Palaeogeography , Palaeoclim atology, Palaeoecology , 54 (1986): 181--195 181 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

C E S I U M - 1 3 7 R E D I S T R I B U T I O N IN T H E

S E D I M E N T S O F T H E P L A Y A , L A K E T Y R R E L L ,

N O R T H W E S T E R N V I C T O R I A .

I. S T R A T I G R A P H Y A N D C E S I U M - 1 3 7

M O B I L I T Y IN T H E U P P E R S E D I M E N T S

M. E. LONGMORE ~ ; T. TORGERSEN2'*; B. M. O 'LEARY 3 and J. G. LULY ~

IDepartment of Biogeography and Geomorphology, Research School of Pacific Studies, Australian National University, Canberra A. C.T. 2601 (Australia) 2Environmental Geochemistry Group, Research School of Earth Sciences, Australian National University, Canberra A. C. T. 2601 (Australia) 3Department of Physics, School o f Applied Science, Queensland Institute o f Technology, Brisbane, Qld. 4001 (Australia)

(Received October 25, 1984;revised and accepted July 22, 1985)

ABSTRACT

Longmore, M. E., Torgersen, T., O'Leary, B. M. and Luly, J. G., 1986. Cesium-137 redistribution in the sediments of the playa, Lake Tyrrelt, northwestern Victoria. I. Stratigraphy and cesium-137 mobili ty in the upper sediments. Palaeogeogr., Palaeo- climatol., Palaeoecol., 54: 181--195.

The stratigraphy of the upper clay sediments in the northern basin of Lake Tyrrell, a saline playa in northwestern Victoria, is described. The 137Cs activity in sediment profiles was measured at two sites and shows redistribution of the radioisotope to depths well below post-bomb sediments, as indicated by 14C dates and surface pollen spectra. The distribution coefficient (KD) of 137Cs on Lake Tyrrell sediments was measured and found to range from ~3600 at 0% NaC1 to - 6 0 0 at 26% NaC1. This low K D for halite- saturated sediments suggests that adsorption sites in the clay lattice are saturated by Na +. An advection--diffusion model with constant input indicates an effective diffusion coefficient of ~1.7 × 10 -8 cm 2 s -~ and a K D of 280, in good agreement with laboratory measurements. More realistic input functions indicate that this is a lower limit on K D. It is concluded that ~37Cs is redistributed in the upper sediments by diffusion due to the low K D resulting from the high salinity. This radioisotope is not useful in locating post- bomb sediments in this and probably other highly saline environments.

*Present address: Department of Marine Sciences, University of Connecticut, Avery Point, Groton CT 06340 (U.S.A.).

0031-0182/86/$03.50 © 1986 Elsevier Science Publishers B.V.

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INTRODUCTION

The nature and rate of accumulation of recent (near-surface) sediments is essential information to aid interpretation of older sedimentary sequences. In order to determine whether recent sediments could be identified by this method in a highly saline playa, the 137Cs profile was measured in Lake Tyrrell in semi-arid northwestern Victoria (35°15'S, 142°50'E).

The radioactive fallout isotope 13~Cs has been used to identify post-bomb accumulations in alluvial soils (Ritchie et al., 1975; McCallan et al., 1980) and lake sediments (e.g. Pennington et al., 1973; Ritchie et al., 1973; Robbins and Edgington, 1975; Jaakkola et al., 1981). Problems in the use of 137Cs have been discussed by several authors (e.g. Pakarinen and Tolonen, 1977; Oldfield et al., 1979; Longmore, 1983; Longmore et al., 1983; Torgersen and Longmore, 1984).

