The distributions of 239,240Pu, 238Pu, 241Am and 137Cs among chemically-defined components of sediments, settling particulates and net plankton of Lake Michigan

J. Environ. Radioactivity 9 (1989) 89-103

The Distributions of 239'24°pu, 238pu, 241Amand 137Cs among Chemically-defined Components of Sediments, Settling Particulates and Net Plankton of Lake Michigan

J. J. A l b e r t s , * M . A . W a h l g r e n , K. A . O r l a n d i n i & C. A . D u r b a h n $ Ecological SciencesSection, Radiological and Environmental Research Division, Argonne National Laboratory, Argonne, Illinois60439, USA (Received 11 April 1988; revised version received 25 July 1988; accepted 4 October 1988)

ABSTRACT The application of selective extraction techniques to samples of consolidated sediments, surficial sediment floc and suspended particulate material collected in sediment traps from Lake Michigan shows that 239'24°pu, 238pu and 2dlAm are removed primarily with the hydrous oxide coatings while Z37Cs is strongly associated with the mineral fraction. Low concentrations of plutonium and americium are found associated with isolated humic acid fractions. The selective extraction of plutonium from net plankton samples results in the observation of a different distribution of plutonium among phases than that observed for sedimentary material, but, in all cases, the major fraction of the plutonium is found in the same extractant as with sediment samples. Attempts to displace Z37Csfrom the mineral fraction by several techniques indicate that the adsorption of Z37Cs is not readily reversible.

INTRODUCTION Wahlgren and Nelson (1975) have shown that approximately 3% of the plutonium which has entered L a k e Michigan as a result of fallout remains *Present address: University of Georgia Marine Institute, Sapelo Island, GA 31327, USA. SResident Associate, January-August 1975, from Gustavus Adolphus College, Minnesota, USA. 89 J. Environ. Radioactivity 0265-931X/89/$03.50 © 1989 Elsevier Science Publishers Ltd, England. Printed in Great Britain

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,I. J. Alberts, M. el. Wahlgren, K. A, Orlandini, C. A. Durbahn

in the water column. Hence, since the lake has acted as a closed system during the timescale of interest ( - 4 0 years), the major portion of the fallout material is associated with lake sediments. The areal deposition and possible relocation of surficial sediments and their associated radionuclides have been studied (Edgington & Robbins, 1975). In addition, studies of sediment trap material collected at many depths in the water column, including samples within a few meters of the sedimentwater interface (Wahlgren et al., 1976), have suggested that considerable resuspension of benthic floc occurs annually in the lake. From these observations, Wahlgren et al. (1976) proposed that the nearly constant plutonium concentration measured in Lake Michigan water between 1971 and 1975 could be accounted for by a continuing exchange reaction between plutonium in the water column and resuspended solid material. A discussion of exchange reactions as a mechanism of control requires a knowledge of how plutonium is distributed among the various solid phases of the sediments. Previous studies of the distributions of these nuclides among chemical fractions of Lake Michigan sediments indicate that plutonium and americium are associated with the hydrous oxide phases of iron and/or manganese in bulk sediments from Lake Michigan (Alberts et al., 1975; Edgington et al., 1976). Those findings are in agreement with results of other studies of transuranic elements in reservoirs (Alberts & Orlandini, 1981) and in soil-ground water systems (Means etal., 1978) and with the general concept of these oxide phases acting as sites of adsorption for many trace elements (Jenne, 1968). However, the previous studies of Lake Michigan sediments dealt only with bulk sediment samples and would detect changes neither in apparent associations as a function of time following deposition nor between floc (the unconsolidated aggregate layer on the sediment surface) and consolidated sediment. This report describes the distribution of 239"24°pu, 241Am and 137Cs among various chemicallydefined fractions of material collected in sediment traps suspended near the lake bottom, in material at different depths in the sediment column and in net plankton collected near the water surface (since these plankton may contribute to the observed transport of plutonium from the epilimnion during late spring and summer).

