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Distribution of Plutonium and its oxidation states in Framvaren and Hellvik fjords, Norway
J. Environ. Radioactivity 22 (1994) 205 -217
Distribution of Plutonium and its Oxidation States in Framvaren and Hellvik fjords, Norway
Arthur L. Sanchez* & Janine Gastaud IAEA Marine Environment Laboratory, 19, avenue des Castellans, MC98000, Principality of Monaco
Elis Holm & Per Roos Department of Radiation Physics, Lund University, Lurid, Sweden (Received 4 November 1992; revised version received 11 February 1993; accepted 3 March 1993)
ABSTRACT Waters from the Skagerrak Sea entering the system of Ijords inside Farsund in the southern tip of Norway (including Lyngsdal, Hellvik and Framvaren I~ords) contain plutonium derived from European nuclear fuel reprocessing facilities in the United Kingdom and France, as shown by the 238pu/e39'24°pu activity ratios measured.]'or water samples from Hellvik and Framvaren.fjords. The shallow sills" interconnecting this series of I)ords, however, severely restrict water exchange. Thus, at the northernmost Framvaren fjord (connected to Hellvik J)'ord by a 500-m channel o1"2 m depth), the 238pu1239'240pusignature for Pu derived from fuel reprocessing is li)und only in the surface layer (upper 20 m) while global fallout Pu ratios are observed in the permanentO, anoxic zone of this'fjord. Plutonium oxidation state measurements in waters collected from Hellvik and Framvaren l)'ords show that water exchange occurs between these two basins by a process 01"interleaving, with subsequent reduction (T]'oxidized Pu species as these reach the anoxic zone. Plutonium serves as a uselul tracer 1or water exchange between these fjords and the Skagerrak Sea. *Present address: Institute of Terrestrial Ecology, Merlewood Research Station, Grangeover-Sands, Cumbria LAI 1 6JU, UK. 2O5 J. Environ. Radioactivity 0265-931X/94/$07-00 'c~ 1994 Elsevier Science Limited, England. Printed in Ireland.
206
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i NTROD[)CTION Plutonimn is a redox-sensitive radionuclide known to exhibit severai oxidation states in the environment. An analytical method based on selective coprecipitation on rare earth fluorides (Lovett & Nelson, 1981i has been used to distinguish the reduced Pu(IIl and IV) from the oxidized Pu(V and V1) species in environmental samples. This technique has shown that Pu exists in both oxidation state categories at varying proportions m various aquatic systems (e.g. Nelson & Lovett, 1978; Nelson et al., 1984, 1989; Cochran e l a l . , 1987: Fukai e t a l . , 1987: Noshkin eted., 1987: Mitchell et al., 1991). in oxic waters, the oxidized species tend to predominate: recent evidence suggests that Pu(V) dominates this fraction (Morse & Choppin, 1986; Orlandini et al., 1986; Choppin & Kobashi, 1990), although it has been suggested that Pu(V) exists in a metastable state which becomes slowly reduced to Pu(IV) in the presence of humic substances or other reducing agents. Our recent work in the Black Sea (Sanchez et al., 1991) has shown that Pu responds to changes in redox conditions in the water column, with oxidized forms found in the suboxic layer gradually becoming reduced in the anoxic zone. We have inferred that these redox state transformations provided evidence for the rapid lateral advective processes occurring at intermediate depths in the Black Sea water column. Thermodynamic calculations (e.g. Rai et al., 1980: Edgington, 1981) also predict that Pu(V) is the predominant form of Pu in oxic natural waters and that the reduced species predominate under reducing conditions. The major objective ot" this study was to assess the processes controlling the distribution of Pu in an anoxic basin. Framvaren, a permanently anoxic 0ord in southern Norway with a maximum depth of 183 m, provided an ideal location for this study because much is known of its hydrochemical regime. The distribution of various chemical species in Framvaren basin is controlled by several factors, including water exchange, supply and decomposition of organic matter, sorption and desorption processes on biotic and abiotic particles, the oxidation-reduction (redox) potential, and for radioactive species, the decay rate (e.g. Jacobs & Emerson, 1985; Anderson & Dyrssen, 1987; Anderson el al., 1988; Landing & Westerlund, 1988; Todd el al., 1988; McKee et at., 1991 ). All these processes are known to influence the geochemical cycling of Pu in natural waters. A sampling site was also selected at the adjacent Hellvik fjord (maximum depth 29.2 m), which is separated from Framvaren f~ord by a 2-m sill and which serves as a source of seawater entering from the Skagerrak Sea. Two possible sources of Pu in Framvaren and Hellvik fjords are global
Plutonium distribution in Framvaren and Hellvik.[jords
207
fallout from nuclear weapons testing and discharges from European nuclear fuel reprocessing facilities at Sellafield and Dounreay (UK) and La Hague (France). Murray and co-workers (1977, 1978, 1979) and Nies (1990) have reported that waters in the North Sea and Skagerrak Sea contain Pu derived from fuel reprocessing facilities, with a characteristic 23Spu/239'Z4°pu activity ratio of about 0.20. This ratio is significantly different from the 238pu/239'24°pu ratio of 0.036 for global fallout Pu in the northern hemisphere (Sholkovitz, 1983). Presumably, Pu derived from reprocessing facilities and advected into the North Sea would occur primarily in the oxidized ( V + V I ) states, as the particle-reactive Pu(III + IV) species tend to be rapidly removed from the water column following their interaction with suspended particulate matter. Mitchell et al. (1991) have shown that 80-90% of soluble Pu in the Irish Sea, derived primarily from Sellafield releases, is in the oxidized state.
S A M P L I N G A N D ANALYSIS Samples were collected during a multidisciplinary cruise organized by the Norwegian Institute for Water Research in May-June 1989. Water profile samples (100 litres volume) were collected for Pu oxidation state measurements from selected depths in Framvaren and Hellvik fjords using a 100 litre Go-Flo sampler (see Fig. 1 for sampling locations). The samples were obtained from the surface down to 60 m in Framvaren fjord (the depth of the O2-H2S interface was at 20 m in June 1989), and from the surface down to 25 m in Hellvik fjord. To minimize changes in redox conditions of the water samples that might affect Pu oxidation states, samples were processed without prior filtration within 2 h after collection, using the rare earth fluoride coprecipitation method (Lovett & Nelson, 1981). This method is based on the selective adsorption of reduced Pu(III + IV) species on LaF3. A sequential separation is performed where the sample is first made oxidizing by adding appropriate quantities of reagents to produce concentrations of 0.8 M in HNO3, 0.25 M in H2SO4, 0.0005 M in K2Cr20 7 (this medium will maintain Pu(V + VI) and the 242pu(VI) tracer in the oxidized state while the reduced Pu(III + IV) and the 236pu(IV) tracer are coprecipitated on LaF3). The oxidized Pu species (and tracer) are subsequently reduced using ferrous ammonium sulphate and coprecipitated with LaF3. The LaF3 precipitates are collected on 0-45-/~m cartridge filters. A distilled water blank sample was also processed similarly. Additional water samples for total Pu and other measurements were also collected. In Framvaren fjord, these included samples down to 175 m
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depth in order to obtain an inventory of Pu in the water column. Plutonium was measured in these samples following coprecipitation with Fehydroxides (Wong, 1971 ). For the anoxic samples from 40 m and below in Framvaren fjord, air was bubbled to remove H2S before coprecipitation. At the laboratory, standard radiochemical procedures were used to isolate and purify Pu from the LaF3 and the Fe-hydroxide precipitates prior to alpha spectrometry. These included purification on anionexchange resins following dissolution of the precipitates in mineral acids (Wong, 1971; Lovett & Nelson, 1981) and electrodeposition on stainless steel planchettes (Talvitie, 1972). For the oxidation state measurements, we found no significant ( < 1%) crossover between the oxidized (V + Vl) and the reduced (IIl + IV) Pu species.
