Victor E. Noshkin

Fallout radionuclides in the Pacific Ocean: Vertical and horizontal distributions, largely from GEOSECS stations

Abstract From GEOSECS stations, largely, the 1974 distributions of Pu and of 137 Cs are described in the Pacific Ocean north of about 20°S latitude. Changes in some of these distributions are described from 1978 cruises by the authors. The Pacific exhibited, everywhere, a shallow subsurface layer of Pu-rich water with its concentration maximum at about 465 m in 1974; over a large portion of the central North Pacific a second layer of Pu-labelled water, less concentrated than the shallow layer, lay just above the bottom. Similar features were not observed in the case of 137 Cs. The inventories of both Pu and 137 Cs in the water column at most 1974 stations are substantially greater than those to be expected from world-wide fallout alone; these inventory excesses appear to be attributable to close-in fallout, but only if the ratio Pu/ 137 Cs in this source was much higher than in world-wide fallout. The North Pacific mean ratio of the inventories is 2.2 times that observed in world-wide fallout. Resolubilization of Pu both from sinking particles and from sediments explains peculiarities of its depth distributions. There is little evidence for tracer movement by sliding downward along density surfaces; 137 Cs appears to have moved to depth by downmixing at the edge of the Kuroshio, and then moved horizontally and upward alongσ t contours. The shallow Pu-rich layer shows no coordination with density, salinity or O 2 isopleths. The deep Pu-rich layer is restricted to a narrow range of O 2 concentrations that confirm its origin in the Aleutian Trench and rapid spread southward and laterally. Near-bottom circulation processes have been much more active than here-to-fore described.

Plutonium and210Pb distributions in northeast Atlantic sediments: subsurface anomalies caused by non-local mixing

Abstract The depth distributions of 210 Pb and 239,240 Pu measured in a suite of box cores collected from water depths of 4000–5000 m in the northeast Atlantic Ocean exhibit pronounced subsurface maxima caused by sediment reworking by benthic infauna. Small-scale spatial heterogeneity in bioturbation rates is indicated by large differences in tracer profiles from duplicate cores separated only by a few centimeters. 210 Pb and 239,240 Pu activity distributions from each subcore exhibit a high degree of correlation, and most tracer profiles exhibit one or more subsurface maxima. One-dimensional, “biodiffusion” analogue models do not adequately simulate the principal features of this data set. However, an inverse “conveyer belt” mixing model which simulates subsurface egestion (or a functionally equivalent process) of surficial material which is enriched both in organic debris and radioactive tracers can reproduce the subsurface tracer maxima. Single-event and continuous subsurface egestion models have been formulated and solved for different “feeding rates” and background biodiffusive fields. The single-event model provides a better fit to the data and, in particular, ensures the observed, high degree of correlation between the 210 Pb and 239,240 Pu activity profiles, regardless of the different tracer input functions. The most likely candidate responsible for subsurface tracer egestion is a large infaunal worm of the phylum Sipunculida which dominates the biomass below a depth of 3 cm.

Budgets and behaviors of uranium and thorium series isotopes in Santa Monica Basin sediments

Abstract Nine natural decay-series isotopes were measured in six box cores collected from a transect across the Santa Monica Basin. The 210 Pb-derived sedimentation rate decreases from ~80 mg/cm 2 -yr at the slope to ~20 mg/cm 2 -yr in the deep central basin. Sediment mixing prevails in sites underlying oxic waters, but is subdued in the anoxic deep basin below the sill depth. Uranium contents in sediments are controlled by levels of authigenic U, which are higher in the more reduced condition in the deep basin. Most of the authigenic U results from precipitation within the sediments. The 232 Th- 228 Th disequilibrium in sediments indicates that 228 Ra is lost from the sediments from a depth of ~ 10 cm upward. Modelling the distribution of excess 228 Th and 234 Th in the surficial layers of the deep basin sediments results in a mean sediment mixing coefficient of 0.2 cm 2 /yr and a sedimentation rate close to that based on 210 Pb. There is no evidence of changing sedimentation rate in the central basin during the past century. Fluxes of excess 210 Pb, 230 Th and 231 Pa to the central Santa Monica Basin sediments are much higher than what can be predicted from local supply. Advective input of open ocean waters coupled with enhanced scavenging of these reactive nuclides at the ocean margin is considered to be the primary cause.