METHODS Samples of sediment, sediment trap material and plankton were taken in 67 m of water at a station approximately 7 km WSW of Grand Haven, Michigan. The sediment cores were collected with a 12.7 cm diameter Benthos-type gravity corer. Sediment floc and overlying water were

Transuranics in Lake Michigan

91

siphoned off and frozen. The cores were extruded, sliced into sections 1 cm thick and frozen. Suspended particulate material was obtained from a set of sediment traps placed at 60 m depth and left in place for one month prior to retrieval; this material was frozen immediately after collection. Net plankton samples were taken with a 1 m, #25 plankton net, towed for 15 min at 3 m depth. The material thus collected was filtered on W h a t m a n #1 filter paper and the resulting cake immediately frozen. All frozen samples were returned to the laboratory and lyophilized. The ~37Cs activity was determined on subsamples of the dried material by gamma-ray spectrometry, after which the subsamples were analyzed for total plutonium and americium after the methods of Golchert and Sedlet (1972), Nelson et al. (1974) and Wahlgren et al. (1976). The remaining material was fractionated into the ion-exchangeable, reductant-soluble, organic and crystalline fractions as described by Gibbs (1973). The detailed fractionation procedure as modified for radionuclides is given in Alberts et al. (1975). Briefly, the procedure involves the sequential extraction of the dried material with 0-1N MgC12, 0-3N sodium citrate in the presence of sodium dithionite, 0.1N NaOH, and finally fusion of the residue with lithium fluoride and boric acid (Biskupsky, 1965). The solutions resulting from the successive extractions and fusion contain, respectively, the ion-exchangeable, reductant-soluble, organic and crystalline fractions of the sediment. It should be r e m e m b e r e d that these fractions are operationally defined. The use of selective extraction procedures for comparisons of regional elemental distributions must be interpreted carefully (Kheboian & Bauer, 1987). We chose this approach because our samples all came from the same lake environment and we wished to compare our current results with previous studies. However, we do not wish to imply that the 'phases' so defined are conclusive proof that the radionuclides are associated with a specific mineral phase. For example, the reductant-soluble fraction is referred to in the text as the hydrousoxide fraction (usually oxides of iron and manganese), even though it may contain other phases which dissolved under the extraction conditions. Nevertheless, the phrase 'hydrous-oxide fraction' will be used interchangeably with 'reductant-soluble fraction' in the text. The dried plankton sample was divided before the f/~actionation procedure. In one aliquot the cell membranes were disrupted by sonification before extraction, while the other aliquot was left undisturbed. The 137Cs concentration in the subsamples was not determined because its concentration in the total samples was insufficient for gamma-ray spectral analysis. Dry sediment from the 1-2 cm section of a second core (sliced, frozen, lyophilized but not chemically treated) from the same station was taken to

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J..I. Alberts, M. A. Wahlgren, K. A. Orlandini, C. A. Durbahn

determine the plutonium and americium concentrations in humic material in the sediments. Approximately t50 g of dry sediment was extracted with 0.IN N a O H to obtain a large amount of the base-soluble organic matter associated with this sediment. The humic acid fraction of this organic matter was isolated by successive acidification, resolubilization and centrifugation. The resulting material was then dialyzed against deionized water until no increase in conductivity of the dialysis water was detected. The resulting humic acid material (HA) was lyophilized and duplicate subsamples of the material were analyzed for 239"24°puand 24~Am. The 137(~S concentration was not determined for these samples.

R E S U L T S A N D DISCUSSION Plutonium, americium and cesium distributions in sediments and suspended particles The depth distribution of 137Csconcentration in the unfractionated samples (Fig. 1) is similar to those for other cores from this station, which are believed to represent undisturbed sediments which accumulate at a rate of approximately 0-08 cm/year (Edgington et al., 1976). The distributions of total 239'24°pu,238pu and 241Am concentrations with depth in the core are shown in Table 1. The 239'24°puand the 137Cs (Table 2) are positively correlated (r = 0.96) in the consolidated sediment, the sediment floc and the 60-m sediment trap material. This positive correlation is consistent with data presented by Edgington and Robbins (1975) for a series of sediment cores from Lake Michigan. Examination of the distribution of the transuranic nuclides in the various chemical fractions (Table 1) shows that at all depths significant quantities of the total concentrations of nuclides (Table 1) are found in the reductant-soluble fraction (85% for 239"24°pu,70% for 238pu and 75% for 241Am). The one low recovery of 241Am in the 4-5 cm subsection (51% in the hydrous oxides) results primarily from the counting errors associated with the very low total concentration at this depth in the sediment. This latter explanation is given support by the fact that no americium was found in any other fraction at that depth. Like 241Am, 238pu appears to have a relatively low recovery (71%) but is not detected in other chemical extracts in one subsection of the core. We feel that the apparently significant discrepancies are really the result of counting and unavoidable handling errors associated with ultratrace analyte concentrations, as evidenced by the apparent 238purecovery of 124% in the 4-5 cm subsection of the core (Table 1).