Plutonium distribution in Framvaren and Hellvik )¢jords
209
RESULTS A N D DISCUSSION Based on dissolved 02 and H 2 S measurements at the time of our sampling, the oxic- anoxic interface in Framvaren fjord was located at 20 m depth; in Hellvik fjord, the water was slightly sulphidic below 11 m (J. Skei, pets. comm.). The results of our Pu measurements are summarized in Tables 1~1. Total Pu (as determined by Fe-hydroxide coprecipitation on unfiltered samples) are reported for the isotopes 23Spu and 239,240puin Tables 1 and 2. The isotope 238pu was detected in all samples analysed for total Pu, but at significantly lower levels than for 239,240pu" In a separate measurement for Pu in suspended particles collected using a 1-/,m cartridge filter, we estimated that between 5 and 60% of the total Pu in the two fjords were associated with the particulate fraction. The oxidized and reduced Pu concentrations (as measured by selective coprecipitation on LaF3) are reported for the isotopes 239,240pu in Tables 3 and 4. In general, we found good agreement (i.e. within the reported error) between the total 239,240pu TABLE 1 Distribution of Total Pu in the Water Column of Framvaren Fjord
Depth
Salinity (%0)
23< 249pu
23# p u
(m)
(mBq m 3)
(mBq m 3)
2 10 20 40 60 80 110 140 175
9 11.3 16-4 18.8 20.4 20-5 21.6 22,4 22,6
7-4 (1.2) 5-6 (1.0) 6-4 (1.4) 27 (3) 61 (6) 113 (8) 290 (20) 362 (25) 410 (30)
0-8 0.4 0.3 0.7 2-5 3-5 8.6 10.6 ll.6
(0.4) (0-2) (0.3) (0,3) (0,6) (0.8) (1.4) (1.8) (1.8)
Activity ratio
0-11 (0-05) 0.07 (0.04) 0.05 (0.04) 0.03 (0.01) 0.04 (0.01) 0-03 (0-01) 0.030 (0.004) 0.029 (0-005) 0-028 (0.005)
Values in parentheses represent one standard deviation counting error. TABLE 2 Distribution of Total Pu in the Water Column of Hellvik Fjord Depth (m)
Salinity (%0)
239"24~P21 (mBq m -~ )
e ~aPu (mBq m 3)
2 7 II 18 25
22 26-5 27.8 28.7 29.6
10-6 (1.3) 11-7 (1.8) 5-4 (0.8) 7-9 (2.4) 7.2 (0-9)
1.5 (0.5) 2-1 (0-8) 1.1 (0.4) 1-4 (1.0) 1-1 (0-3)
Values in parentheses represent one standard deviation counting error.
Activity ratio
0.14 0-18 0.20 0.18 0-15
(0.04) (0.07) (0.08) (0.14) (0-04)
210
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TABLE 3 Distribution of 2~,~ ,4 ~Pu Oxidation Sta~es in the Water Column of Framxarcla F j(~rd Depth (m)
P u ( H I ~ 1l'~ ( m B q m ~ ,'
P u / lIt1) ( m B q m ~)
R e d u c e d Pu (!!4,)
( ) x i d i z e d 1-'~ % J
2 10 20 40 60
6-8 (3.6) 3-5 (1.9) 2-5 (t .4) 12.1 (6.4) 18.(/(9.5)
6.1 (1.1) 4.0 (0.9) 3.5 (0.7) 8.3 11.3) 11 (2)
53 47 42 59 62
47 53 58 41 38
Values in parentheses represent one standard deviation counting error. TABLE 4 Distribution of 239, 24Opu Oxidation States in the Water Column of Hellvik Fjord Depth (m)
2 7 11 18 25
Pu(Ill + IV) ( m B q m 3 j~
2.7 5-3 1.2 1.7 3.0
(1.5) (0-5) (0-7) (l.11 (1.7)
P u t V + ~'7) (mBqm ;)
9-5 10.7 10.2 4.2 4-5
(1-5) (1.6) (2.0) (0.8) (0.9)
Reduced Pu (%)
Oxidized Pu Jo ' ( '~"
22 33 11 29 40
78 67 89 71 60
Values in parentheses represent one standard deviation counting error.
