Research Decline of Radionuclides in the Nearshore Environment Following Nuclear Reactor Closure: A U.K. Case Study A N D R E W B . C U N D Y , * ,† IAN W. CROUDACE,† P H I L L I P E . W A R W I C K , †,‡ A N D MICHAEL E. D. BAINS‡ School of Ocean and Earth Science, Southampton Oceanography Centre, European Way, Southampton, SO14 3ZH, U.K., and AEA Technology, Winfrith Technology Centre, Dorchester, Dorset, DT2 8DH, U.K.
Radioactive discharges from nuclear facilities are frequently made into the marine environment and their fate during and after cessation of discharges is a matter of interest and concern. This study examines the decline of the radionuclides 60Co and 65Zn along the southern U.K. coast, over the period 1988-1998, following the closure of the steam-generating heavy water (SGHW) reactor at AEA Winfrith, Dorset, UK. 60Co and 65Zn (and other activation products such as 63Ni and 55Fe) were widely dispersed in the marine environment off the central south coast of England, due to authorized releases from AEA Winfrith. Significant interaction occurred with clay-rich sediments and biota. A general exponential decline in 60Co activities (and in 65Zn activity) is found in intertidal mudflat sediments, seaweed and marine fauna in different areas along the south coast following closure of the reactor in 1990. Effective half-lives are determined which vary from 1 to 4 years in surface sediments (60Co only), 1-4 years in seaweed and 0.52.5 years in crustaceans, bivalves and molluscs. Physical mixing and bioturbation largely control the rate at which 60Co declines in surface sediments. Both 60Co and 65Zn show a relatively slow rate of decline in seaweed and in marine fauna, showing that even after the virtual cessation of discharge from nuclear facilities, contamination of these organisms may persist for a number of years, albeit at reduced activities. Reasons for this persistence are likely to include absorption of radionuclides from sediment, and release and recycling of radionuclides via breakdown of contaminated organic material.
Introduction Radioactive discharges from nuclear facilities are frequently made into the marine environment and their fate both during and after the cessation of discharges is a matter of interest and concern. A considerable amount of literature exists on the behavior and dispersion patterns of radionuclides * Corresponding author phone: +44 (0)1703 592780; fax: +44 (0)1703 596450; e-mail:
[email protected]. † Southampton Oceanography Centre. ‡ Formerly of AEA Technology, Winfrith Technology Centre. 10.1021/es9811694 CCC: $18.00 Published on Web 07/14/1999
1999 American Chemical Society
released from nuclear facilities (e.g. Sellafield (1, 2), La Hague (3-5), and Hanford (6)) and on the adsorption and uptake of these radionuclides by sediments and marine organisms (e.g. refs 7-9). The impact of nuclear facility discharges depends largely on the scale of input, the chemical form of the waste products and the physical, chemical and biological characteristics of the environment into which the waste is discharged. The dispersion of radionuclides and the rate at which measured radionuclide activities decline following discharge is thus extremely variable, depending on both the chemical behavior of the radionuclide of interest and local dispersion/transport pathways. It is important to evaluate the rate at which radioactivity declines in the marine environment following cessation of discharge (the effective half-life), and the processes that control the rate of decline in sediments and marine organisms. While these may be assessed in laboratory studies of adsorption/desorption (e.g. refs 8 and 10) and uptake/depuration (e.g. refs 11 and 12), such studies are necessarily limited in terms of duration and complexity. An alternative method is to use empirical activity data collected as part of long-term statutory and public monitoring programs for localities where nuclear facilities have been closed down. Where spatial and temporal coverage is sufficient, the data allow both the rate of decline, and the mechanisms causing this decline, to be evaluated. This study therefore uses data (13, 14) collected as part of long-term monitoring programs to examine the decline of the radionuclides 60Co and 65Zn along the southern U.K. coast, over the period 1988-1998, during and following the closure of the steam-generating heavy water (SGHW) reactor at AEA Winfrith. While a number of radionuclides (including 241Am, 137Cs, and Pu isotopes) were measured as part of the statutory monitoring programs associated with this facility, the most comprehensive data-set is for the activation product 60Co and, to a lesser extent, 65Zn. Although 60Co and 65Zn form only a small component of reactor liquid effluents, they may contribute significantly to local radiation dose through seafood pathways. 