Radionuclide characterization and associated dose from long-lived radionuclides in close-in fallout delivered to the marine environment at Bikini and Enewetak Atolls.

Between June 1946 and October 1958, Enewetak and Bikini Atolls were used by the US as testing grounds for 66 nuclear devices. The combined explosive yield from these tests was 107 million t (million t TNT equivalents). This testing produced close-in fallout debris that was contaminated with quantities of radioactive fission and particle activated products, and unspent radioactive nuclear fuel that entered the aquatic environment of the atolls. Today, the sediments in the lagoons are reservoirs for tens of TBq of the transuranics and some long-lived fission and activation products. The larger amounts of contamination are associated with fine and coarse sediment material adjacent to the locations of the high yield explosions. Radionuclides are also distributed vertically in the sediment column to various depths in all regions of the lagoons. Concentrations greater than fallout background levels are found in filtered water sampled over several decades from all locations and depths in the lagoons. This is a direct indication that the radionuclides are continuously mobilized to solution from the solid phases. Of particular importance is the fact that the long-lived radionuclides are accumulated to different levels by indigenous aquatic plants and organisms that are used as food by resident people. One might anticipate finding continuous high contamination levels in many of the edible marine organisms from the lagoons, since the radionuclides associated with the sediments are not contained and are available to the different organisms in a relatively shallow water environment. This is not the case. We estimate that the radiological dose from consumption of the edible parts of marine foods at Enewetak and Bikini is presently approximately 0.05% of the total 50-year integral effective dose from all other exposure pathways that include ingestion of terrestrial foods and drinking water, external exposure and inhalation. The total radiological dose from the marine pathway is dominated by the natural radionuclides, 210Po and 210Pb. Man-made radionuclides presently contribute < 0.3% of the dose from these natural radionuclides in the marine food chain and within approximately 90 years only 0.15%.

Past and present levels of some radionuclides in fish from Bikini and Enewetak atolls

Bikini and Enewetak were the sites in the Northern Marshall Islands that were used by the United States as testing grounds for nuclear devices between 1946 and 1958. The testing produced close-in fallout debris that was contaminated with different radionuclides and which entered the aquatic environment. The contaminated lagoon sediments became a reservoir and source term of manmade radionuclides for the resident marine organisms. This report contains a summary of all the available data on the concentrations of 137Cs, 60Co and 207Bi in flesh samples of reef and pelagic fish collected from Bikini and Enewetak Atolls between 1964 and 1995. The selection of these three radionuclides for discussion is based on the fact that these are the only radionuclides that have been routinely detected by gamma spectrometry in flesh samples from all fish for the last 20 y. Flesh from fish is an important source of food in the Marshallese diet. These radionuclides along with the transuranic radionuclides and 90Sr contribute most of the small radiological dose from ingesting marine foods. Some basic relationships among concentrations in different tissues and organs are discussed. The reef fish can be used as indicator species because their body burden is derived from feeding, over a lifetime, within a relatively small contaminated area of the lagoon. Therefore, the emphasis of this report is to use this extensive and unique concentration data base to describe the effective half lives and cycling for the radionuclides in the marine environments during the 31-y period between 1964 and 1995. The results from an analysis of the radionuclide concentrations in the flesh samples indicate the removal rates for the 3 radionuclides are significantly different. 137Cs is removed from the lagoons with an effective half life of 9-12 y. Little 60Co is mobilized to the water column so that it is depleted in both environments, primarily through radioactive decay. The properties of 207Bi are different at Enewetak and Bikini. At Enewetak the radionuclide is lost from the environment with an effective half live of 5.1 y. At Bikini only radioactive decay can account for the rate at which the radionuclide is lost from the lagoon. The difference in the binding properties of the sedimentary materials for 207Bi among the two Atolls is not understood.