concentration and the sum of (oxidized + reduced) 239'24°pu species at those depths where samples for both measurements were collected. The error terms associated with these measurements are mainly from counting statistics, with corrections for the distilled water blank values; we note that some measurements showed rather significant ( > 50%) error terms. Three samples (2 m in Framvaren; 7 and 1 1 m in Hellvik fjord) showed slightly higher levels of (oxidized + reduced) Pu compared to total Pu. One sample (60 m in Framvaren) showed significantly less (oxidized + reduced) Pu than total Pu; we suspect that the rare earth fluoride separation method might not be applicable under the highly reducing condition of this particular sample (it contained 0.5 mM HzS). Most of the discussion to follow will be based on the trends observed for the isotopes 239'24°pu. The levels of total Pu in the upper 20 m of Framvaren fjord and in the entire water column of Hellvik fjord are similar, ranging between 5.4 and 11.7 mBq m -3. In general, these values are low and comparable to measurements reported for surface waters in the North Sea (Nies, 1990) and in waters around Denmark (Aarkrog et al., 1982). In contrast, Pu levels in the permanently anoxic layer of Framvaren are significantly higher, showing a trend of increasing Pu
Plutonium distribution in Framvaren and Hellvik J)ords
211
concentration with depth. The sample from 175 m showed the highest Pu concentration; samples from 40 m to 140 m showed intermediate levels. Similar high concentrations of Pu were found in the anoxic waters of a meromictic lake in Washington State (Sanchez et al., 1986). The water column inventory of total 239,240pu in Framvaren fjord based _9 on the profile measurements is estimated to be 35-7 Bq m ", with over 99% of this inventory contained in the permanently anoxic zone (20 m and below). The expected global fallout 239,240pu delivery to the 50-60"N latitude band is 48 Bq m -2 (Hardy et al., 1973); thus, around 75% of the expected Pu fallout delivery to Framvaren fjord is within the water column, primarily in the anoxic zone. This observation is in sharp contrast to our results from the Black Sea, where we estimated that less than 10% of the Pu inventory is found in the water column (Sanchez et al., 1991). Similar to Framvaren fjord, the Black Sea water column is permanently anoxic (from about 100 m depth), although the levels of H2S are much higher in Framvaren. The elevated levels of Pu found in the anoxic waters of Framvaren are presumably due to complexing with dissolved organic matter and/or the enhanced solubility of the Pu(IlI) oxidation state. The b o t t o m waters of this fjord contain up to 17.5 mg litre ~ of humic substances (Haraldsson & Westerlund, 1988). Nelson et al. (1985.) have shown that colloidal organic carbon at concentrations commonly encountered in natural waters (1-20 mg litre -l) interact strongly with reduced Pu(IlI + IV) species and inhibit their sorption onto suspended particles. Mudge et al. (1988) have also shown that natural colloidal organic carbon and humic substances remobilize Pu from contaminated estuarine sediments. Nash et al. (1988) also suggest that Pu(III), which tends to be more soluble compared to Pu(IV), would predominate under reducing conditions. Pu speciation calculations predict that Pu(III) is strongly complexed by humic acids (Morse & Choppin, 1986; Choppin et al., 1986). We noted a distinct difference between the 238pu/239'24°pu ratios in the two fjords. In Hellvik fjord, the ratios ranged between 0-14 and 0.20 (average: 0.17 ± 0.02) for the entire water column. In contrast, the 238pu/239'24°pu ratios in Framvaren fjord waters at and below the oxicanoxic interface ranged between 0.028 and 0.04; the ratios were slightly higher in the top 20 m samples (Table 1). The 238pu/239'24°pu activity ratios found in Hellvik fjord are similar to ratios reported for North Sea waters in 1987 (Nies, 1990), suggesting that exchange of waters between this fjord and the North Sea/Skagerrak Sea must be occurring. As discussed earlier, these activity ratios indicate Pu derived from European nuclear fuel reprocessing plants. According to Murray and co-workers (1977, 1978, 1979), actinides released from these reprocessing facilities are
212
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transported to the North Sea by currents from the northern tip of Scotland and from the English Channel. The distributions of Pu oxidation states in the two fjords, shown ili Tables 3 and 4, are also distinctly different and showed some interesting features. We emphasize, however, that due to the magnitude of the error terms in some of these measurements, we can only note these as apparem trends. In Hellvik (jord, oxidized Pu species predominate in the enthe water column, even though the water below 11 m was apparently anoxic at the time of sampling. We did note a decreasing trend in the proportion of oxidized Pu species in the bottom 18 and 25 m samples. In contrast, the Framvaren samples showed the following Pu oxidation state distributions: (1) at the surface (2 in), almost equal proportions of reduced and oxidized Pu species were found; (2) at 10 and 20 m, oxidized Pu at amounts slightly higher than for the surface were measured; and (3) at 40 and 60 m. reduced Pu species predominate. We did not collect samples from below 60 m as we expected that oxidized Pu would be unstable to reduction under the highly reducing conditions below this depth (the reported sulphide concentrations at 60 m and below in Framvaren range between 0.5 and 8 mM H2S; Skei, 1988). In addition, as noted earlier, we suspected that the rare earth fluoride separation method might not be applicable under these conditions. To explain the Pu oxidation state profile in Framvaren fjord, it is helpful to consider the process of water exchange between Hellvik and Framvaren fjords, as described by Stigebrandt & Molvaer (1988). According to these authors, water transport between the two basins is driven primarily by fluctuating sea-level differences (the varying sea level acting as a pump). Waters drawn from the upper few metres of the two basins are exchanged following these fluctuations in sea level. Furthermore, water entering Framvaren from Hellvik fjord would be normally denser (more saline) than the surface water in Framvaren. This water would thus tend to flow along the bottom as a density current, and in the process entrain surrounding water (and become lighter). When this water attains the same density as the surrounding water, it is interleaved into the interior of Framvaren fjord. Stigebrandt & Molvaer (1988) distinguished four water masses in Framvaren fjord based on the salinity profile: (1) the surt:acc water (O 2 m) with low salinity, (2) the intermediate water (2 18 m) with strong vertical gradients in O2 content and salinity, (3) the deep water (18--80 m), and (4) the bottom water (80-183 m). The features of our Pu oxidation state profile suggest that this type of water exchange might be occurring. We could envisage a simple linear mixing model, taking as end members surface water at Hellvik fjord ( S = 22%0, containing 78% oxidized Pu) and surface water of Framvaren
Plutonium distribution in Framvaren and Hellvik fiords
213
fjord (S -- 9%0, containing 47% oxidized Pu). These two end members mix to produce waters of intermediate salinity, containing intermediate amounts of oxidized Pu. Assuming that no significant Pu reduction occurs after mixing, this model would predict that water at 10 m (S = 11-3%o) should contain about 52-53% oxidized Pu, and water at 20 m (S = 16-4%o) should contain about 64-65% oxidized Pu. Our actual measurements show 53% and 58% oxidized Pu species at these depths, respectively. Although these measurements are not significantly different (within the error terms), it is interesting to note that the profiles support the mixing model envisaged here. The proportion of oxidized Pu species measured at 20 m is less than the model prediction, but this intuitively makes sense considering that oxidized Pu is unstable to reduction. Without this type of mixing, it would be difficult to explain how we obtain a proportionately greater a m o u n t of oxidized Pu species at the subsurface depths of 10 and 20 m compared to the surface water. If wind mixing were important (unlikely due to the salinity gradients), we should expect to find similar proportions of oxidized and reduced Pu species at the top 20 m layer. In the permanently anoxic zone below 20 m, the high levels of sulphide should favour the reduction of oxidized Pu. However, the water samples collected from these depths still contained around 40% oxidized Pu. Again, the mixing model could partly explain this observation, with Hellvik-derived water as the source of oxidized Pu at these depths. These results would also suggest that reduction of oxidized Pu species must be occurring with slow kinetics even in the presence of sulphides; however, at the time of sampling, we had no indication as to how long it has been since these waters have interleaved into the anoxic layer. This type of water mixing also appears to be occurring in Hellvik fiord. We note that, as in Framvaren fjord, oxidized Pu species are at a maxim u m at an intermediate depth (11 m) in Hellvik fjord. Although we do not have any measurements for water entering from the Skagerrak Sea, it is reasonable to assume that this water, being more saline, would mix to intermediate depths at Hellvik fjord, bringing in a high proportion of oxidized Pu species in the process. At greater depths, the reduction of these oxidized Pu species also seems to become important. Since reduced Pu species tend to be particle-reactive, our results suggest that Pu found in the bottom waters of Hellvik fjord could be immobilized into the sediments and would not be available for transport into Framvaren fjord. Preliminary measurements do indicate that the inventory of Pu in the surface sediments of Hellvik fjord is significantly higher than in Framvaren (P. Roos, pers. comm.). In general, our observations suggest that Pu and its oxidation states prove to be a useful tracer for the water exchange between these fjords and
214
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5'J,~hc_: . l {.~/wam/. /(. It~J#,l. P. Roo.s
with tile Skagerrak Sea. Better constraints on the Pu oxidation state dat,~ would help to delineatc the processes clearly. We have also t~oted that Jqi derived from reprocessing facilities appears to be effectivel3 trapped i~, Hellvik (jord and is not transported significantly into Framvaren 17iota!, due to the limited exchange of surface waters between the two basin>,. Based on our data. it appears possible to investigate the extent and history of water exchange and transport between these l]ords and the Skagerrak Sea by looking at the sediment record of Pu in these basins.