60Co (t1/2 ) 5.27 years) is a common discharge product present in low-level aqueous waste from nuclear facilities. The post-discharge behavior of 60Co depends on its chemical form. Uncomplexed Co shows high particle reactivity in the marine environment (having a distribution coefficient of the order of 104 (15)), and so is rapidly sorbed onto sedimentary material mainly through interaction with Mn and Fe-oxyhydroxides (e.g. ref 16). Organically complexed Co, in contrast, may be significantly less particle reactive (e.g. ref 17 and 18). Even after sorption onto sediments, remobilization and release of 60Co by earlydiagenetic processes may potentially occur (8, 10). Bioaccumulation of 60Co has been observed in both benthic algae and shellfish in the vicinity of nuclear facilities. Co is an essential element for marine organisms, being a component of vitamin B12 and an important cofactor for several enzymes (19). 60Co is accumulated by passive absorption and active uptake that is dependent on environmental factors and metabolic processes in benthic algae (11, 20, 21) and shellfish (e.g. refs 12 and 22). Concentration factors (CF, where CF ) activity in organism/activity in seawater) of 60Co vary according to type of organism - concentration factors in the brown alga Fucus vesiculosus have been reported in the range 103-104 (15, 23), while shellfish CFs of around 5000 have been reported (15). 65Zn (t1/2 ) 245 days) is also a common discharge product from nuclear facilities, and has received VOL. 33, NO. 17, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Area of study along the southern U.K. coast and major sediment and water transport paths (after refs 36-38). Filled circles show location of sampling sites, filled square shows location of AEA Winfrith discharge pipeline at Arish Mell. Inset graphs show annual discharges of 60Co and 65Zn from Winfrith between 1985 and 1995. particular interest due to its high concentration factor by (edible) shellfish. Oysters (Crassostrea) in particular concentrate Zn up to 105 times the seawater concentration (e.g. refs 7 and 15). Marine crustaceans and molluscs typically show CFs of 3-5 × 104 (15). Zn is an essential element, being required by certain enzymes and structural proteins (19), and is also accumulated by benthic algae, with CFs in the range 1140-2500 (for Fucus vesiculosus (23)). Like 60Co, uncomplexed 65Zn is particle reactive (distribution coefficient ) 104 (15)) and so is rapidly sorbed onto sediments, e.g. in a study of the Bradwell Power station in SE England, Preston (24) found a distribution of 65Zn in oyster tissue consistent with particle-associated transport of 65Zn. Discharges from the AEA Winfrith Reactor. The AEA Winfrith site in Dorset (Figure 1) began discharging radioactive effluent in 1970. Wastes were produced during the operation of the SGHW reactor and other on-site radioactive facilities. After a period of holding and dilution, the effluents were discharged under authorization into the marine environment via a 3 km pipeline in Weymouth Bay (Figure 1). The major activation products of Fe, Co, Cr, Mn, Ni, and Zn were released to sea following annual cleaning of the SGHW reactor cooling circuit. The main annual discharge of 60Co and 65Zn occurred between January and March. A partial record of the discharges since 1970 is published in annual reports, with maximum discharge of activation products occurring in 1980/81. The highest annual discharges of 60Co 2842
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and 65Zn were approximately 20 and 3 TBq, respectively. The reactor was closed in 1990, and while discharges have continued since that date the activity levels are a small fraction of what they were during the reactor life (inset, Figure 1). 1991 and 1992 discharges were less than 10% and 0.1% of the 1990 discharge, respectively.
