4A • III ! 1 • 5 1

2 - - o o o 4 l

IS — > 1 I

12 - -

t.t9 —

A 1 I

0J2 - - U 3 t tt 16 * LinMrdtpth AM ••PI -

- , . . i . .. .1 • . . i i 11 il i i i i i • 11 ti H2 U II 21 M 1t 2t St Dry m— dtpth kg nf» l37 FIGURE 6 Vertical distribution of Cs in undisturbed carpets of Cladonia alpestris and underlying material from the Rogen area (62.3°N, 12.U°E) in Septemberp , 1969, and September, 1970. The careful separation of the different components was made in the field in immediate connection with the sampling-procedure. excluded. The vertical distribution of ll7Cs in the different layers is presented in Figure 6. The mean depth of the layer studied is here expressed in terms of dry mass per unit area (kg n"a) instead of linear depth (cm), 22

which has been traditionally used. The use of dry mass depth as a parameter gives a more reproducible vertical distribution from place-to-place and from time to time, mainly because of the difficulties of making a reproducible linear length-separation of the plants and because the radionuclide-concentration at a given point is shown to be better correlated to the mass, depth of the point than to the linear depth. This is shown in Figure 6. The figure also shows that the decrease of 137Cs-concentration with depth is continuous, without any significant steps at the interfaces between such different materials as living lichen and dead lichen, dead lichen and humus, humus and soil, etc.

4.3 Mathematical description of the vertical distribution of 137Cs

From a simple passive transport model it can be expected that the amount of 137Cs retained at a given depth in the lichen-carpet will be related to the dry mass depth z (kg m"2) of this layer as expressed by the equation

c = o e"$ z (1)

where c is the concentration of 137Cs in a layer at dry mass depth z and where a and $ are constants.

If the vertical distribution of l37Cs can be explained by pure diffusion from a thin layer at the top of the carpet, the l37Cs-concentration could be related to the mast-depth z by means of the equation -B z' 23 c = a e (2) where a and B are constants.

There is always a possibility for an active uptake of

137Cs in living lichen. It is impossible to predict how such a process influences the vertical distribution of the radionuclide unless one has information on the metabolic processes in different parts of the plants. When trying different ways of representing the vertical distribution of 137Cs in the lichen- carpet and in the underlying material it was found that the dry mass concentration of 137Cs at a dry mass depth z in September, 1969, and 1970, could be written as

-6 c = ct e (3) where a and B are constants, and the constant p lies between 1/2 and 1.

The constant p has been evaluated numerically for the different sampling-plots and times of collection by means of the method of least squares. The best agree- ment of the model to the experimental points was obtained with p = 0.75. For this p-value, the constants a and B were then determined by means of the same method for the different sampling-plots and different times of collection, according to Table 3, where the

integrated area-content down to the dry mass depth z = 2.0 kg m"2 is also given, as calculated with

Equation 3. 24

TABLE 3

Parameters for the mathematical description of vertical distribution of 137Cs according to Equations 3 and 9.

2. 0 Sampling- Date a r 0 7 5 8 f -ft?Bz • 1 ale dz plot nCi kg" kg-0.7 5 nl.S 0 nCi m"2

Sept. 90.7 1.57 Rogen 2a 1969 0.90 51 .9 Sept. 70.1 1 .39 1 .06 Rogen 1 1970 45.0 Sept. 75.1 1 .45 1.00 Rogen 2a 1970 45.9 Sept. 117.7 1 .96 0.67 Rogen 2b 1970 52.9

Let us now consider two different vertical distributions, K and L, which can be described by means of Equation 3, using the same p-value but different values for the constants o and 6:

cK = ctK

L = oL (5)

Let us also consider the relative vertical distributions

The relation between depths zK and zL, at which the two relative concentrations c^ and c£ are the same, is given by (6) 25 which gives

ZL The ratio — is called the relative penetration, r, zK of distribution L in comparison to distribution K, so

K where r = hp (9)

The relative penetration, r, of l37Cs at different plots and times are given in Table 3. From this table and from Figure 6 it is evident that the vertical distribution of lS7Cs in carpets of Cladonia alpestris from Rogen 2a changed very little from September, 1969 to Septem- ber, 19 70. The small relative penetration at Rogen 2b reflects the high top denseness of the lichen-carpet at the plot.