The Northern Marshall Islands radiological survey: Data and dose assessments

Fallout from atmospheric nuclear tests, especially from those conducted at the Pacific Proving Grounds between 1946 and 1958, contaminated areas of the Northern Marshall Islands. A radiological survey at some Northern Marshall Islands was conducted from September through November 1978 to evaluate the extent of residual radioactive contamination. The atolls included in the Northern Marshall Islands Radiological Survey (NMIRS) were Likiep, Ailuk, Utirik, Wotho, Ujelang, Taka, Rongelap, Rongerik, Bikar, Ailinginae, and Mejit and Jemo Islands. The original test sites, Bikini and Enewetak Atolls, were also visited on the survey. An aerial survey was conducted to determine the external gamma exposure rate. Terrestrial (soil, food crops, animals, and native vegetation), cistern and well water samples, and marine (sediment, seawater, fish and clams) samples were collected to evaluate radionuclide concentrations in the atoll environment. Samples were processed and analyzed for 137Cs, 90Sr, 239+240Pu and 241Am. The dose from the ingestion pathway was calculated using the radionuclide concentration data and a diet model for local food, marine, and water consumption. The ingestion pathway contributes 70% to 90% of the estimated dose. Approximately 95% of the dose is from 137Cs. 90Sr is the second most significant radionuclide via ingestion. External gamma exposure from 137Cs accounts for about 10% to 30% of the dose. 239+240Pu and 241Am are the major contributors to dose via the inhalation pathway; however, inhalation accounts for only about 1% of the total estimated dose, based on surface soil levels and resuspension studies. All doses are computed for concentrations decay corrected to 1996. The maximum annual effective dose from manmade radionuclides at these atolls ranges from .02 mSv y(-1) to 2.1 mSv y(-1). The background dose in the Marshall Islands is estimated to be 2.4 mSv y(-1). The combined dose from both background and bomb related radionuclides ranges from slightly over 2.4 mSv y(-1) to 4.5 mSv y(-1). The 50-y integral dose ranges from 0.5 to 65 mSv.

Radionuclide concentrations, fluxes, and residence times at Santa Monica and San Pedro Basins

Our participation in the California Basin Study (CaBS) in 1986–1988 has produced a radionuclide data base that allows us to trace the particle and water movement in the Santa Monica and the San Pedro Basins. These data enable us to calculate the radionuclide inventories in the three compartments of the basin, the water column, the settling particles, and the sediments. We have also determined the fluxes of several radionuclides associated with the sinking particles, the residence time in the water column, and the rate of deposition in the sediments. The biogeochemical environment of the Santa Monica Basin is uniquely suited to the application of tracer techniques to study the sedimentation processes. Subtle, but measurable changes in the distribution of fallout of 137Cs from 1977 to 1988 were noted in the water column. Compared to the decay corrected measurement in 1973–1977 at the GEOSECS and Farallon Islands stations off the coast of California, 137Cs inventory in the water column in Santa Monica Basin has not changed significantly for the past decade. Increases in the deep water concentration profiles of 137Cs particle transport and diagenetic release of cesium in the deep water. The major 137Cs inventory (>70%) resides in the upper 400m of water, with a mean residence time of 480 ± 90 years. Less than 11% of the total 137Cs inventory was found in the sediments at water depths of 900m in 1987. Unlike 137Cs, more than 80% of the fallout 239+240Pu has been deposited in the Santa Monica Basin sediments. A mean residence time of 13 ± 2 years was measured for 239+240Pu in the basin water. The ubiquitous 239+240Pu subsurface concentration maximum observed in the Pacific oceanic waters at about 350m depth is also evident in the basin water. We have no definitive explanation for the persistence of the subsurface maximum except that it appears to be correlated with the density gradient at 26.5 to 26.9 sigma-t. The 239+240Pu flux for settling particles correlates remarkably well with mass flux, and it increases with depth in the water column. Plutonium deposition in anoxic sediments also appears to be useful as a time marker for age-dating of sediments and estimating the sediment-accumulation rates. The sedimentation rates determined by plutonium are shown to be comparable to those estimated by excess 210Pb in the cores. Radionuclide fluxes measured from sediment trap particles in 1986–1988 were lower by a factor of 0.6 ± 0.2, compared to the average fluxes determined from the box core samples. Radionuclides, such as 210Pb, 137Cs, and 239+240Pu have been shown to be useful tools in the study of biogeochemical processes in the Santa Monica Basin.