DISTRIBUTION OF RADIOACTIVE FALLOUT IN AUSTRALIA

Detectable levels of 137Cs in the atmosphere on a global scale occurred in late 1952, when the first high-yielding nuclear explosions (Gibbs et al., 1965; Moroney, 1979) generated significant atmospheric fallout. Escalation of nuclear testing, until the 1958--1961 moratorium, caused increased fall- out of the isotope. Testing escalated again during 1961/1962 and 1963, when the Test Ban Treaty of August 1963 banned all but underground testing of nuclear devices. Following the Treaty, radioactive fallout was greatly reduced, but several small peaks occurred during the 1970's as a result of French tests in the Pacific Ocean. Fallout in the last decade has been very low, and is still declining.

137Cs, attached to fine debris hurled into the stratosphere by nuclear explosions, is carried into the troposphere at mid- and polar latitudes through the tropopause gap (Davis, 1961). 137Cs is principally carried to the ground in rain and snowfall (Libby, 1956; Welford and Collins, 1960). The radio- isotope then attaches itself to particles, and becomes concentrated in the surface layers of soil profiles (Walton, 1963; Hakkinen and Lakanen, 1968; Ritchie et al., 1970; Cline and Rickard, 1972; McHenry et al., 1973; Ritchie et al., 1974; McCallan et al., 1980). ~37Cs falling on lakes and reservoirs adheres to suspended particles (McHenry et al., 1973), and is carried to the sediment surface. A further contribution to the 137Cs contained in lake sediments is a component attached to soil particles carried into the lake from the catchment (McHenry et al., 1973; Stiller and Assaf, 1973). As a result and in combination with sediment-focusing in some permanent lake basins (e.g. Lehman, 1975), and areal concentration of 137Cs (the integrated lSTCs profile expressed as concentration per unit area of soil or sediment surface) in alluvial plains (McCallan et al., 1980) and lake basins (McHenry et al., 1973) tends to be greater than fallout.

The estimated and measured 137Cs fallout for Adelaide is illustrated in

183

Fig.1. Until 1981 , the estimated total fallout of 137Cs at Adelaide, corrected to 1981, is 657 .86 MBq km -~. Temporal variation of fallout in Adelaide, indeed throughout Australia, is represented by a relatively flat curve with two to three small peaks of similar magnitude, fol lowed by a period of declining fallout to the present. Levels of radioactive fallout in Australia have, in general, been a factor of ten or more smaller than those of the northern hemisphere.

The temporal variation of 137Cs fallout is frequently recorded in lake sediment profiles and can be used to date the corresponding sedimentary layers (Pennington et al., 1973). In the northern hemisphere, both the first appearance of 137Cs in the mid-1950's and the peak o f fallout in 1963 are

1 ~!8(:I

l!),zi~

1! ,,()

I D ( S

! ) l i{)

I A I I O t J [ 'n A [ ) E [ A I I D [

k,

~>

i

G~

7

: 13,' PTq ~ 1 0 k,/', '

Fig.1. ~37Cs fallout in Adelaide 1954--1979 (corrected for radioactive decay to 1981). ~37Cs fallout for the period 1954--1963 was estimated from extrapolated and measured 9°Sr data for Adelaide (Gibbs et al., 1965) using a 137Cs/g°Sr ratio of 1.7 prior to 1961 and 1.5 from 1961 (Comar and Lengemann, 1966). For the period 1964--1973, ~37Cs was measured in Adelaide by the Australian Radiation Laboratory (A.R.L.)(refer- ences in Bullock et al., 1975), and in the period 1974--1979 in Melbourne by A.E.R.E., Harwell. (References in Cambray et al., 1980.) Data from the A.R.L. for the latter period were not available (J. R. Moroney, personal communication, 1978).

184

used as dating horizons. In the southern hemisphere, only the first appear- ance of '37Cs can be used, as the 1964 fallout peak is too small to be con- fidently identified (Fig.l) . In addition, many variables within the lake catchment and within the lake itself can affect the eventual position of 13~Cs peaks measured in the sediment, such as location of sediment source, depth of erosion during an erosion event, temporal-spacing of storms with erosion potential etc.