93

Transuranics in Lake Michigan 1370S Bq / g dry wt. 0.1

~a

1~1 4i

0.3

0.5

0.7

0.9

///" ////// TOTAL SEDIMENT

5

H

EXTRACTION RESIDUE

6i Fig. l. Variation of 137Csconcentration with depth in Lake Michigan sediment.

The concentration of 137Cs in samples which have undergone the sequential extraction procedure but have not undergone fusion has the same depth distribution in the sediments as the unfractionated samples (Fig. 1). The fact that the plots of the fractionated and unfractionated samples are the same (albeit displaced because the extraction procedure has removed approximately 10% of the mass of the sample) shows that 137Cs is not removed by the extraction procedure. Thus the 137Cs appears to be associated with the crystalline fraction (presumably clay minerals) present in the sediment, as shown to be the case for river sediments (Jenne & Wahlberg, 1968): However, two other explanations may be proposed for the apparent associations of 137Cs with the crystalline fraction reported here, both of these being artifacts of the method. The first requires that ion-exchangeable 137Cs simply re-adsorbs on the crystalline fraction by a reversible ion-exchange reaction during the experiment. The second requires that the 137Cs be associated with the reductant-soluble fraction of the sediment and again re-adsorbs to the crystalline fraction when this fraction dissolves. Desorption and resorption of 137Csdoes occur in reservoirs under

94

]. ]. Alberts. M. A. Wahlgren, K. A. Orlandini, C. A. Durbahn TABLE I

239'24°pu, 238pu and 241Am Concentrations (/~Bq/g Dry Weight) and Distributions among Chemical Fractions a in a Lake Michigan Sediment Core and Solids from a 60 m Sediment Trap

Sample

Total concentration

Reductant soluble

Organic

% Recovery in extracts

239,24Opu 0-1 cm 1-2 2-3 _3-4 4-5 Sed. trap

4 366 + 411 4 836 + 396 6 408 + 459 4455_+311 729 _+81 3 700 _+ 171)

3 963 + 122 4 040 + 122 5 683 _+ 115 3830_+78 648 _+41 3 811 _+370

(90.8) (83.5) (88.7) (86.0) (88.9) (103)

56 (1.3) -37 (0.5) 74(1.7) 52 (7-1) 222 (6.0)

92.1 83.5 89.3 87.6 96-0 109

0-1 cm 1-2 2-3 3-4 4-5 Sed. trap

307 _+ 30 181 _+70 340_ 100 192 _+ 18 33_+ 15 ND b

278_+ 11 185 _+22 266_+ 22 137 + 4 41 + 11

(90.6) (102) (78-2) (71-4) (124)

------

90,6 102 78.2 71.4 124

241Am 0-1 cm 1-2 2-3 3--4 4-5 Sed. trap

651 + 118 1 147_+ 111 1 427 ___181 1 110 + 148 363_+ 67 555 _+ 107

733 + 48 851 + 89 1 332 _+59 1 036 + 66 185__+4 555 _+52

(113) (74-2) (93.3) (93.3) (51:0) (100)

<~19 ~<19 ~<19 ~<19 ~<19 ~
113 74-2 93-7 93.3 51.0 100

238pu

~Fhe concentrations of 239'24°pu,238puand 241Am in the 0-1N MgCI2 extracts were ~<19in all samples. Parentheses after concentrations indicate percentage of total found in that extract. bNot determined.