Knowledge of the vertical distribution of radionuclides in the lichen-carpet and underlying material is of primary interest when estimating the radionuclide-content in the reindeer-fodder and when calculating the absorbed dose from external radiation in individuals (animals or men), living in districts where the ground is covered with lichen. Different investigations of the depth- distributions of fallout radionuclides in soil have been carried out, and different mathematical models concerning the depth-distribution have been proposed and used for 26

calculations of the exposure in the air above the ground i ! (GALE et al, 1964; ALEKSAKHIN, 1965; IZRAEL* and STUKIN, ,' 1970). Fallout-radionuclides, which have been present in the soil for some time, are normally assumed to be distributed exponentially as a function of linear depth, | with a "half-value-layer" for the radionuclide concentra- j rion of around 2 cm (WALTON, 1963; BECK, 1966; BECK and j DE PLANQUE, 1968; KOGAN et al, 1971). This model must be considered as rough and uncertain, and only valid as a mean value for different radionuclides, different types of soils, different soil-densities, etc.

The proposed equation has been used to compare some reported data on the vertical distribution of 137Cs. The 137Cs-content at various depths in vertical strata of lichen-covered ground, as reported by a small number of measurements from Alaska (HANSON and EBERHARDT, 1971), is described by means of Equation 3 (p = 0.75) with a 0-value of 1.2. This value shows that the penetration- depth of 13 7Cs was somewhat higher than the mean value found for Rogen 1, 2a and 2b. A study of the distribution of 137Cs in the vegetation and surface-soil covering the tundra has been carried out in USSR (MOLTJANOVA and KULI- KOV, 1970; KULIKOV, 1972). For depths £ 6 kg m"2, the vertical distribution of 137Cs recorded at three different sampling-places can be described by means of Equa- tion 3 (p = 0.75), with a 3-value of 0.9, which means a penetration about two times greater than in the lichen-covered ground in the Rogen area. The vegetation covering the tundra area studied, consisted of a dominant 27 part of moss and lichen but also of a small number of higher plants, with root-systems at various depths. This, of course, makes a direct comparison with the Rogen-data difficult, especially at greater depths.

4.4 Factors affecting the vertical distribution

of 1 37Cs

It is known that the retention of cations in organic matter decreases with decreasing pH-value. The pH-value of the precipitation over most parts of north-western

Europe has been decreasing since the middle 1950's. In the Rogen area, however, no significant changes in the pH-value of the precipitation seem to have occurred up to 1970 (N.N., 1972).

Even if 137Cs were completely fixed in the lichen-material, the vertical distribution of the radionuclide changes with time due to the continuous growth of the lichen thallus which is combined with a continuous decomposition of its older parts. The rate of growth is influenced by a number of parameters, such as amount of precipitation, temperature, humidity, age of the lichen-carpet, etc.

(KÄRENLAMPI, 1970, 1971). The growing-rate of Cladonia alpestris in the Rogen area was determined to be about

3.5 mm a"1 (SKUNCKE, 1963). This experimentally determined value is the same as was found for Cladonia alpestris in Northern Canada (SCOTTER, 1963). The growing-rate of 3.5 mm a"1 (0.074 kg m'2 a'1) as an effect on the 137Cs- content in the entire lichen-carpet (2.0 kg m"2, dry weight) 28

has been studied for a supposed initial radionuclide- distribution as of September, 1970. Fallout of 137Cs after 1970 has been neglected, and it is assumed that no re-distribution of the radionuclide occurs from the gelatinous layer to the lichen-carpet itself. On the basis of these assumptions the l37Cs-content, expressed in relation to the value for 1970, will be as follows: 1972 - 98%, 1974 - 95%, 1976 - 92%, 1978- 89%, 1982 - 81%, 1986 - 69% and 1990 - 53%. The amount of 137Cs present in the lichen-carpet at the time of maximal fallout-rate, 196 3-1964, is supposed to be somewhat more concentrated to the top layers than in 1970. The decrease of the 137Cs-content in the entire lichen- carpet (2.0 kg m"2, dry weight) from September, 1965, to September, 1970, explained by growth of lichen, is thus estimated to be less than 7% under the assumptions mentioned above. During the same period, 196 5 to 1970, physical decay is responsible for a decrease of around 12%. In the field-study, the reduction of the 137Cs-area-content in the lichen-carpet from 1965 to 1970 has been determined to be 33% for Rogen 1 and 21% for Rogen 2a. The lichen-carpet is also subject to physical damage caused by snow, ice, wind, animals, etc. Damage in which parts of the top layer having high *37Cs-content fall down in the carpet, works as a non-physiological downward transport of radioactivity. 29

4.5 Mechanisms of retention and transport for

ls7Cs in lichen

The uptake of cations in organic substances may occur ei- ther by a simple ion-exchange or by complex- or chelate- formation. In all these cases negatively charged organic groups act as ligands, but very little is known about the exact mechanisms responsible for the retention of cations

(cf. RUHLING and TYLER, 1971). Monovalent cations are normally taken up to a greater extent than divalent cations.