THE STUDY SITE

Lake Tyrrell is the largest playa lake in Victoria, with an area of some 150 km 2 , and lies in the Mallee district 45 km west of Swan Hill (Fig.2). The lake floor is some 40 m above sea level, with a slight tilt to the west (northern basin) and south to west in the southern basin (Teller et al., 1982).

The western shore is steep, with cliffs rising 15--20 m, in strong contrast to the smoothly sloping clay lunettes lying along the southern and eastern sides.

Water is supplied to the lake in three ways. (1) Sporadic rainfall, averaging 325 mm per annum, falling on the lake surface and in the relatively small catchment, provides the major control on lake water Ievels. Annual evapora- tion is in excess of 1350 mm. (2) Tyrrell Creek, which enters the lake at its southern extremity, is an overflow channel from the Avoca River, and contr ibutes little water to the lake, except in years of abnormally high rainfall. (3) Groundwater outcrops on the lake floor and around the shorelines as springs, some of which are perennial (Teller et al., 1982; Macumber, 1983), but groundwater seepage contr ibutes little surface water to the lake. It does, however, maintain halite-saturated sediments and a watertable at or near the sediment surface (Teller et al., 1982; Macumber, 1983). The amount of water in the lake (maximum depth 50--75 cm in winter) is variable, depending upon how recently heavy rains have occurred.

Clastic sediments within the lake are dominated by silt and clay derived from the re-working of marginal deposits, from occasional f loods down Tyrrell Creek, and from dust storms (Teller et al., 1982). The surface sedi- ments, away from the lake and island margins, are primarily halite and clays dominated by illite and kaolinite (Teller et al., 1982).

Groundwater hydrology in and around Lake Tyrrell is complex, and has had a significant effect on the geochemistry of the lake (Teller et al., 1982; Macumber, 1983). Whereas Lake Tyrrell is effectively a closed system with respect to surface water movement , this is not the case for groundwater. Discharge of groundwater onto the lake surface effectively lowers the regional piezometric surface in areas immediately surrounding the lake, thereby producing a low groundwater divide on the down-basin (western) side of the lake (Macumber, 1983). Groundwaters associated with the lake basin have ionic ratios similar to those of sea water, but vary considerably in salinity. Regional groundwater averages ~4% TDS (total dissolved solids),

185

Fig.2. Location of Lake Tyrrell and sampling sites.

but brines beneath the lake floor reach 24% TDS. Na ÷ and C1- ions comprise up to 97% of TDS in the present surface and groundwaters, with subsidiary ions including SO~-, Mg 2÷, K ÷, Ca 2÷, and HCO~ (Teller et al., 1982; Macumber, 1983). Cl-, Na ÷, and K ÷ concentrations increase with increasing salinity until the saturation level of NaC1 is reached. Except in the marginal seepage areas, the pH of the sediments ranges from highly alkaline (pH 9.0) to weakly alkaline (Teller et al., 1982; Macumber, 1983).

Evaporative concentrat ion of brine allows the denser surface water to move down into a high salinity pool below the lake floor, transferring salts

186

from the surface. Loading of the lake surface by water at times of high rain- fall or flooding increases the piezometric head enough to overcome the divide, and a proport ion of the stored water and salts can be exported down-basin into the regional watertable (Macumber, 1983). Evaporation of surface brine also results in deposit ion of a halite crust. Salt thicknesses reach their maxima in the deepest parts of the lake basin, bu t the short- term surface distribution of salt is affected by movement of brine bodies blown across the lake floor by strong winds (Teller et al., 1982; Macumber, 1983; Torgersen, 1984).

S e d i m e n t s t r a t i g r a p h y

A number of stratigraphic profiles have been determined across the northern basin of Lake Tyrrell (Figs.2 and 3). Of these, three have been analysed in detail; Site 1, where 14C dates have been obtained (Luly et al., 1986), and Site 2, 4 and 4b, where samples were collected for 137Cs deter- minations. The results of '4C determinations are also shown on Fig.3 and discussed in Luly et al. (1986).

depth (cm)

A N U 2390 ~ 0 l

1263 + 12% modern ~ , I A N U 2391 / 107 6 + 2 0 % modern

5-

10.