c e r t a i n n a t u r a l a n a e r o b i c c o n d i t i o n s ( A l b e r t s et al., 1979; E v a n s et al., 1983; A l b e r t s et al., 1987). T o t e s t t h e first a l t e r n a t i v e , t h e e x t r a c t e d s a m p l e s w h i c h h a d n o t b e e n f u s e d ( w h o s e 137Cs c o n c e n t r a t i o n s w e r e s h o w n in Fig. 1) w e r e e q u i l i b r a t e d o v e r n i g h t w i t h s o l u t i o n s c o n t a i n i n g 1 m g o f s t a b l e c e s i u m p e r 50 m l , a f t e r which the solid material was separated, dried and counted. This solid material was then equilibrated overnight with a much more concentrated s o l u t i o n c o n t a i n i n g 1 g o f s t a b l e c e s i u m p e r 50 m l a n d t h e a n a l y t i c a l p r o c e d u r e r e p e a t e d . T h e r e s u l t s f o r t h e d i f f e r e n t s a m p l e s ( T a b l e 2) i n d i c a t e t h a t , w i t h i n c o u n t i n g e r r o r , n o 137Cs w a s r e m o v e d f r o m t h e s o l i d

_+ 0.01 _+ 0.01 + 0.01 + 0.01 + 0-004 + 0-003

0.28 + 0.01

0-30 0.31 0-40 0.23 0.04 0-01

Total

_+ 0-02 + 0.02 + 0-03 + 0.03 _+ 0.02 + 0-02

0.35 + 0-05

0.34 0.36 0.44 0.25 0-05 0-01

Fusion fraction

ND

0.32 + 0.01 0.35 + 0.01 0.46 + 0-01 0-27 + 0.01 0.05 + 0-01 ND b

0-36 + 0-I0

0.30 + 0-0l 0-33 + 0-01 0-41 + 0.01 0-27 + 0-01 0.03 + 0.01 ND

Equil. with stable Cs solution (1 rag~50 ml) (1 g/50 ml)

0.29 + 0-05

0.30 + 0-02 0-32 + 0-02 0.36 + 0.01 0.23 + 0-01 0.04 + 0.01 ND

Extracted with CsCl

TABLE 2 (Bq/g Dry Weight) in May 1975 S e d i m e n t C o r e and in Material f r o m a S e d i m e n t T r a p a

~The stated errors are one standard deviation in the counting statistics. bNot d e t e r m i n e d .

Successive d e p t h s in s e d i m e n t cores 0-1 cm 1-2 cm 2-3 cm cm 4-5 cm 5~5 cm Material from sediment trap s u s p e n d e d at 60 m during M a y - J u n e 1975

Sample

137Cs C o n c e n t r a t i o n s

t~

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J. J. Albert~', M. A. Wahlgren, K. A. Orlandini, C. A. Durbahn

material by either procedure. A fresh subsample of sediment from each depth was treated with 0-1N CsCt instead of MgCI2 to determine if the 137Cs could be removed from the whole sediment. The results again clearly show that no reversible ion-exchange reaction occurs between ~37Cs and these sediments (last column of Table 2). To test the second alternative, namely that the 137Cs was sorbed on the hydrous oxide coatings of the clay particles instead of directly on the clay surfaces themselves, the sediment which had been treated with 0.1N CsCI was treated by the normal citrate-dithionite extraction procedure, except that the extraction solution contained 50 mg of stable cesium per ml. If the 137Cs were associated with the hydrous oxides, the stable cesium would be expected to exchange with the 137Cs following the dissolution of the hydrous oxides but prior to re-adsorption of the cesium onto the solid surfaces present. Even if equilibrium were not obtained, a significant decrease in the 137Csconcentration in the solid material would be expected if the 137Cs were associated with hydrous oxides. However, as with all other treatments, the 137Cs did not exchange with the stable cesium in this experiment but remained quantitatively with the solid component. When the procedures were applied to the material from the sediment traps (bottom line of Table 2), the results, as with the floc and the consolidated sediment, were that no re-equilibration or loss of 137Cs was observed. It appears from these investigations that 137Cs in the sediments of Lake Michigan is tightly bound to the crystalline fraction of these sediments and is not readily released from the solid phase to the lake water, this being consistent with estimates of high distribution coefficients for this nuclide and 'cleaned' clays (Garder & Skulberg, 1964). The distributions of 239"24°pu and 241Am in the various chemical fractions of the sediment trap material are similar to those for the consolidated sediment (Table 1). Furthermore, the concentrations of 239"24°pu, 241Am and 137Cs and the 239"24°pu/241Am ratio (Table 3) are the same in both the sediment trap material and the top centimeter of sediment. Since 241Am is formed by the decay of 241pu, which has a radioactive half-life of 13 years, sediment trap material representative of one month's collection of only new input would not be expected to have 241A m concentrations similar to that of material which had been accumulating at the sediment surface for approximately 5 years. Therefore, the concentration and ratio data indicate that the sediment trap collection must be strongly influenced by resuspension of material from the bottom. The 238pu/239'24°pu activity ratio (Table 3) agrees with values previously reported for this ratio in Lake Michigan sediment (Edgington & Robbins, 1975). The 239"24°pu/241Am activity ratio decreases with depth at the rate expected as a result of the growth of 241Am from its parent 241pu in an