CONCLUSIONS The measurements reveal that several processes influence the overall balance and geochemical cycling of Pu in the water column of Hellvik and Framvaren fjords: (1) Water exchange with the Skagerrak Sea. This water exchange process brings in Pu derived from European nuclear fuel reprocessing plants, primarily in the oxidized state. A major fraction of this Pu, however, appears to be trapped inside Hellvik 0ord, and reaches Framvaren only in the surface layer. Thus, global fallout Pu ratios in the anoxic zone of Framvaren fjord are relatively unaffected by Pu derived from reprocessing facilities. (2) Oxidation-reduction reactions. The measurements reveal that in general, oxidized Pu species entering from the Skagerrak Sea would become reduced in the water column. In Hellvik fjord, the reduced species are presumably deposited to the sediments and cannot reach Framvaren fjord, particularly since water exchange occurs via a shallow (2 m) channel. (3) Interactions with dissolved/colloidal organic matter. The enhanced levels of Pu in the anoxic waters of Framvaren fjord are most likely a result of complexing with dissolved organic matter. There is sufficient experimental evidence from several workers showing the importance of such interactions. In addition, dissolved organic carbon could serve potentially as a reducing agent for oxidized Pu species entering Framvaren from Hellvik fjord.
ACKNOWLEDGEMENTS We thank Dr Jens Skei (Norwegian Institute for Water Research, Oslo) for coordinating the sampling programme at Framvaren fjord. We also thank Kjell-Ake Carlsson (Lund University, Sweden) for help with sample
Plutonium distribution in Framvaren and Hellvik.[jords
215
collection and processing. We gratefully acknowledge travel and partial financial support from the Swedish Radiation Protection Institute through a grant to Dr Elis H o l m (Department of Radiation Physics, Lund University). The I A E A - M a r i n e Environment L a b o r a t o r y operates under an agreement between the International Atomic Energy Agency and the government of the Principality of M o n a c o .
REFERENCES Aarkrog, A., Dahlgaard, H. & Nilsson, K. (1982). Studies on the distribution of transuranics in the Baltic Sea, the Danish Belts, the Kattegat and the North Sea. In Transuranic Cycling Behaviour in the Marine Environment, IAEATECDOC-265. IAEA, Vienna, pp. 23-32. Anderson, L. G. & Dyrssen, D. (1987). Formation of chemogenic calcite in superanoxic seawater - - Framvaren, southern Norway. Marine Chem., 20, 361-76. Anderson, L. G., Dyrssen, D. & Hall, P. O. J. (1988). On the sulfur chemistry of a super-anoxic fjord, Framvaren, south Norway. Marine Chem., 23, 283-93. Choppin, G. R. & Kobashi, A, (1990). Distribution of Pu(V) and Pu(VI) in seawater. Marine Chem. 30, 241-7. Choppin, G. R., Roberts, R. A. & Morse, J. W. (1986). Effects of humic substances on plutonium speciation in marine systems. In Organic Marine Geochemistry, ed. M. L. Sohn. American Chemical Society, Washington, D.C., pp. 382-8. Cochran, J. K., Livingston, H. D., Hirschberg, D. J. & Surprenant, L. D. (1987). Natural and anthropogenic radionuclide distributions in the North West Atlantic Ocean. Earth Planetary Sci. Letters, 84, 135-52. Edgington, D. N. (1981). Characterization of transuramic elements at environmental levels. In Proceedings of the Symposium on Techniques for Identifying Transuranic Speciation in Aquatic Environments, 24--28 March 1980, Ispra, Italy. IAEA, Vienna, pp. 3-25. Fukai, R., Yamato, A., Thein, M. & Bilinski, H. (1987). Oxidation states of fallout plutonium in Mediterranean rain and seawater. Geochem. J., 21, 51-7. Haraldsson, C. & Westerlund, S. (1988). Trace metals in the water columns of the Black Sea and Framvaren Fjord. Marine Chemistry, 23, 417-24. Hardy, E. P., Krey, P. W. & Volchok, H. L. (1973). Global inventory and distribution of fallout plutonium. Nature, 241,444-5. Jacobs, L. & Emerson, S. (1985). Partitioning and transport of metals across the O2/H28 interface in a permanently anoxic basin: Framvaren Fjord, Norway. Geochim. Cosmochim. Acta, 49, 143344. Landing, W. M. & Westerlund, S. (1988). The solution chemistry of iron(II) in Framvaren Fjord. Marine Chem., 23, 32943. Lovett, M. B. & Nelson, D. M. (1981). Determination of some oxidation states of plutonium in seawater and associated particulate matter. In Proceedings of the Symposium on Techniques for Identifying Transuranic Speciation in Aquatic Environments, 24-28 March 1980, Ispra, Italy. IAEA, Vienna, pp. 27-35. McKee, B. A., Todd, J. F. & Moore, W. S. (1991). Particle/solution partitioning
21~,
t : S~mc&:../. ¢~a~/Jmi. 1:. llo/m, t'. Roo.~