The alkali metal ions which most often appear "free" are in fact always associated with the water molecules in their environment. The extent of such a hydration of the ions plays a central role for their properties. The extent of hydration of the alkali metal ions is, however, very uncertain. It is also known that the alkali metal ions complex only weakly, so that an active process of uptake is necessary to accumulate them in lichen tissue.

Metabolically mediated accumulation of phosphate, amino acids and sugars has been demonstrated in the lichen

Peltigera polydactyla (SMITH, 1962). The uptake of carrier-free I37Cs by Ramalina reticulata does not, however, appear to be directly linked to the metabolism

(HANDLEY and OVERSTREET, 1968). Laboratory experiments with this lichen also showed that the uptake of the radionuclide from a surrounding solution was a very rapid process in which conditions of equilibrium were reached in one to two houre. Laboratory experiments on the uptake of ll7Cs in Cladonia rangiferina (NEVSTRUEVA 30

et al, 1967) showed that the radionuclide remained chiefly (more than 9 5%) in the part, which was kept in an aqueous 137Cs solution, when 90Sr under the same conditions was evenly distributed in the lichen. The radionuclide-distributions became constant after four to eleven days. When the entire lichen was im- mersed into the solution, both 137Cs and 90Sr were distributed relatively evenly in the plant, with a tendency toward primary sorption in the older parts. In an interesting study on the rate of transport of

lt| NaH C03 in Cladonia rangiferina (BARASHKOVA, 1968) it was found that the rate of transport from the bottom to the top of the plants was higher than from the top to the bottom, when the bottoms of the plants were put in water and the rest were in air of different humidity. It was also shown that the rates of transport were dependent on the humidity of the parts of the plants which were in the air. The extent to which dead Cladonia alpestris is able to take up and translocate cations from solutions of 137Cs- and 90Sr- ions has been studied under laboratory conditions (TUOMINEN, 1967, 1968). The translocation of Cs and Sr from the bottom to the top of the dead plants seemed to fit a simple diffusion-equation, complicated especially with respect to Sr with an ion-exchange.

Results from laboratory experiments, especially when they are carried out on dead lichen, are not fully applicable when discussing the situation under natural 31 conditions.

The relationship between vertical distribution of 137Cs in the Cladonia alpestris-carpet and the amount of precipitation during the months preceeding collection which has been found in this work may be explained by an in- creased upward net transport of 137Cs during periods when precipitation is low and evaporation high. The result is similar to results registered for the transport of NaHCO3 in Cladonia rangiferina (BARASHKOVA, 1968) cited above. The change in vertical l37Cs-distribution by humidity of the lichen-carpet indicates that a considerable amount of 137Cs must be readily moveable in the carpet. This result contradicts the traditional idea that 137Cs was well fixed in the lichen-material (NEVSTRUEVA et al, 1967 and others). Recently, the existence of an internal cycling of 137Cs in the lichen-carpet was proposed (HAN- SON and EBERHARDT, 1971) but no convincing lichen-data was presented.

Information on specific activity is often very valuable when studying metabolism and food-chain concentrations of radionuclides (KAYE and NELSON, 1968; PERSSON, 1972 a). The content of stable Cs in lichen, gelatinous matter, humus and soil is, however, very small and it has not been possible to make reliable determinations of Cs by means of the techniques available for this investigation. By means of flame-spectrophotometry the contents of two other alkali metals» K and Na, have been studied in the samples The elements Cs and K have obvious characterise 32

tics in common, although the behaviour of the two nuclides in plants, animals and man is not as closely related as, for example, Sr and Ca. The l37Cs/K-ratio (nCi l37Cs per g of K) in the Cladonia alpestris-carpet from Rogen 1 during the period 1967-1970 is given in Figure 7.

wCa/K-ratlo nCI gr* 100

Quarterly precipitation

-Tln-i-l

FIGURE 7 137Cs/K-ratio (nCi 137Cs per g of K) in the different layers A (0-3 cm), B (3-6 cm) and C (6-12 cm) of the Cladonia alpestris- carpet at the sampling-place Rogen 1 during the period 1967-1970.