Methods Intertidal surface sediments and marine organisms were collected and analyzed prior to and following the closure of the SGHW reactor at AEA Winfrith. Sampling was carried out as part of a statutory monitoring program (13, 14). The sampling locations discussed here (Figure 1) cover over 100 km of coastline, east and west of the discharge pipe. Sediments were collected from fixed sampling sites by taking a 2 cm surface scrape over 1 m2. Samples (minimum 1 kg wet weight) of benthic algae (Fucoid species, hereafter referred to as seaweed) were collected and identified, and measured as either bulk samples or new growth only (new growth was clearly distinguishable by its lighter color). Seaweed activities are reported on a wet weight basis. Marine crustaceans were caught by pot, molluscs collected by sampling intertidal areas and bivalves sampled by divers. A minimum of 25 (Maia squinado and Cancer pagurus) or 12 (Homarus gammarus) individuals were analyzed in each marine crustacean sample, 600 g (wet weight) for each mollusc sample, and 12 individuals for each bivalve (Pecten maximus) sample. Activities are
FIGURE 2. 60Co activity (in Bq/kg dry weight) vs time in surface sediment from intertidal mudflats. Data shown are normalized to a standard 40K activity of 400 Bq/kg to correct for grain size variability between samples. Effective half-lives (EHL) following reactor closure in 1990 (with (2σ range) are shown for each site, calculated using linear regression analysis of the natural logarithm of the activity vs time. See Figure 1 for site locations (numbered on each graph). reported on a wet weight basis. 60Co and 65Zn activities were determined using high resolution gamma spectrometry. Errors were typically in the order of 5% (2σ) and detection limits were 0.5 Bq/kg. All activities shown have been decay-corrected to the date of sample collection. Sediment activities used for the calculation of effective half-life (see below) have been normalized to 40K to reduce the effect of compositional variability between samples. Coastal and estuarine sediments in the study area show a geochemical composition which is consistent with derivation from the same (dominantly Tertiary) source material (25), where potassium is mainly present in the clay fraction due to the low feldspar content of the Tertiary deposits (26). Potassium has been shown to be a reliable indicator of mineralogy in these sediments (26). Activities were arbitrarily corrected to a standard 40K activity of 400 Bq/kg (a typical value for fine-grained sediments in the areas studied) using the following equation:
corrected activity ) (measured activity of 60Co/40K activity) × 400 Bq/kg Statistical analyses of data were performed using SPSS 6.1 for Windows.
Results and Discussion Dispersion of 60Co. Preclosure (1988-1990) activities of 60Co in seaweed (and marine fauna) are highest in the area around,
and immediately east of, the discharge pipeline, at Weymouth, Bowleaze Cove, Lulworth Cove, Kimmeridge, and Swanage (Figures 3 and 4). Efficient mixing and dispersion of discharges is indicated by monthly seaweed data (Figure 5). These data show the 60Co activity in new growth during, and following, main annual release of effluents in January and February. Samples from Kimmeridge and Swanage show a peak in 60Co activity within ca. 1 month of the release of the discharge pulse. (NB. This peak in activity is superimposed on a general decline in seaweed activity over the period 19891991, related to the reduction in annual discharges of 60Co (discussed below)). Seaweed from the Weymouth site, west of the discharge point, shows lower activities and little monthly variation in activity, probably related to the site’s more sheltered location upstream of the discharge point against the main direction of littoral drift (Figure 1). 60Co distribution beyond Weymouth Bay is generally consistent with regional water and sediment transport paths, which are dominantly west to east (Figure 1). Low to negligible 60Co activities are found at sites to the west of Weymouth Bay (13), while there is a slow decline in 60Co activity with distance eastward from the discharge pipeline (Figures 2-4). Detectable activities are still present in seaweed and sediment over 100 km east of the discharge point. The wide dispersal of 60Co from the Winfrith site measured here in both seaweed and sediment is due to eastward transport of 60Co from Weymouth Bay, the high concentration VOL. 33, NO. 17, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. 60Co activity (in Bq/kg wet weight) vs time in brown algae (Fucus sp.) samples. Note change of y-axis scale for Kimmeridge, Weymouth, Bowleaze Cove, and Swanage samples. At Weymouth, Kimmeridge, and Swanage, activities were measured in new growth only. All other sites show activities for bulk samples. Effective half-lives (EHL) following reactor closure in 1990 (with (2σ range) are shown for each site, calculated using linear regression analysis of the natural logarithm of the activity vs time. See Figure 1 for site locations (numbered on each graph).