20-

Site no

10 5 2 11 4 3

]5- ANU 2392

197o +_ 8o ~ #P I

sS/t crust

laminated su/phtde clay

dark grey gypsum poor clay

[ ~ pale grey oxldlsed clay

P ~- dlsplaove millet - seed gypsum

. . . . trans~tlona/ - - abrup t " ~ oxJdssed layer

Fig.3. Lake Tyrrell : stratigraphy and radiocarbon dates.

187

The main sites are on the elevated northeastern sector of the lake, and are covered by a thin salt crust at irregular intervals. The crust is frequently absent at one or all sites. At these times a thin surficial salt efflorescence is often present. When a salt crust is absent, the surface appears uniformly dark brown, and is firmly compacted.

Immediately below the salt crust (where present) is a thin (0--1 cm) algal mud layer, followed by a zone of laminated, black, sulphide-rich clays. This laminated horizon is commonly 2--5 cm in depth, and grades into dark grey, non-laminated, gypsum-poor clays. Below 5--11 cm these clays grade into pale grey oxidised clays containing abundant secondary gypsum. The gypsum-rich sequence grades into a zone of laminated gypsum--clay "couplets" which can be traced across the whole northern basin of the lake. Clays of the upper part of the sequence are dominated by illite and kaolinite (Teller et al., 1982; Bowler and Teller, 1986).

Sulphide content of the sediment falls rapidly with increasing depth, from 5% in surficial clays to 0.3% at 6 cm. Sulphate content , on the other hand, increases, being 3% and 15% for depths as above (Teller et al., 1982).

In other cores, located on a transect across the lake, the pattern of salt and algal mud followed by sulphide clay, dark grey gypsum-poor clay and pale grey oxidised clay is repeated, supporting the correlation between widely separated Sites 1, 2 and 4 (Fig.3).

METHODS

Surface sediment samples were obtained in November 1981 and February 1982 from a 40 × 50 cm pit at Tyrrell Site 4 and 4b, excavated to a depth of ~ 11 cm. (Pit samples were taken in order to obtain sufficient material over shallow depths for '37Cs measurement). The surface salt crust, ~1 cm deep, and the algal layer, ~0.25 cm deep, were sampled as distinct units. The underlying black laminated clay and grey clay were sampled at ~0.75 cm intervals down to and into the pale grey oxidised clay. In December 1981, 8 cores were taken at Site 2. In order to obtain sufficient material for 137Cs measurement, the salt crust, algal mud and black laminated clay from all cores were combined. The dark grey gypsum-poor clay, the pale grey oxidised clay to iron-rich oxidised layer at 18 cm, the oxidised layer and pale grey clay to 35 cm were sampled as separate layers in all cores and corresponding layers were combined. The samples were dried at 105°C to constant weight, ground to pass through a 2-mm screen and analysed for 137Cs"

137Cs measurements were carried out on a lithium-drifted germanium [Ge(Li)] semi-conductor detector, by counting the 662 keV gamma rays emitted. The lowest specific activities detectable in 750 g of sample, using 40-hr counting times and the 2¢ limit, are ~0.35 mBq g-1. The average weight of sample measured was ~700 g dry weight. Additional details of the counting technique are in McCallan et al. (1980).

188

Experiments to determine the distribution coefficient (KD) [ a ratio of the amount of bound radionuclide per unit volume (or unit weight) of dry sediment to the amount of radionuclide per unit volume (or unit weight) of liquid; Duursma and Gross (1971)] were conducted using a bulk surface- sample of Lake Tyrrell sediment from the vicinity of Sites 4 and 4b. ~37Cs- spiked solutions of varying salinity (0--27% NaC1) and acidity (pH 4 and 9) were equilibrated with the mud and K D was determined by methods de- scribed in Torgersen and Longmore (1984).