The highest ratio was found in the middle layer of the . carpet. The l37Cs/K-ratio in the same layer was, within limits of uncertainty, the same in 1967, 1969 and 1970. In 1968, however, all layers showed a higher ratio, which is supposedly the effect of the extremely dry sum- mer this year, as was earlier discussed in this work. The comparison between l37Cs and K shows that 137Cs moves more freely in the lichen-carpet than K.

ratio nCig* 1 1 1 1 i i i | 1 I 1 1 • i 11 K 1 1 1 1 1II K» — 1000 m

•Xw • m - \^ • Cs/K-ralio • mGm/m-nMo -

• •* ••

—^_* • • ^\ - i • . • I i

1 • ••i l i 10 i • 1

- X **x^ \ N ^ - \ X \

1 0.1 \ \ \ : is 3 e « 16 i *• 1 • åtmm i i i cm Hen•ft •OH —

• lit 1 It | , ... i 1 1 1 1 1 1 1 i 1 1 1 1 1 11 0.1 1.0 10 100 Or» *****

lf7 ll7 FIGURE 8 Cs/K-ratio (nCi Cs per g of K) and ll7Cs/Na-ratio (nCi 1S7Cs per g of Na) as a function of dry mass depth at the sampling-place Rogen 1 in September, 1970. 3

Figure 8 gives the l37Cs/K-ratio (nCi 137Cs per g of K) in vertical profiles of ground covered by Cladonia alpestris at Rogen 1 in September, 1970. As was earlier pointed out, the ratio has a maximum value in the middle layer (B) of the lichen-carpet, at a depth of around 1 kg m"2. In the soil, the ratio is drastically reduced due to the high K-content in this fraction. The varying 137Cs/K-ratio in the lichen-carpet may be explained in several ways:

1. The top layer consists, to a considerable degree, of material, which was not exposed to the high deposition- rates of l37Cs in 1963-1965. This gives, together with a slow circulation of 137Cs in the lichen thallus, a low ratio in the top layer because K is continuously deposited.

2. Differences exist between the behaviour of Cs- and K- ions in the lichen-carpet (different diffusion-rates, varying degree of metabolical uptake, etc.).

3. Still undissolved fallout-particles are present pre- ferentially in the B-layer (because the lichen-plants have grown after the deposition and because large particles are not easily attached to the fine top branches while the C-layer is efficiently shadowed from big fallout-particles by means of overlying layers).

The naturally produced radionuclide *10Pb is continuously deposited on the lichen-carpet (PERSSON, 1970 b). Measure- wents of the 210Pb/Pb-ratio in the A, B and C-layers of the lichen-carpet have recently been carried out (PERSSON 35 and HOLM, 1972). In the regions available for comparison, the variation with depth of the two ratios, 137Cs/K and

2loPb/Pb, are surprisingly similar. Both ratios have their maximum values in the B-region of the carpet in spite of the constant deposition-rate of 210Pb and the chemical similarity between the different Pb-isotopes.

This result supports explanation number 3. It is known that 137Cs condenses late in the particle formation process after a nuclear detonation and is therefore pre- ferentially attached to the surfaces of the particles.

210Pb, which is formed by the decaying gas 222Rn, is attached to the surfaces of atmospheric aerosol-particles.

The particle-size distribution for these aerosols is assumed to be very similar to the size-distribution of fission-products found at ground-level (LOCKHART et al, 1965)

Thus, the fractionation of 137Cs and 210Pb between particles of different volumes seems to be very similar.

The results obtained support the theory that undissolved fallout-particles are still present in the lichen-carpet at a depth which, at the deposition, was determined by the particle-size and the geometrical properties of the lichen-carpet.

The measurements of Na show that the penetration of this element in the lichen-carpet is considerably greater than that of both 1S7Cs and K. 4.6 l37 Cs/90Sr-activity ratio ir> different parts

of the lichen-carpet and in underlying material

From the point of radiological health, 90Sr has always

been pointed out as being the most hazardous radiönuclide

in the fallout from nuclear weapons-tests. The retention

of 90Sr in the lichen-carpet has, however, been reported

to be less effective than the retention of 137Cs (NEVSTRU-

EVA et al, 1967). A comparison between vertical distribu-

tions of 137Cs reported in this work and previously re-

ported 90Sr-distributions for the same material (PERSSON,

1971) confirms this statement. Some of the fractionated

samples, which were analysed for 137Cs in this work, have

also been analysed for 90Sr (PERSSON, 1972 b). Figure 9

gives the l37Cs/90Sr-activity ratio as a function of mass

depth for vertical strata for Rogen 2a in

September, 1969. The 137Cs/9oSr-activity ratio in the top

layer of the lichen-carpet, which is eaten by reindeer

(around 3 cm, which means around 0.6 kg m"2, dry weight)