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FIGURE 4. 60Co activity (in Bq/kg wet weight) vs time in crustaceans, molluscs, and bivalves. Note change of scale on y-axes. Effective half-lives (EHL) following reactor closure in 1990 (with (2σ range) are shown for each site, calculated using linear regression analysis of the natural logarithm of the activity vs time. See Figure 1 for site locations (numbered on each graph).
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FIGURE 5. Monthly discharges of 60Co (GBq) from AEA Winfrith and 60Co activity (Bq/kg wet weight, new growth only) for Fucus serratus from Kimmeridge, Weymouth, and Swanage (see Figure 1 for site locations), over the period January 1989 to December 1990. Arrows on lower graphs mark periods of peak 60Co discharge. factors for 60Co in fucoid seaweeds, and possibly the persistence of discharged 60Co-complexes in solution. 60Co is discharged from AEA Winfrith in two forms, 60Co(II) and 60Co(III) picolinate. While significant fractions of both species are soluble in seawater, Leonard et al (17) note that particle reactivity tends to be enhanced for 60Co(II) and the presence of the complexed form may enhance mobility and dispersion of 60Co. The presence of 60Co at activities well-above background in seaweed samples taken at large distances from the discharge point may be a result of this enhanced transport, or alternatively of efficient recycling of 60Co in the coastal environment (discussed further in the section below). While the AEA Winfrith reactor is the major source of 60Co along the south-central UK coast (25), other sources of 60Co may 2846
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(arguably) affect the observed distance-activity distribution. Both Sellafield and La Hague have discharged 60Co over the period 1988-1998. Transport and dispersion of discharges from La Hague is predominantly eastward, along the French coastline toward the Dover Straits and the North Sea (3). The vertical distribution of 60Co in saltmarsh deposits around the study area shows a pattern which correlates well with periods of peak discharge from Winfrith (25), and the distribution of 137Cs, which has been widely dispersed in the marine environment following release from both La Hague and Sellafield, is consistent with derivation from atmospheric fallout, rather than from nuclear facility discharges (27). Hence, the influence of discharges from both Sellafield and La Hague is inferred here to be insignificant. The influence of discharges from Ministry of Defence operations (nuclear submarine servicing) is localized and small-scale (typically, activities released are 500 to 1000 times lower than those released by AEA Winfrith, based on data for the Devonport dockyard, southwest U.K. (28)). Decline of 60Co Following Reactor Closure. 60Co activities measured in intertidal mudflat sediments, seaweed and marine fauna show a general exponential decline following closure of the SGHW reactor at AEA Winfrith in 1990 ( Figures 2-4). Effective half-lives for 60Co following reactor closure were calculated using linear regression analysis of the natural logarithm of the activity versus time. Effective half-lives are shown in Figures 2-4. Radionuclide activities are decaycorrected to the date of sample collection, hence the effective half-life includes decline in activity due to physical decay of nuclides. Only sites where the linear regression fit was significant at 95% are included. The effective half-life calculation assumes complete cessation of discharges at the end of 1990. While small-scale discharges following reactor closure in 1990 may increase the effective half-lives calculated here, this is likely to be a minor effect given the rapid decrease in discharge after 1990 (Figure 1). Effective half-lives show considerable variability between sites, related to local mixing and