of thorium isotopes in Framvaren (jord: insights into sorption kinetic> ~:,, super-anoxic environment. In Radionuelide,s in the Study o/,~tarine t~/',~ ~ ses, eds P. J. Kershaw & I) S. Woodhead. Elsevier Applied Scic~,~c London, pp. 130 41. Mitchell, P. I., Vives Batlle, ,I.. Ryan, T. P., Schell, W. R., Sanchez-Caheza, .I " , & Vidal-Quadras. A. (1991) Studies on the speciation of piutonium aJ~d americium in the v~estern Irish Sea. In Radionuelides in the S:ud3" o[ Ma~'/n,' Processes, eds P. ,I. Kershaw & D. S. Woodhead. Elsevier aq~plied Science, London, pp. 37-51 Morse, J. W. & Choppm, G. R. (1986). Laboratory studies of plutonium *n marine systems. Marine Chem., 20, 73 89. Mudge, S., Hamilton-Taylor, J.. Kelly, M. & Bradshaw, K. (1988). Laboratory studies of the chemical behaviour of plutonium associated with contaminated estuarine sediments. J. Environ. Radioactivity, 8, 217-37. Murray, C. N. & Kautsky, H. (1977). Plutonium and americium activities in the North Sea and German coastal regions. Estuarine Coastal Mar. Sci.. 5, 319 28. Murray, C. N., Kautsky, H., Hoppenheit, M. & Domina, M. (1978). Actmide activities in water entering the northern North Sea. Nature, 276, 225 30. Murray, C. N., Kautsky, H. & Eicke, H. F. (1979). Transfer of actinides from the English Channel into the southern North Sea. Nature, 278, 617-20. Nash, K. L., Cleveland, J. M. & Rees, T. E. (1988). Speciation patterns of actinides in natural waters: a laboratory investigation. J. Environ. Radioactivity, 7. 131 57. Nelson, D. M. & Lovett, M. B. (1978). Oxidation state ot" plutonium in the Irish Sea. Nature, 276, 599 601. Nelson, D. M., Carey, E. A. & Bowen, V. T. (1984). Plutonium oxidation state distributions in the Pacific Ocean during 1980-1981. Earth Pkmetarv Sci. Letters, 68, 422 30. Nelson, D. M., Penrose, W. R., Karttunen, J. O. & Mehlhaft, P. (1985). Effects of dissolved organic carbon on the adsorption properties ot" plutonium in natural waters. Environ. Sei. Teehnol., 19, 127 31. Nelson, D. M., Orlandini, K. A. & Penrose, W. R. (1989). Oxidation states of plutonium in carbonate-rich natural waters. J. Environ. Radioactivity, 9, 189 98. Nies, H. (1990). The contamination of the North Sea by artificial radionuclides during the year 1987. J. Environ. Radioactivity, I1, 55 70. Noshkin, V. E., Wong, K. M., Jokela, T. A., Brunk, J. L. & Eagle, R. J. (1987). Plutonium and americium behavior in coral atoll environments. In Oceanic Processes in Marine Pollution, vol. 2, Physieoehemical Processes and Wasles in the Ocean, eds T. P. O'Connor, W. V. Burr & I. W. Duedall, R. E. Krieger Publ. Co., Malabar, Florida, pp. 157 74. Orlandini, K. A., Penrose, W. R. & Nelson, D. M. (1986). Pu(V) as the stable form of oxidized plutonium in natural waters. Marine Chem., 18, 49-57. Rai, D., Serne, R. J. & Swanson, J. L. (1980). Solution species of plutonium in the environment. J. Environ. Quat., 9, 417-20. Sanchez, A. L., Murray, J. W., Schell, W. R. & Miller, L. G. (1986). Fallout plutonium in two oxic-anoxic environments. Limnol. Oceanogr., 31, 1 110-:-21. Sanchez, A. L., Gastaud, J., Noshkin, V. & Buesseler, K. O. (1991). Plutonium
Plutonium distribution in Framvaren and Hellvik.[/ords
217
oxidation states in the southwestern Black Sea: evidence regarding the origin of the cold intermediate layer. Deep-Sea Res. 38, $845-53. Sholkovitz, E. R. (1983). The geochemistry of plutonium in fresh and marine water environments. Earth Sci. Rev., 19, 95-161. Skei, J. M. (1988). Formation of framboidal iron sulfide in the water of a permanently anoxic fjord Framvaren, south Norway. Marine Chem., 23, 345-52. Stigebrandt, A. & Molvaer, J. (1988). On the water exchange of Framvaren. Marine Chem., 23, 219-28. Talvitie, N. A. (1972). Electrodeposition of actinides for alpha spectrometric determination. Analyt. Chem., 44, 280-3. Todd, J. F., Elsinger, R. J. & Moore, W. S. (1988). The distributions of uranium, radium and thorium isotopes in two anoxic fjords: Framvaren fjord (Norway) and Saanich Inlet (British Columbia). Marine Chem., 23, 393~415. Wong, K. M. (1971). Radiochemical determination of plutonium in sea water, sediments and marine organisms. Analyt. Chim. Acta, 56, 355-64.
Distribution of Plutonium and its Oxidation States in Framvaren and Hellvik fjords, Norway
Arthur L. Sanchez* & Janine Gastaud IAEA Marine Environment Laboratory, 19, avenue des Castellans, MC98000, Principality of Monaco
Elis Holm & Per Roos Department of Radiation Physics, Lund University, Lurid, Sweden (Received 4 November 1992; revised version received 11 February 1993; accepted 3 March 1993)
ABSTRACT Waters from the Skagerrak Sea entering the system of Ijords inside Farsund in the southern tip of Norway (including Lyngsdal, Hellvik and Framvaren I~ords) contain plutonium derived from European nuclear fuel reprocessing facilities in the United Kingdom and France, as shown by the 238pu/e39'24°pu activity ratios measured.]'or water samples from Hellvik and Framvaren.fjords. The shallow sills" interconnecting this series of I)ords, however, severely restrict water exchange. Thus, at the northernmost Framvaren fjord (connected to Hellvik J)'ord by a 500-m channel o1"2 m depth), the 238pu1239'240pusignature for Pu derived from fuel reprocessing is li)und only in the surface layer (upper 20 m) while global fallout Pu ratios are observed in the permanentO, anoxic zone of this'fjord. Plutonium oxidation state measurements in waters collected from Hellvik and Framvaren l)'ords show that water exchange occurs between these two basins by a process 01"interleaving, with subsequent reduction (T]'oxidized Pu species as these reach the anoxic zone. Plutonium serves as a uselul tracer 1or water exchange between these fjords and the Skagerrak Sea. *Present address: Institute of Terrestrial Ecology, Merlewood Research Station, Grangeover-Sands, Cumbria LAI 1 6JU, UK. 2O5 J. Environ. Radioactivity 0265-931X/94/$07-00 'c~ 1994 Elsevier Science Limited, England. Printed in Ireland.