Transuranics in L a k e Michigan

97

TABLE 3 R a t i o s of P l u t o n i u m a n d A m e r i c i u m Activities in S e d i m e n t a n d S e d i m e n t T r a p M a t e r i a l f r o m L a k e Michigan Sample Successive d e p t h s in s e d i m e n t cores 0-1 cm 1-2 cm 2-3 cm 3 - 4 cm 4 - 5 cm M a t e r i a l from s e d i m e n t trap s u s p e n d e d at 60 m d u r i n g M a y - J u n e 1975

238pu?39"2 4°Pu

0-074 0.037 0.053 0-043 0-046

_ 0.025 _+ 0.015 +_ 0.016 + 0.014 + 0.021

ND"

239'24°pu/2 41A m

6.7 4-2 4-5 4-0 2-0

_+ 1.4 _ 0-5 +_ 0.7 + 0.6 + 0.4

6.7 _+ 1.3

"Not d e t e r m i n e d .

undisturbed system. This trend supports the earlier assumption that the core under investigation is representative of undisturbed sediment below the floc layer. In addition, the agreement with the expected decrease of the 2 3 9 ' 2 4 ° p u / Z 4 t A m ratio with depth indicates that neither americium nor plutonium is moving independently, even a decade after deposition of the sediment. This observation contradicts Livingston and Bowen (1977), who claimed that plutonium was diffusing out of Lake Ontario sediments, perhaps via interaction with humic acids, while americium was not. Plutonium and americium associations with humic acids

Cleveland and Rees (1976) have shown that plutonium and americium are only slightly solubilized by fulvic acids in soils and that the solubilized material is unstable in solution. Edgington et al. (1976), who measured grab samples of Lake Michigan sediment, found that some of the plutonium and americium was associated with the humic and fulvic acids. In our study, purified humic acid from the sediment was analyzed for plutonium and americium to determine the possible role humic material in Lake Michigan sediments might play in mobilization. On a relative weight basis (Table 4), the nuclide concentrations in the organic matter are approximately 2-3 times those in the whole sediments; this is consistent with the results of other researchers (Alberts & Orlandini, 1981). However, this organic matter (the N a O H fraction of the sequential extraction) is a very minor constituent of the total sediment

98

J. J. Alberts, M. A. Wahlgren, K, A. Orlandini, C. A. Durbahn

TABLE 4 239'24°pu and 24tAm Concentrations'~ in the Purified Humic Acid Component of Lake Michigan Sediment

239'24°pu 241Am 239"24°pu/24tAm

Sample A b

Sample B

12,0 _+0.89

14.7 + 1.03

2.41 + 0-41 5.2 -+ 1.t)

2.89 +_0,41 5-1 __+l.O

"All values given as mBq/g dry wt of humic acid + Itr counting error. bSamples A and B are replicate humic acid samples obtained from one isolation procedure on a sediment sample.