RESULTS

13~Cs results for Site 4 and 4b are shown in Fig.4.a and site 2 in Fig.4.b. The 137Cs profile from the surface sediments of Site 4 and 4b (Fig.4.a) shows an exponential decrease in specific activity with depth to at least 11 cm. The surface salt crust had a count rate o f < 0 . 7 mBq g-1 (not signifi- cant (N/S)). The underlying brown algal layer had a specific activity of 18.9 + 0.7 mBq g-~. The underlying laminated clay horizon had a specific activity (39.6 + 0.7 mBq g-l) double that of the algal layer. The corre- sponding algal layer and laminated clay at Site 1, collected in 1980, gave ~4C activities of 126% (ANU-2390) and 107% (ANU-2391) of the modern standard respectively (Fig.3). From the surface to 11 cm at Site 4 and 4b there is an "exponent ia l" decrease of ~37Cs specific activity with depth from 37.4 + 1.1 to 2.14 + 0.26 mBq g-1. The depth at which 13~Cs is no longer detectable in the sediment profile at this site has not yet been deter-

LAKE TYRRELL Site 4 and 4b

LAK£ TYRRELL Site 2

0

2

6 & ~ ox,d;sodlarerNiS

8 N/S

10 5 10 15

CS--137 mBq9 -1 dry weight

0 10 20 30 40

Cs-137 m8q g-1 dry weight

a b

Fig.4. Variation of specific activity (mBq g-Z) of '37Cs as a function of depth in the Lake Tyrrell upper sediments at (a) Site 4 and 4b, and (b) Site 2. N/S = not significant count rate (< 0.7 mBq g-' ).

t • pH 4

4000 L • pH 9

1 2000. I ~:. t,::,,~

,y,

~s~• ~ o o o

b O0

189

i

(/ 10 20 30

'~ NaC

Fig.5. Distribution coefficient K D as a function of NaC1 concentration in interstitial water and pH.

mined. At 16--17 cm, the sediments from Site 1 gave a 14C date of 1970 + 80 yr B.P. (ANU-2392) (Fig.3).

The 13~Cs profile from the core samples at Site 2 (Fig.4.b) again shows an exponential decrease in 137Cs activity with depth down to ~ 18 cm, where material in the iron-rich oxidised layer and the pale grey clay below gave a "no t significant" count rate. At Site 2 the ~4C age for 44--48 cm was 3150 + 500 yr B.P. (ANU-3066).

The results of the distribution-coefficient experiment are illustrated in Fig.5. There is an exponential decrease in the distribution coefficient (KD)

with increasing salinity. The pH has a negligible effect o n K D compared with salinity. The K D falls from ~ 3600 at zero salinity to ~ 600 at the saturation level (26.5%) for NaC1. This result is consistent with the relationship:

In K D = m (%NaC1) + b

where rn = --0.0674 + 0.021, and b = 8.18 -+ 0.091.

DISCUSSION

The 137Cs from Sites 2, 4 and 4b extends deeper into the sediment than would be expected. Radiocarbon results from Site 1 (Luly et al., 1986) indicate an average rate of sediment accumulation of ~0.01 cm yr -1, a figure supported by pollen spectra in which evidence of European distur- bance of vegetation (late 1880s and early 1890s) is restricted to the upper

190

1--2 cm of black clay at Sites 1 and 2. As ~37Cs only became a component of the environment in the mid 1950's, the penetrat ion of ~37Cs to at least 11 cm at Site 4 and 4b, and 18 cm at Site 2, therefore implies considerable redistribution of the radioisotope in the sedimentary profile.