206
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./. l;~i,~t{:lld, t:. tied;It, t ~, Ro¢~s
i NTROD[)CTION Plutonimn is a redox-sensitive radionuclide known to exhibit severai oxidation states in the environment. An analytical method based on selective coprecipitation on rare earth fluorides (Lovett & Nelson, 1981i has been used to distinguish the reduced Pu(IIl and IV) from the oxidized Pu(V and V1) species in environmental samples. This technique has shown that Pu exists in both oxidation state categories at varying proportions m various aquatic systems (e.g. Nelson & Lovett, 1978; Nelson et al., 1984, 1989; Cochran e l a l . , 1987: Fukai e t a l . , 1987: Noshkin eted., 1987: Mitchell et al., 1991). in oxic waters, the oxidized species tend to predominate: recent evidence suggests that Pu(V) dominates this fraction (Morse & Choppin, 1986; Orlandini et al., 1986; Choppin & Kobashi, 1990), although it has been suggested that Pu(V) exists in a metastable state which becomes slowly reduced to Pu(IV) in the presence of humic substances or other reducing agents. Our recent work in the Black Sea (Sanchez et al., 1991) has shown that Pu responds to changes in redox conditions in the water column, with oxidized forms found in the suboxic layer gradually becoming reduced in the anoxic zone. We have inferred that these redox state transformations provided evidence for the rapid lateral advective processes occurring at intermediate depths in the Black Sea water column. Thermodynamic calculations (e.g. Rai et al., 1980: Edgington, 1981) also predict that Pu(V) is the predominant form of Pu in oxic natural waters and that the reduced species predominate under reducing conditions. The major objective ot" this study was to assess the processes controlling the distribution of Pu in an anoxic basin. Framvaren, a permanently anoxic 0ord in southern Norway with a maximum depth of 183 m, provided an ideal location for this study because much is known of its hydrochemical regime. The distribution of various chemical species in Framvaren basin is controlled by several factors, including water exchange, supply and decomposition of organic matter, sorption and desorption processes on biotic and abiotic particles, the oxidation-reduction (redox) potential, and for radioactive species, the decay rate (e.g. Jacobs & Emerson, 1985; Anderson & Dyrssen, 1987; Anderson el al., 1988; Landing & Westerlund, 1988; Todd el al., 1988; McKee et at., 1991 ). All these processes are known to influence the geochemical cycling of Pu in natural waters. A sampling site was also selected at the adjacent Hellvik fjord (maximum depth 29.2 m), which is separated from Framvaren f~ord by a 2-m sill and which serves as a source of seawater entering from the Skagerrak Sea. Two possible sources of Pu in Framvaren and Hellvik fjords are global
Plutonium distribution in Framvaren and Hellvik.[jords
207
fallout from nuclear weapons testing and discharges from European nuclear fuel reprocessing facilities at Sellafield and Dounreay (UK) and La Hague (France). Murray and co-workers (1977, 1978, 1979) and Nies (1990) have reported that waters in the North Sea and Skagerrak Sea contain Pu derived from fuel reprocessing facilities, with a characteristic 23Spu/239'Z4°pu activity ratio of about 0.20. This ratio is significantly different from the 238pu/239'24°pu ratio of 0.036 for global fallout Pu in the northern hemisphere (Sholkovitz, 1983). Presumably, Pu derived from reprocessing facilities and advected into the North Sea would occur primarily in the oxidized ( V + V I ) states, as the particle-reactive Pu(III + IV) species tend to be rapidly removed from the water column following their interaction with suspended particulate matter. Mitchell et al. (1991) have shown that 80-90% of soluble Pu in the Irish Sea, derived primarily from Sellafield releases, is in the oxidized state.
S A M P L I N G A N D ANALYSIS Samples were collected during a multidisciplinary cruise organized by the Norwegian Institute for Water Research in May-June 1989. Water profile samples (100 litres volume) were collected for Pu oxidation state measurements from selected depths in Framvaren and Hellvik fjords using a 100 litre Go-Flo sampler (see Fig. 1 for sampling locations). The samples were obtained from the surface down to 60 m in Framvaren fjord (the depth of the O2-H2S interface was at 20 m in June 1989), and from the surface down to 25 m in Hellvik fjord. To minimize changes in redox conditions of the water samples that might affect Pu oxidation states, samples were processed without prior filtration within 2 h after collection, using the rare earth fluoride coprecipitation method (Lovett & Nelson, 1981). This method is based on the selective adsorption of reduced Pu(III + IV) species on LaF3. A sequential separation is performed where the sample is first made oxidizing by adding appropriate quantities of reagents to produce concentrations of 0.8 M in HNO3, 0.25 M in H2SO4, 0.0005 M in K2Cr20 7 (this medium will maintain Pu(V + VI) and the 242pu(VI) tracer in the oxidized state while the reduced Pu(III + IV) and the 236pu(IV) tracer are coprecipitated on LaF3). The oxidized Pu species (and tracer) are subsequently reduced using ferrous ammonium sulphate and coprecipitated with LaF3. The LaF3 precipitates are collected on 0-45-/~m cartridge filters. A distilled water blank sample was also processed similarly. Additional water samples for total Pu and other measurements were also collected. In Framvaren fjord, these included samples down to 175 m
2(}~
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0
2
Scale 2000,
l NOrwa~i 1
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%-,.
Oftefjord
Hetlvok~
Farsund
Fig. 1. Map of the area showing sampling sites FI in Framvaren fjord and HI in Hellvik t~jord.
depth in order to obtain an inventory of Pu in the water column. Plutonium was measured in these samples following coprecipitation with Fehydroxides (Wong, 1971 ). For the anoxic samples from 40 m and below in Framvaren fjord, air was bubbled to remove H2S before coprecipitation. At the laboratory, standard radiochemical procedures were used to isolate and purify Pu from the LaF3 and the Fe-hydroxide precipitates prior to alpha spectrometry. These included purification on anionexchange resins following dissolution of the precipitates in mineral acids (Wong, 1971; Lovett & Nelson, 1981) and electrodeposition on stainless steel planchettes (Talvitie, 1972). For the oxidation state measurements, we found no significant ( < 1%) crossover between the oxidized (V + Vl) and the reduced (IIl + IV) Pu species.
Plutonium distribution in Framvaren and Hellvik )¢jords
209
RESULTS A N D DISCUSSION Based on dissolved 02 and H 2 S measurements at the time of our sampling, the oxic- anoxic interface in Framvaren fjord was located at 20 m depth; in Hellvik fjord, the water was slightly sulphidic below 11 m (J. Skei, pets. comm.). The results of our Pu measurements are summarized in Tables 1~1. Total Pu (as determined by Fe-hydroxide coprecipitation on unfiltered samples) are reported for the isotopes 23Spu and 239,240puin Tables 1 and 2. The isotope 238pu was detected in all samples analysed for total Pu, but at significantly lower levels than for 239,240pu" In a separate measurement for Pu in suspended particles collected using a 1-/,m cartridge filter, we estimated that between 5 and 60% of the total Pu in the two fjords were associated with the particulate fraction. The oxidized and reduced Pu concentrations (as measured by selective coprecipitation on LaF3) are reported for the isotopes 239,240pu in Tables 3 and 4. In general, we found good agreement (i.e. within the reported error) between the total 239,240pu TABLE 1 Distribution of Total Pu in the Water Column of Framvaren Fjord
Depth
Salinity (%0)
23< 249pu
23# p u
(m)
(mBq m 3)
(mBq m 3)
2 10 20 40 60 80 110 140 175
9 11.3 16-4 18.8 20.4 20-5 21.6 22,4 22,6
7-4 (1.2) 5-6 (1.0) 6-4 (1.4) 27 (3) 61 (6) 113 (8) 290 (20) 362 (25) 410 (30)
0-8 0.4 0.3 0.7 2-5 3-5 8.6 10.6 ll.6
(0.4) (0-2) (0.3) (0,3) (0,6) (0.8) (1.4) (1.8) (1.8)
Activity ratio
0-11 (0-05) 0.07 (0.04) 0.05 (0.04) 0.03 (0.01) 0.04 (0.01) 0-03 (0-01) 0.030 (0.004) 0.029 (0-005) 0-028 (0.005)
Values in parentheses represent one standard deviation counting error. TABLE 2 Distribution of Total Pu in the Water Column of Hellvik Fjord Depth (m)
Salinity (%0)
239"24~P21 (mBq m -~ )
e ~aPu (mBq m 3)
2 7 II 18 25
22 26-5 27.8 28.7 29.6
10-6 (1.3) 11-7 (1.8) 5-4 (0.8) 7-9 (2.4) 7.2 (0-9)
1.5 (0.5) 2-1 (0-8) 1.1 (0.4) 1-4 (1.0) 1-1 (0-3)
Values in parentheses represent one standard deviation counting error.