circulation dynamics, rather than being controlled by initial activity or by distance from the discharge point (seaweed samples do, however, show a slight tendency for effective half-life to increase with distance). Effective halflives range from 0.8 to 3.8 years in surface sediments, 1.13.2 years in seaweed and 0.9-2.4 years in marine fauna, mean half-lives are 2.5 years ((1.0, 1σ), 1.9 years ((0.5, 1σ), and 1.5 years ((0.5, 1σ), respectively. The rate of decline of 60Co is therefore slowest in surface sediment, slightly more rapid in seaweed, and most rapid in crustaceans, molluscs and bivalves. The exponential decline of 60Co in sediments and the lack of relationship between effective half -life and distance is consistent with the decline in activity being caused by mixing and dilution of 60Co-labeled sediment with uncontaminated or less-contaminated sediment, rather than the eastwards transport of a discrete plume of contaminated sediment. Early-diagenetic remobilization of 60Co into sediment porewaters and release into the overlying water column may arguably cause a similar decline in sediment activity. Some authors report increased mobility of 60Co under suboxic conditions (e.g. refs 8 and 10) but this is unlikely to be significant in these sediments. This is because sediment cores show a subsurface maximum in 60Co activity which corresponds to the peak AEA Winfrith discharge in 1980/1981 (25). The retention of this activity maximum, and the subsequent decline in activity toward the sediment surface indicate limited early-diagenetic remobilization and release over the time period examined here. Hence, mixing and dilution of contaminated sediment, rather than desorption, is likely to be the main cause of the observed decline in 60Co activity. Effective sediment half-lives are thus highly dependent on local sediment dynamics, and so are site-specific. While
FIGURE 6. 65Zn activity (in Bq/kg wet weight) vs time in Fucus serratus and Cancer pagurus. Note change of scale on y-axes. Activities for Fucus serratus are measured using new growth only. Effective half-lives (EHL) following reactor closure in 1990 (with (2σ range) are shown for each site, calculated using linear regression analysis of the natural logarithm of the activity vs time. See Figure 1 for site locations (numbered on each graph). the effective half-life obtained in this study for sediments is similar to the value of 1.5 years obtained by Patel and Patel (29) for sediments from Tarapur, nr. Bombay, India, Nakamura and Nagaya (30) examined 60Co in sediments from Urazoko Bay, Japan, and found that the sedimentary load of 60Co remained practically constant over a 5-year period despite a decrease in discharge of almost 2 orders of magnitude. This was most likely due to limited desorption from sediments, insignificant input of fresh sediment and negligible sediment mixing and redistribution. It is important to note that due to varying sediment accretion rates each surface sediment sample used in the present study does not represent a uniform time period. While surface sediment data are therefore useful in examining the general decline of radionuclide activity in sediments, the dependence of measured activities on local sediment dynamics and on sediment accretion rate means that use of either sediment traps or of dated sediment cores is a much more reliable indicator of annual particle-bound radionuclide input. The effective half-life of 60Co in seaweed (Figure 3) is much greater than 60Co-excretion rates based on laboratory studies (e.g. refs 11 and 31, etc.). This trend is seen for both wholeplant and recent-growth determinations (Figure 3). The decline of 60Co observed in this study is therefore not simply due to dilution of 60Co activity by new plant growth (as