(organic carbon .content < 0.5% ), so it contains only a small percentage of the total plutonium and americium despite the higher concentrations. The 239"24°ptl/241Am ratio in humic material is approximately 5.0 (Table 4), in agreement with the value for the corresponding section (1-2 cm) of the sediment core (Table 3). Thus the plutonium and americium isotopes in the humic acid fractions are bound to the organic molecules to the same extent and apparently do not move relative to each other after deposition. Formation of complexes is not the only mechanism by which natural organic matter may interact with metals to affect solubility; another is by oxidation-reduction reactions. Many investigators (Szilagyi, 1971; Schnitzer & Khan, 1972; Alberts et al., 1974; Nash et al., 198t) have shown that humic acids are capable of reducing certain elements and stabilizing them in soluble form in the natural environment. Plutonium has been reduced by fulvic acids under laboratory conditions (Bondietti et al., 1976) and naturally occurring organic matter appears to control the reduction of plutonium in Lake Michigan sediments (Penrose et al., 1987). However, reduction of Pu to the IV oxidation state will make it more immobile than in higher oxidation states (Nelson & Lovett. 1981).

Phase d i s t r i l m ~ ~ plutemmm in net ptankton Plutonium and americium in Lake Michigan consolidated sediment, floc and sediment trap material exist primarily in the citrate-dithionite extractable phases, Three possible mechanisms may be envisioned for the mechanism of association offatlout nuclides with these coatings. First, the nuclides may be actively or passivelytaken up by plankton, transported to the sediments when the organisms die and there resorbed on existing hydrous oxides. Second, hydrous oxides may form on detrital particles and

Transuranics in Lake Michigan

99

then act as sorption sites as these parti¢les fall through the water column. Third, hydrous oxides may form amorphous precipitates which either coprecipitate or sorb the nuclides in the water column. The latter two mechanisms represent chemical scavenging while the first is a case of biological scavenging. Planktonic organisms concentrate plutonium in the surface waters of Lake Michigan (Yaguchi et al., 1974). Since these organisms represent the initial concentration step of plutonium by particles in the epilimnion, plankton samples were subjected to sequential extraction to determine how the plutonium was distributed among the different fractions of these organisms. Only 50% of the plutonium in plankton was associated with the reductant-soluble extracts (Table 5), while at least 90% was associated with this extractant in sediments. The MgCI2 extract, which contains approximately 14% of the plutonium, presumably represents material that was inside the cell wall or loosely associated with surfaces. Approximately 6% of the plutonium was found in the extraction residue of both the intact and disrupted cells, presumably associated with the siliceous test of the plankton or attached clay minerals. The only fraction for which the concentrations in these two samples differed was that obtained with NaOH. The difference may represent plutonium incorporated in organic coatings which remained on the tests of intact cells but were disrupted and removed by the ultrasonic dispersion. The plutonium concentrations in the four fractions add up to the total, within the errors of measurement, while americium was not detectable in the plankton sample. The distribution of the plutonium among the fractions from the plankton samples may not be directly comparable to sediments (Kheboian & Bauer, 1987). However, the distribution of plutonium in plankton is unlike that in the sediment (the MgC12 fraction accounting for <0-5% of TABLE 5 Distribution of 239'24°pu Concentration (/zBq/g Dry Weight) in Extractable Fractions of Lake Michigan Net Plankton (Station 5, October 1975) a

Total sample

MgCle fraction

Reductantsoluble fraction

NaOH fraction

Extraction residue

Intact cells

226_+30

Disrupted cells

226+30

30+7 (13.3) h 33+7 (14-6)

115+11 (50-9) 111+11 (49-1)

10+3 (4.4) <2 (<0-9)

15+4 (6-6) 15+4 (6.6)

aThe stated errors are one standard deviation in the counting statistics. h% of total sample.

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.I.J. Alberts, M. A. Wahlgren, K. A. Orlandini, C. A. Durbahn

the plutonium in suspended material caught in sediment traps and surficial sediments) and is different from previously reported sedimentary distributions of this element (Alberts et al., 1975; Edgington etal., 1976; Alberts & Orlandini, 1981). The fact that 4.4% of the plutonium in intact cells is in the organic fraction resembles the situation in sediment trap material (6.0%, Table 1) more than that in surface sediments (0-1 cm, 1.3%. Table 1) and may be indicative of a real shift from organic to inorganic association of plutonium from water column to sediments. However, the relative importance of biological versus chemical scavenging in the transport of nuclides from the epilimnion to the sediments cannot be assessed until more information is obtained. Nevertheless, it does seem that plutonium associated with plankton is not simply adsorbed onto the organisms and subsequent changes in plutonium distribution must occur before the distribution in chemically-defined fractions of the bulk sediments is obtained.