Activity ratio
0.14 0-18 0.20 0.18 0-15
(0.04) (0.07) (0.08) (0.14) (0-04)
210
4 I..';aHchcz, J. ¢~a.vtaud, E. t t o / m . P. Roo.v
TABLE 3 Distribution of 2~,~ ,4 ~Pu Oxidation Sta~es in the Water Column of Framxarcla F j(~rd Depth (m)
P u ( H I ~ 1l'~ ( m B q m ~ ,'
P u / lIt1) ( m B q m ~)
R e d u c e d Pu (!!4,)
( ) x i d i z e d 1-'~ % J
2 10 20 40 60
6-8 (3.6) 3-5 (1.9) 2-5 (t .4) 12.1 (6.4) 18.(/(9.5)
6.1 (1.1) 4.0 (0.9) 3.5 (0.7) 8.3 11.3) 11 (2)
53 47 42 59 62
47 53 58 41 38
Values in parentheses represent one standard deviation counting error. TABLE 4 Distribution of 239, 24Opu Oxidation States in the Water Column of Hellvik Fjord Depth (m)
2 7 11 18 25
Pu(Ill + IV) ( m B q m 3 j~
2.7 5-3 1.2 1.7 3.0
(1.5) (0-5) (0-7) (l.11 (1.7)
P u t V + ~'7) (mBqm ;)
9-5 10.7 10.2 4.2 4-5
(1-5) (1.6) (2.0) (0.8) (0.9)
Reduced Pu (%)
Oxidized Pu Jo ' ( '~"
22 33 11 29 40
78 67 89 71 60
Values in parentheses represent one standard deviation counting error.
concentration and the sum of (oxidized + reduced) 239'24°pu species at those depths where samples for both measurements were collected. The error terms associated with these measurements are mainly from counting statistics, with corrections for the distilled water blank values; we note that some measurements showed rather significant ( > 50%) error terms. Three samples (2 m in Framvaren; 7 and 1 1 m in Hellvik fjord) showed slightly higher levels of (oxidized + reduced) Pu compared to total Pu. One sample (60 m in Framvaren) showed significantly less (oxidized + reduced) Pu than total Pu; we suspect that the rare earth fluoride separation method might not be applicable under the highly reducing condition of this particular sample (it contained 0.5 mM HzS). Most of the discussion to follow will be based on the trends observed for the isotopes 239'24°pu. The levels of total Pu in the upper 20 m of Framvaren fjord and in the entire water column of Hellvik fjord are similar, ranging between 5.4 and 11.7 mBq m -3. In general, these values are low and comparable to measurements reported for surface waters in the North Sea (Nies, 1990) and in waters around Denmark (Aarkrog et al., 1982). In contrast, Pu levels in the permanently anoxic layer of Framvaren are significantly higher, showing a trend of increasing Pu
Plutonium distribution in Framvaren and Hellvik J)ords
211
concentration with depth. The sample from 175 m showed the highest Pu concentration; samples from 40 m to 140 m showed intermediate levels. Similar high concentrations of Pu were found in the anoxic waters of a meromictic lake in Washington State (Sanchez et al., 1986). The water column inventory of total 239,240pu in Framvaren fjord based _9 on the profile measurements is estimated to be 35-7 Bq m ", with over 99% of this inventory contained in the permanently anoxic zone (20 m and below). The expected global fallout 239,240pu delivery to the 50-60"N latitude band is 48 Bq m -2 (Hardy et al., 1973); thus, around 75% of the expected Pu fallout delivery to Framvaren fjord is within the water column, primarily in the anoxic zone. This observation is in sharp contrast to our results from the Black Sea, where we estimated that less than 10% of the Pu inventory is found in the water column (Sanchez et al., 1991). Similar to Framvaren fjord, the Black Sea water column is permanently anoxic (from about 100 m depth), although the levels of H2S are much higher in Framvaren. The elevated levels of Pu found in the anoxic waters of Framvaren are presumably due to complexing with dissolved organic matter and/or the enhanced solubility of the Pu(IlI) oxidation state. The b o t t o m waters of this fjord contain up to 17.5 mg litre ~ of humic substances (Haraldsson & Westerlund, 1988). Nelson et al. (1985.) have shown that colloidal organic carbon at concentrations commonly encountered in natural waters (1-20 mg litre -l) interact strongly with reduced Pu(IlI + IV) species and inhibit their sorption onto suspended particles. Mudge et al. (1988) have also shown that natural colloidal organic carbon and humic substances remobilize Pu from contaminated estuarine sediments. Nash et al. (1988) also suggest that Pu(III), which tends to be more soluble compared to Pu(IV), would predominate under reducing conditions. Pu speciation calculations predict that Pu(III) is strongly complexed by humic acids (Morse & Choppin, 1986; Choppin et al., 1986). We noted a distinct difference between the 238pu/239'24°pu ratios in the two fjords. In Hellvik fjord, the ratios ranged between 0-14 and 0.20 (average: 0.17 ± 0.02) for the entire water column. In contrast, the 238pu/239'24°pu ratios in Framvaren fjord waters at and below the oxicanoxic interface ranged between 0.028 and 0.04; the ratios were slightly higher in the top 20 m samples (Table 1). The 238pu/239'24°pu activity ratios found in Hellvik fjord are similar to ratios reported for North Sea waters in 1987 (Nies, 1990), suggesting that exchange of waters between this fjord and the North Sea/Skagerrak Sea must be occurring. As discussed earlier, these activity ratios indicate Pu derived from European nuclear fuel reprocessing plants. According to Murray and co-workers (1977, 1978, 1979), actinides released from these reprocessing facilities are
212
I. L. ~,tlcltc:. ,/. (,ts/ata/. E ttohn. P. Roov
transported to the North Sea by currents from the northern tip of Scotland and from the English Channel. The distributions of Pu oxidation states in the two fjords, shown ili Tables 3 and 4, are also distinctly different and showed some interesting features. We emphasize, however, that due to the magnitude of the error terms in some of these measurements, we can only note these as apparem trends. In Hellvik (jord, oxidized Pu species predominate in the enthe water column, even though the water below 11 m was apparently anoxic at the time of sampling. We did note a decreasing trend in the proportion of oxidized Pu species in the bottom 18 and 25 m samples. In contrast, the Framvaren samples showed the following Pu oxidation state distributions: (1) at the surface (2 in), almost equal proportions of reduced and oxidized Pu species were found; (2) at 10 and 20 m, oxidized Pu at amounts slightly higher than for the surface were measured; and (3) at 40 and 60 m. reduced Pu species predominate. We did not collect samples from below 60 m as we expected that oxidized Pu would be unstable to reduction under the highly reducing conditions below this depth (the reported sulphide concentrations at 60 m and below in Framvaren range between 0.5 and 8 mM H2S; Skei, 1988). In addition, as noted earlier, we suspected that the rare earth fluoride separation method might not be applicable under these conditions. To explain the Pu oxidation state profile in Framvaren fjord, it is helpful to consider the process of water exchange between Hellvik and Framvaren fjords, as described by Stigebrandt & Molvaer (1988). According to these authors, water transport between the two basins is driven