~37Cs in the sediment profile

The adsorption capacity of a sediment is determined in part by the inter-relationships between the following factors:

(1) The variations in the mineralogical composition o f the sedimentary particles and the ion-exchange capacity o f the particles Illite is known to have a high affinity for 13~Cs (Fredrikkson et al., 1958;

Tamura and Jacobs, 1960; Tamura, 1964; Aston and Duursma, 1973) and to be the most efficient clay-mineral adsorber (on a weight basis) at low con- centrations of ~3~Cs, despite its low ion-exchange capacity (Tamura, 1964). Kaolinite owes its adsorptive capacity to a high ion-exchange capacity (Tamura and Jacobs, 1960).

(2) The size, and thus surface area, o f the sedimentary particles A direct correlation between decreasing particle size and ~37Cs adsorption

capacity has been observed, with clay-size particles being the most adsorptive (De Bortoli and Gaglione, 1969; McHenry et al., 1973; Ritchie et al., 1973; Brisbin et al., 1974; Ritchie and McHenry, 1976).

(3) pH and Eh concentration Several authors have observed that H ÷ ions compete with 137Cs for adsorp-

tion sites (Lerman and Lietzke, 1975) and can displace ~37Cs from adsorp- tion sites when the medium becomes acidic (Agre and Korogodin, 1960). Other authors have observed diffusion of 137Cs in highly organic sediments of an acidic perched lake (Longmore et al., 1983) bu t were not able to demon- strate experimentally that the acidic environment had any effect on the dif- fusion rate (Torgersen and Longmore, 1984). Duursma and Gross (1971) observed that the distribution coefficient of ~37Cs between sediments and water was greater under anoxic conditions in the marine environment.

(4) The presence o f competing ions Aston and Duursma (1973) observed that as the salinity increased from

0--4%, the amount of ~37Cs adsorbed on to marine sediment decreased by a factor of ten. There was a corresponding increase in the adsorption of Na ÷. They hypothesised that this was due to the compet i t ion of Na + and K ÷ with ~37Cs for sites on the clay crystal-lattice. Duursma and Eisma (1973) observed suppression in the adsorption of 137Cs by the illitic fraction with increases in the quanti ty of extractable Na ÷ and K ÷ in the sediments. Other workers (Coleman et al., 1963) noted that ~37Cs was easily displaced from mont-

191

morillonite pre-treated by saturation with NaC1 and drying. In addition Na+-saturated illite and kaolinite adsorbed little 137Cs, and the adsorbed fraction was easily displaced on leaching.

Whereas it is difficult to quantitatively evaluate the individual effect of all these factors in Lake Tyrrell, the net effect of adsorption-moderated diffusion can be quantitatively modelled and the (deduced) effective K D can be compared to the measured K D.

Diffusion

The mathematics of early diagenesis, including the adsorption of dissolved species on to sedimentary particles, has been reviewed by Berner (1976). The descriptive equation for simple linear adsorption with radioactive decay, constant sedimentary properties with time and depth, and no compaction or advective flow, is:

(~C/~t) = [¢D/(1 +K ' ) ] (~2C/~Z2)--S(~C/~Z)--kC, K'=KD[(1--¢)/¢] (1)

where C is concentration, t is time, ¢ is porosity, D is the molecular diffusion coefficient, Z is depth, S is sedimentation rate, k is the decay constant, and KD is the distribution coefficient. For Lake Tyrrell, Eq. 1 was solved by substituting D' = CD/(1 + K') and using central difference technique for ~2C/~Z2 and OC/aZ, forward differences for OC/Ot, a space step of 1 cm and a time step of one day.

With the fallout of 137Cs modelled as constant for 25 years, Fig.6 shows

LAKE TYRRELL Sile 4 and 4b

normahs~d 137Cs actwltV

02 04 06 0 8 1 0 0

2,

4 ,

0-

2,~

Fig.6. Measured, and predicted 137Cs curve for various effective diffusion coefficients (from Eq. 1).

192

the mathematical results with the 13VCs data for Site 4 and 4b normalized to a value of 39.6 mBq g-~ at Z = 0 (Fig.4.a). The fi t ted ~37Cs curve (Fig.6) from the Lake Tyrrell sediments gives D' = 1.7 × 10 :s cm 2 s -~ and a KD of 280 ( D ' = C D / K D ( 1 - - ¢)/¢), which is within an acceptable range of experi- mentally determined values.