is more than ten times higher than the value 1.6, which

is the average ratio of 137Cs/90Sr in the fallout (UNSCEAR,

1966). This selective accumulation of the two radionuclides

in the lichen-carpet is then combined with a natural dis-

crimination of 90Sr in the second link of the food-chain,

because this radiönuclide is concentrated in the reindeer-

skeleton, which is normally not consumed by man. This

leads to a much greater contribution of 137Cs in relation

90 v I to Sr to the absorbed dose in Lapps (PERSSON, 1970 a). 6 t

^»-activity ratio

1 1 1 1 Mil r-—i—i | i 111

- -

If 1

K i 1 1J

t.5 -

lichen l igillvl aoil 11* t.1 1 III i • i i i 11 MS 0.1 t.5 It Pry ma»» dopth kg nr* FIGURE 9 137 Cs/90Sr-activity ratio as a function of dry mass depth as registered in carefully field- fractionated samples of lichen, gelatinous matter and soil from the sampling-place Roeen 2a in September, 1969. The average 137Cs/9*Sr- activity ratio of 1.6, which has been found in the fallout (UNSCEAR, 1966), is also indicated in the figure.

»4.7 Estimated effects of reindeer-grazing

The results and the discussion in this work are all valid for undisturbed lichen-carpets. In practice, most areas are grazed by reindeer and in several places the areas are heavily grazed, which means that considerable amounts of l97Cs in the lichen-carpet are lost through reindeer- grazing. This parameter varies very much from place to place and is difficult to estimate. The extent to which 137Cs is transported to new branches from parts of the carpet which are not grazed is a subject for further studies in the field and in the laboratory. The results of this work, however, indicate that such a transport of unbound 137Cs exists. This free 137Cs may then be metabolically absorbed by the living parts of the carpet. The renewal-time of a lichen-carpet which has been grazed down to 3 cm below the top surface is 8-9 a, assuming a growing-rate of 3.5 mm a"1, which was determined for Cladonia alpestris in this area (SKUNCKE, 196 3). Assuming a constant relative distribution of 137Cs in the lichen-carpet, this means a half-time for 137Cs in an arbitrary layer of the carpet of 6-7 a due to grazing. This value corresponds to an effective half-time of 5-6 a for I37Cs in reindeer-food. During the period 196 5-1971 the decrease in 137Cs- concentration in reindeer-meat from the Rogen area corresponds to an effective half-time of 4.5 - 5.5 a (LIDEN, 1972). This value directly reflects the decrease of l37Cs-content in reindeer-food and is, within limits of uncertainty, the same as was calculated above from lichen data. An effective half-time of 5-6 a in lichen-carpets grazed by reindeer is also in agree- ment with data from Finland (MIETTINEN and HÄSÄNEN, 1967). Thus, in areas frequented by reindeer, grazing is the dominant factor for elimination of l37Cs from the lichen- carpet. The next most important factor is physical decay of the radionuclide, then continuous growth of the lichen- carpet and finally internal movement of 137Cs in the carpet. I 5. CONCLUSION

The fallout-radionuclide 137Cs is to a high degree accumulated and retained in undisturbed natural carpets of the reindeer-lichen Cladonia alpestris, as well as in underlying layers of gelatinous matter, humus and soil. The deeper mechanisms behind this behaviour are not known, and the elimination of 137Cs from a lichen-carpet is difficult to describe. As a rough approximation it is possible to assume a first order elimination during short periods. 1968-1970 the elimination of 137Cs was characterized by an effective half-time of 8-1H a for different undisturbed Cladonia alpestris-carpets (2 kg m~2) which were studied in the Rogen area. The initial vertical distribution is subject only to a small long-term variation, mainly explained by the continuous growth of the lichen-carpet. However, varying amounts of precipitation have been shown to cause considerable short-term changes in the vertical l37Cs-distribution in the lichen-carpet, so that the radionuclide in dry periods is transferred up to the top regions of the carpet, and in periods of high precipitation-rate it is washed down in the carpet. In the meteorologically normal years 1969 and 1970, the relative depth distribution of 137Cs in September in undisturbed areas covered by lichen followed the func- 0.75 tion exp(-1.6 z ). Different depth distributions may be compared by means of the concept "relative penetra- tion11. The ls7Cs/K-ratio shows a maximum value at a depth in the lichen-carpet of around 1 kg m"2. This probably indicates the presence of undissolved fallout- particles in this layer to a higher degree than in the other layers.

6. ACKNOWLEDGEMENTS

The author wishes to extend his thanks to Prof. K. LIDEN for his support and encouragement, and for many instruc- tive and stimulating discussions. The valuable comments and suggestions of Dr. B. PERSSON are also gratefully acknowledged. Thanks are due to Mrs. B. AMILON and Mrs. C. LINGÅRDH for several years of valuable help with sample-preparation and analytical work. This investigation has been supported by grants from the Swedish Atomic Research Council. •f I

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