proposed by Dahlgaard and Boelskifte (32)). It is generally accepted that activities of radionuclides in brown seaweeds reflect ambient dissolved concentrations, and hence these species have been used as bioindicators of radionuclides, due to their high concentration factors and their wide distribution (e.g. refs 32 and 33). In the absence of appreciable post-1990 discharge from Winfrith, continued persistence of 60Co in both whole-plant and new-growth analyses up to, and beyond, 1996 is likely to be a result of recycling of 60Co into the water column, or of leaching from sediments. The latter mechanism has been proposed by Luoma et al. (34), who argue that brown seaweed may scavenge heavy metals from particulates when it contacts bed and suspended sediments. Soluble 60Co may be present in the water column due to recycling of 60Co from decaying organic matter or due to early diagenetic remobilization from sediments. As discussed previously, early diagenetic remobilization of 60Co from sediments is unlikely to be a major pathway for release of 60Co into the water column, and so remobilization of 60Co from decaying organic matter is a more likely potential uptake route. Bryan (20) studied 65Zn uptake and depuration in brown seaweeds, and found that 65Zn was rapidly lost following death of the plant. Hence, despite cessation of discharge, seaweed (and, by implication, seawater) concentrations may remain elevated for some time due to recycling VOL. 33, NO. 17, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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of radionuclides already present in the marine environment. The average effective half-life of 60Co in marine crustaceans, molluscs and bivalves in this study is lower than in sediments and seaweed (Figure 4) but is still significantly higher than laboratory- and semi-field-based studies of individual species depuration (e.g. refs 22 and 35). The likely uptake route of 60Co into these species is a combination of passive and metabolic absorption from seawater, and from particulate material (including organic matter) filtered or ingested from the sediment surface or from benthic algae. Continued ingestion of 60Co, even after cessation of discharges, is therefore likely, due to ingestion of contaminated particles and/or uptake of 60Co that has been recycled into the water column. The more rapid decline of 60Co in marine fauna than is found in sediment or seaweed may be caused by the active uptake and depuration of both stable and radioactive cobalt by these organisms during use in metabolic processes. Pre-closure activities of 60Co show a degree of species-dependence, with higher pre-1990 activities being found in spidercrab (Maja squinado) from Lulworth Cove and Kimmeridge compared to brown crab (Cancer pagurus). The effective half-lives for marine fauna, however, are not strongly influenced by species type. Comparison of 60Co and 65Zn in Seaweed and Marine Fauna. The rate of decline of 65Zn in Fucus serratus following closure of the Winfrith reactor in 1990 (based on measurements of new growth) is similar to that of 60Co (Figure 6), although activities prior to 1990 are much lower, reflecting the lower total discharge of 65Zn into Weymouth Bay. Effective half-lives range from 1.9 to 3.9 years, which are significantly longer than the physical half-life of 65Zn (245 days). This indicates substantial recycling of 65Zn (enough to offset the physical decay of 65Zn). As sediments are likely to contain the major fraction of environmental 65Zn (Kd for uncomplexed 65 Zn ) 104 (15)), early-diagenetic release of 65Zn from sediments is a potential mechanism for recycling of 65Zn into the water column. The extent of early-diagenetic remobilization of 65Zn is not clear from the present data. Alternatively, since the same effect is not seen in marine fauna (see below), the long effective half-life may be a metabolic effect. Activities shown in Figure 6 are for new growth only, and since the concentration