primarily by fluctuating sea-level differences (the varying sea level acting as a pump). Waters drawn from the upper few metres of the two basins are exchanged following these fluctuations in sea level. Furthermore, water entering Framvaren from Hellvik fjord would be normally denser (more saline) than the surface water in Framvaren. This water would thus tend to flow along the bottom as a density current, and in the process entrain surrounding water (and become lighter). When this water attains the same density as the surrounding water, it is interleaved into the interior of Framvaren fjord. Stigebrandt & Molvaer (1988) distinguished four water masses in Framvaren fjord based on the salinity profile: (1) the surt:acc water (O 2 m) with low salinity, (2) the intermediate water (2 18 m) with strong vertical gradients in O2 content and salinity, (3) the deep water (18--80 m), and (4) the bottom water (80-183 m). The features of our Pu oxidation state profile suggest that this type of water exchange might be occurring. We could envisage a simple linear mixing model, taking as end members surface water at Hellvik fjord ( S = 22%0, containing 78% oxidized Pu) and surface water of Framvaren
Plutonium distribution in Framvaren and Hellvik fiords
213
fjord (S -- 9%0, containing 47% oxidized Pu). These two end members mix to produce waters of intermediate salinity, containing intermediate amounts of oxidized Pu. Assuming that no significant Pu reduction occurs after mixing, this model would predict that water at 10 m (S = 11-3%o) should contain about 52-53% oxidized Pu, and water at 20 m (S = 16-4%o) should contain about 64-65% oxidized Pu. Our actual measurements show 53% and 58% oxidized Pu species at these depths, respectively. Although these measurements are not significantly different (within the error terms), it is interesting to note that the profiles support the mixing model envisaged here. The proportion of oxidized Pu species measured at 20 m is less than the model prediction, but this intuitively makes sense considering that oxidized Pu is unstable to reduction. Without this type of mixing, it would be difficult to explain how we obtain a proportionately greater a m o u n t of oxidized Pu species at the subsurface depths of 10 and 20 m compared to the surface water. If wind mixing were important (unlikely due to the salinity gradients), we should expect to find similar proportions of oxidized and reduced Pu species at the top 20 m layer. In the permanently anoxic zone below 20 m, the high levels of sulphide should favour the reduction of oxidized Pu. However, the water samples collected from these depths still contained around 40% oxidized Pu. Again, the mixing model could partly explain this observation, with Hellvik-derived water as the source of oxidized Pu at these depths. These results would also suggest that reduction of oxidized Pu species must be occurring with slow kinetics even in the presence of sulphides; however, at the time of sampling, we had no indication as to how long it has been since these waters have interleaved into the anoxic layer. This type of water mixing also appears to be occurring in Hellvik fiord. We note that, as in Framvaren fjord, oxidized Pu species are at a maxim u m at an intermediate depth (11 m) in Hellvik fjord. Although we do not have any measurements for water entering from the Skagerrak Sea, it is reasonable to assume that this water, being more saline, would mix to intermediate depths at Hellvik fjord, bringing in a high proportion of oxidized Pu species in the process. At greater depths, the reduction of these oxidized Pu species also seems to become important. Since reduced Pu species tend to be particle-reactive, our results suggest that Pu found in the bottom waters of Hellvik fjord could be immobilized into the sediments and would not be available for transport into Framvaren fjord. Preliminary measurements do indicate that the inventory of Pu in the surface sediments of Hellvik fjord is significantly higher than in Framvaren (P. Roos, pers. comm.). In general, our observations suggest that Pu and its oxidation states prove to be a useful tracer for the water exchange between these fjords and
214
I....
5'J,~hc_: . l {.~/wam/. /(. It~J#,l. P. Roo.s
with tile Skagerrak Sea. Better constraints on the Pu oxidation state dat,~ would help to delineatc the processes clearly. We have also t~oted that Jqi derived from reprocessing facilities appears to be effectivel3 trapped i~, Hellvik (jord and is not transported significantly into Framvaren 17iota!, due to the limited exchange of surface waters between the two basin>,. Based on our data. it appears possible to investigate the extent and history of water exchange and transport between these l]ords and the Skagerrak Sea by looking at the sediment record of Pu in these basins.
CONCLUSIONS The measurements reveal that several processes influence the overall balance and geochemical cycling of Pu in the water column of Hellvik and Framvaren fjords: (1) Water exchange with the Skagerrak Sea. This water exchange process brings in Pu derived from European nuclear fuel reprocessing plants, primarily in the oxidized state. A major fraction of this Pu, however, appears to be trapped inside Hellvik 0ord, and reaches Framvaren only in the surface layer. Thus, global fallout Pu ratios in the anoxic zone of Framvaren fjord are relatively unaffected by Pu derived from reprocessing facilities. (2) Oxidation-reduction reactions. The measurements reveal that in general, oxidized Pu species entering from the Skagerrak Sea would become reduced in the water column. In Hellvik fjord, the reduced species are presumably deposited to the sediments and cannot reach Framvaren fjord, particularly since water exchange occurs via a shallow (2 m) channel. (3) Interactions with dissolved/colloidal organic matter. The enhanced levels of Pu in the anoxic waters of Framvaren fjord are most likely a result of complexing with dissolved organic matter. There is sufficient experimental evidence from several workers showing the importance of such interactions. In addition, dissolved organic carbon could serve potentially as a reducing agent for oxidized Pu species entering Framvaren from Hellvik fjord.
ACKNOWLEDGEMENTS We thank Dr Jens Skei (Norwegian Institute for Water Research, Oslo) for coordinating the sampling programme at Framvaren fjord. We also thank Kjell-Ake Carlsson (Lund University, Sweden) for help with sample
Plutonium distribution in Framvaren and Hellvik.[jords
215
collection and processing. We gratefully acknowledge travel and partial financial support from the Swedish Radiation Protection Institute through a grant to Dr Elis H o l m (Department of Radiation Physics, Lund University). The I A E A - M a r i n e Environment L a b o r a t o r y operates under an agreement between the International Atomic Energy Agency and the government of the Principality of M o n a c o .
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