CONCLUSIONS A study of the distribution of 238'239"24°pu, 241Am and 137Cs among the different chemically-defined constituents of the surficial floc layer, of different depth increments in the consolidated sediment and of material collected in sediment traps in Lake Michigan shows that the bulk of the transuranic elements is found in the citrate-dithionite extract. This fraction is believed to represent hydrous oxides, which coat sedimentary particles. These ubiquitous particle coatings may act as a sedimentary sink for these radionuclides in the aerobic waters of Lake Michigan. In contrast to the transuranics, 137Cs was found to be strongly associated with the chemically defined 'fusion" fractions of the sediments--most likely clay minerals. The distribution of plutonium among the constituents of a net plankton sample was more evenly divided than in either bulk sediment or sediment trap material, Since plankton decompose and constitute only a small portion of the permanent sediment, additional chemical processes apparently occur between the initial plutonium concentration by the biota of the epilimnion and the final deposition of plutonium in the abiotic sedimentary sink. The concentrations of plutonium and americium are about twice as high in sedimentary humic material on a relative weight basis as in bulk sediment but the nuclides exhibit the same activity ratio in both materials. Based on total concentrations, it appears that associations of transuranic elements with organic components are relatively unimportant in these Lake Michigan sediments.

Transuranics in Lake Michigan

101

The similarities in total concentrations of the nuclides for the chemically-defined fractions and in activity ratios of the isotopes within the ~oc, consolidated sediments and sediment trap material are consistent with the concept of resuspension of unconsolidated sediment into the water column. There is no indication that plutonium, americium or cesium are diffusing from the sediment into the overlying waters, unless the diffusion rates are identical for these different chemical elements.

ACKNOWLEDGEMENTS W e wish to thank Captain E. D u n s t e r and Earl Wilson of the University of Michigan's R/V Mysis for their patience and invaluable assistance in the field program. W e also wish to acknowledge the assistance of G. T. Tisue, D. M. Nelson and J. O. Karttunen in various aspects of the field and laboratory studies and the Center for Educational Affairs at A r g o n n e National L a b o r a t o r y for providing partial support for C. A. D u r b a h n . This research was s u p p o r t e d in part by the US D e p a r t m e n t of Energy, Office of Health and Environmental Research contract W-31-109-ENG38 with A r g o n n e National Laboratory. This is contribution No. 610 of the University of Georgia's Marine Institute, Sapelo Island, G A .

REFERENCES Alberts, J. J., Schindler, J. E., Miller, R. W. & Nutter, D. E. (1974). Elemental mercury evolution mediated by humic acid. Science, 184, 895-7. Alberts, J. J., Wahlgren, M. A., Reeve, C. A. & Jehn, P. J. (1975). Sedimentary 239'24°pu phase distributions in Lake Michigan sediments. In Radiological and Environmental Research Division Annual Report, ANL-75-3, Part IIl, JanuaryDecember 1974, pp. 103--12, Argonne National Laboratory. Alberts, J. J., Tilly, L. J. & Vigerstad, T. J. (1979). Seasonal cycling of cesium-137 in a reservoir. Science, 203,649-51. Alberts, J. J. & Orlandini, K. A. (1981). Laboratory and field studies of the relative mobility of 239"24°pu and 241Am from lake sediments under oxic and anoxic conditions. Geochim. Cosmochim. Acta, 45, 1931-9. Alberts, J. J., Bowling, J. W. & Orlandini, K. A. (1987). The effect of seasonal anoxia on the distribution of the radionuclides 2 3~8 Pu, 2 3-9 '24(1Pu, 2 4 l Am, 2 4 4 Cm and 137Cs in pond systems of the southeastern United States. In Environmental Research on Actinide Elements, ed. by J. E. Pinder, III, J. J. Alberts, K. W. McLeod & R. G. Schreckhise, pp. 371-90, US DOE CONF-841142, NTIS, Springfield, VA. Biskupsky, V. A. (1965). Fast and complete decomposition of rocks, refractory silicates and minerals. Anal. Chim. Acta, 33,333-4. Bondietti, E. A., Reynolds, S. A. & Shanks, M. H. (1976). Interaction of

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.I.J. Alberts, M. A. Wahlgren. K. A. Orlandini, C. A. Durbahn

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