of Zn in the older parts may significantly exceed that in the recent growing tips (e.g. 21), the long effective half-life may be due to metabolic transport of 65Zn from the older parts of the thallus to new tissue during plant growth. The effective half-life of 65Zn, in contrast, is much lower than that of 60Co for the brown crab; the effective half-live for 65Zn is less than 7 months at both Lulworth Cove and Kimmeridge, compared with around 17 months for 60Co. This may simply reflect the shorter radioactive half-life of 65Zn (or alternatively may be due to a more rapid turnover of Zn than is found for Co in brown crab). Pre-1990 65Zn activities in brown crab are also significantly higher than 60Co activities (Figures 4 and 6) despite lower discharge of 65Zn. This is probably a consequence of the high concentration factor for Zn in crustaceans (5 × 104 for Zn compared to 5 × 103 for Co (15)), and in the molluscs which form a major part of its diet. The results of this study indicate that 60Co and 65Zn are widely dispersed in the marine environment around the central south coast of England, due to their release from AEA Winfrith and subsequent eastward transport and dispersion along regional sediment and water transport pathways. The rate at which 60Co has declined in surface sediments following reactor closure in 1990 has largely been controlled by the rate of sediment mixing resulting from (i) dilution, removal, or burial of 60Co-labeled material, and (ii) the extent to which older, more contaminated sediment (labeled during the 1980/ 81 peak discharge period) has been reworked. Sediment mixing may act to increase or decrease 60Co activity in surface 2848
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sediments - the exponential decline shown in Figure 2 indicates that regionally, dilution is the major process. There is no evidence for large-scale desorption of 60Co in these sediments (25). While activities of 60Co and 65Zn in seaweed, crustaceans, molluscs and bivalves have declined significantly following reactor closure, reworking of these radionuclides has caused continued persistence in marine organisms. In the case of 65Zn in seaweed, the effective half-life exceeds the radioactive half-life of 65Zn. The data indicate that even after virtual cessation of discharge from nuclear facilities, contamination of seaweed and marine fauna by particle-reactive radionuclides may persist for a number of years, albeit at reduced activities. The mechanisms causing this persistence are not clear, but are likely to involve absorption of radionuclides from sediments, and release and recycling of radionuclides via breakdown of contaminated organic matter.
Acknowledgments The authors acknowledge Sonia Bryant from the Southern England Radiation Monitoring Group for technical support and data collation, Callum Firth of Brunel University for advice on statistical analyses, and the environmental monitoring section of AEA Technology, Winfrith, Dorset. The authors also wish to thank three anonymous referees for helpful and constructive comments.
Literature Cited (1) Leonard, K. S.; McCubbin, D.; Brown, J.; Bonfield, R.; Brooks, T. Mar. Pollut. Bull. 1997, 34, 628-636. (2) MacKenzie, A. B.; Cook, G. T.; McDonald, P.; Jones, S. R. J. Environ. Radioact. 1998, 39, 5-53. (3) Salomon, J. C.; Guegeuniat, P.; Breton, M. Mathematical model of 125Sb transport and dispersion in the Channel. In Radionuclides in the study of marine processes; Kershaw, P. J., Woodhead, D. S., Eds.; Elsevier: New York, 1991; pp 74-84. (4) Guegueniat, P.; Bailly du Bois, P.; Salomon, J. C.; Masson, M.; Cabioch, L. J. Mar. Sys. 1995, 6, 483-494. (5) Boust, D. Cont. Shelf Res. 1999, in press. (6) Cutshall, N. H.; Larsen, I. L.; Olsen, C. R.; Nittrouer, C. A.; deMaster, D. J. Mar. Geol. 1986, 71, 125-136. (7) Bryan, G. W.; Preston, A.; Templeton, W. L. Accumulation of radionuclides by aquatic organisms of economic importance in the United Kingdom. Symposium on the disposal of radioactive wastes into seas, oceans and surface waters; IAEA-SM-72/39; IAEA: Vienna, 1966. (8) Fukai, M. Health Phys. 1990, 59, 879-889. (9) Nicholson, M. D.; Hunt, G. J. J. Environ. Radioact. 