Concentration of 210Po and 210Pb in the diet at the Marshall Islands

The concentrations of 210Po and 210Pb have been determined in many local foods consumed by societies residing on different atolls in the Marshall Islands. The average daily intake of these two naturally occurring radionuclides from local and imported food is estimated to be 2.18 and 0.36 Bq, respectively. Local foods contribute 87% of the 210Po and 47% of the 210Pb associated with the diet. The items contributing the majority of the activity to the diet are derived from the marine environment and include parts of fish, invertebrates, seabirds and eggs of seabirds. The committed effective dose from ingestion of 210Po and 210Pb is approximately 2 mSv/year (200 mrem/year). This pathway now contributes 83% of the natural background irradiation received by residents in the Marshall Islands. Because the naturally occurring radionuclides are omnipresent in terrestrial and marine foods at all atolls, the annual intake and computed dose can be considered as typical values for individuals with comparable diets and inhabiting other islands in the Pacific.

Behavior of plutonium isotopes in the marine environment of Enewetak atoll

There continue to be reports in the literature that suggest a difference in the behavior of239+240Pu and238Pu in some aquatic environments. Plutonium isotopes have been measured in marine samples collected over 3 decades from Enewetak atoll, one of the sites in the Marshall Islands used by the United States between 1946 and 1958 to test nuclear devices. The plutonium isotopes originated from a variety of complex sources and could possibly coexist in this environment as different physical-chemical species. However results indicate little difference in the mobility and biological availability of239+240Pu and238Pu.

Plutonium Concentrations in Fish and Seawater From Kwajalein Atoll

A follow-up study has been made to assess the concentrations of z3y+24’Pu and Cs in the marine environment of Kwajalein Atoll. Fish collected from the atoll in 1972 had body burdens of plutonium that were substantially higher than concentrations in similar species from locations contaminated only with global fallout. Our recent results, however, indicated that Kwajalein lagoon seawater contained levels of plutonium more similar to global fallout levels found in north equatorial Pacific surface waters. N o satisfactory explanation for the reported plutonium levels in fish from Kwajalein collected in 1972 could be deduced from the available data. The highest plutonium concentrations reported for the 1972 reef species of fish could expose man, through ingestion of marine foods, to a dose rate as high as 25% of the proposed EPA guideline for annual total transuranic dose rate to bone (3 mrad/yr over 70 yr). Our present results show the dose rate from the marine food pathway is nearer to 0.005% of the recommended EPA value and is consistent with the view that Kwajalein Atoll contains plutonium concentrations that are expected from global fallout. The magnitude of the plutonium levels reported in fish collected from Kwajalein lagoon during 1972 was excessively high. and these results appear to be inconsistent with other environmental data from the lagoon. Our results also show that concentration factors for plutonium in fish muscle and bone tissues appear to be independent of species, trophic level and location, which leads us to believe that there is a great deal of validity in the concept of a concentration factor for estimating concentrations of plutonium in fish. I37