The use of a constant surface concentration is mathematically convenient and does tend to "mimic" the smoothing of the input function boundary condition caused by the high mobili ty of ~37Cs in these sediments (see Torgersen and Longmore, 1984). The other end of the spectrum is a boundary condit ion that rigidly follows the input function (Fig.l) . With this boundary condition, a reasonable fit to the data was obtained with D' = 3--10 × 10 -~ cm 2 s -~. A D ' v a l u e of 6 × 10 -9 cm 2 s -~ (which is a lower limit estimate on D') gives a K D < 791 with large errors. It was further shown that S > 0.05 cm yr -1 gave unrealistic 137Cs profiles.

Given the uncertainty of the applicable boundary conditions, the mathe- matical modelling of the 137Cs profiles yields effective K D in the range 280--791 which compares quite well with the experimentally determined value of K D = 598 for ¢ = 0.40 and halite saturation. It is concluded, there- fore, that a significant redistribution of '37Cs occurs in the sediment of Lake Tyrrell due to the relatively low value of KD which is probably the result of Na + saturation of adsorption sites.

CONCLUSIONS

Illite and kaolinite have a high affinity for 137Cs and the anoxic, alkaline conditions in the sediments may favour adsorption of 137Cs. Nevertheless, pre-saturation of the adsorption sites by Na + may reduce 137Cs adsorption to sediment particles. This hypothesis is confirmed by the exponential decline of KD, measured with Tyrrell clays from ~3600 to ~600, as the salinity of '37Cs-spiked solutions is increased from 0 to 26.5% NaC1 respec- tively (Fig.5). Na + forms up to 37% of the TDS of the surface and ground- water brines (Teller et al., 1982; Macumber, 1983), and K ÷ would be the only one of the subsidiary ions likely to desorb substantial amounts of '37Cs from soil particles (Schulz, 1965). The pre-saturation of sediments by NaC1 before the first fallout of ~ 37Cs would reduce Cs adsorption and thereby promote its mobil i ty by advective and diffusive processes.

Therefore, it is concluded that the t37Cs profile presented from Lake Tyrrell reflects downward diffusion of '37Cs through the upper portion of the sediment column, due to the low K D resulting from the high salinity. In the case of Lake Tyrrell, extreme salinity reduces the fraction of Cs adsorbed (K D ~ 600) and therefore increases the fraction of 137Cs available for diffusion in the sediment. The downward redistribution of 137Cs within the surface sediments may be facilitated by the downward movement of concentrated brine or by the rise and fall of the groundwater due to the passing of an atmospheric high-pressure system over the region (Macumber,

193

1 9 8 3 ) . T h e v e r t i c a l p r o f i l e c a n n o t b e u s e d t o d a t e L a k e T y r r e l l s e d i m e n t s , a n d m o s t p r o b a b l y s e d i m e n t s in s i m i l a r s a l i ne e n v i r o n m e n t s .

ACKNOWLEDGEMENTS

We w i s h t o t h a n k B. J. T h o m a s , Q . I . T . , f o r t h e u se o f his Ge (L i ) d e t e c t o r t o c o u n t s o m e o f t h e s a m p l e s . G a m m a - c o u n t i n g e q u i p m e n t f o r t h e K D e x p e r i m e n t s w a s p r o v i d e d in p a r t b y M S T g r a n t 8 0 / 2 0 2 9 t o T. T o r g e r s e n , J . M. B o w l e r k i n d l y c o m m e n t e d o n a n e a r l y d r a f t o f t h e m a n u s c r i p t . M a n y t h a n k s a r e d u e t o J e n n y T o d e , C a r o l i n e T w a n g a n d M a r c i a M u r p h y f o r t y p i n g t h e m a n u s c r i p t .

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