1995, 28, 43-56. (10) Mahara, Y.; Kudo, A. Health Phys. 1981, 41, 645-655. (11) Nakahara, M.; Koyanagi, T.; Saiki, M. Concentration of radioactive cobalt by seaweeds in the food chain; IAEA-SM-198/20; IAEA: Vienna, 1973. (12) Nolan, C.; Dahlgaard, H. Mar. Ecol. Prog. Ser. 1991, 70, 165174. (13) Croudace, I. W.; Higgins, A. Annual reports of the Southern England Radiation Monitoring Group; Portsmouth Civic Offices: 1988-1998; 90 pp. (14) UKAEA. Radioactive discharges and environmental monitoring; Annual reports; UKAEA Winfrith: 1985-1997. (15) International Atomic Energy Agency. Sediment Kds and concentration factors for radionuclides in the marine environment; Technical report series no. 247; IAEA: Vienna, 1985. (16) Cundy, A. B.; Croudace, I. W. J. Environ. Radioact. 1995, 29, 191-212. (17) Leonard, K. S.; McCubbin, D.; Harvey, B. R. J. Environ. Radioact. 1993, 20, 1-21. (18) Zhang, H.; Van Den Berg, C. M. G.; Wollast, R. Mar. Chem. 1990, 28, 285-300. (19) Lehninger, A. E. Biochemistry, 2nd ed.; Worth Publishers, Inc.: New York, 1976. (20) Bryan, G. W. J. Mar. Biol. Assoc. U.K. 1969, 49, 225-243. (21) Forsberg, Å.; So¨derlund, S.; Frank, A.; Petersson, L. R.; Pederse´n, M. Environ. Pollut. 1988, 49, 245-263. (22) Dahlgaard, H. Variation in radionuclide loss rates from Baltic Mytilus edulis. In Trace metals in the environment 1: heavy metals in the environment; Vernet, J.-P., Ed.; Elsevier: Amsterdam, 1991; pp 261-271.
(23) Fleming, A. A study of the uptake and potential uptake of zinc and cobalt by marine organsims with specific relevance to marine modelling and critical group assessment. RSD Tech. Memo 9/87, AEA Technology, Winfrith: U.K., 1987. (24) Preston, A. Helgolander Meeresunters 1968, 17, 269-279. (25) Cundy, A. B.; Croudace, I. W. Est. Coast. Shelf Sci. 1996, 43, 449-467. (26) Cundy, A. B. Radionuclide and geochemical studies of recent sediments from the Solent estuarine system. Ph.D. Thesis, University of Southampton, Southampton, U.K., 1994; unpublished. (27) Cundy, A. B.; Croudace, I. W.; Thomson, J.; Lewis, J. T. Environ. Sci. Technol. 1997, 31, 1093-1101. (28) Ministry of Agriculture, Fisheries and Food. Radioactivity in surface and coastal waters of the British Isles, 1993; Aquatic environment monitoring report no. 42; MAFF: Lowestoft, 1994. (29) Patel, B.; Patel, S. Radioecology of cobalt-60 under tropical environmental conditions. In Radionuclides in the study of marine processes; Kershaw, P. J., Woodhead, D. S., Eds.; Elsevier: New York, 1991; pp 276-282. (30) Nakamura, K.; Nagaya, Y. J. Oceanogr. Soc. Jpn. 1977, 33, 1-5. (31) Guimaraes, J. R. D.; Penna-Franca, E. Mar. Environ. Res. 1985, 16, 77-93.
(32) Dahlgaard, H.; Boelskifte, S. J. Environ. Radioact. 1992, 16, 4963. (33) Boelskifte, S. J. Environ. Radioact. 1985, 2, 215-227. (34) Luoma, S. N.; Bryan, G. W.; Langston, W. J. Mar. Pollut. Bull. 1982, 13, 394-396. (35) Van Weers. A. W. Uptake and loss of 65Zn and 60Co by the mussel Mytilus edulis. Radioactive contamination of the marine environment (Proc. Symp. Seattle 1972); IAEA: Vienna, 1973. (36) Bowles, P.; Burns, R. H.; Hudswell, F.; Whipple, R. T. P Exercise “Mermaid”; Report AERE E/R 2625; HMSO: London, 1958. (37) Dyer, K. R. Sedimentation and sediment transport. In The Solent estuarine system, an assessment of present knowledge; NERC Public Series C 22, 1980; pp 20-24. (38) Bray, M. J.; Carter, D. J.; Hooke, J. M. J. Coastal Res. 1995, 11, 381-400.
Received for review November 12, 1998. Revised manuscript received May 19, 1999. Accepted May 26, 1999. ES9811694
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