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The Effect of Carbonate Soil on Transport and Dose Estimates for Long-Lived Radionuclides at a U.S. Pacific Test Site

ABSTRACT

The United States conducted a series of nuclear tests from 1946 to 1958 at Bikini, a coral atoll, in the Marshall Islands (MI). The aquatic and terrestrial environments of the atoll are still contaminated with several long-lived radionuclides that were generated during testing. The four major radionuclides found in terrestrial plants and soils are cesium-137, strontium-90, plutonium-239+240 and americium-241. Cesium-137 in the coral soils is more available for uptake by plants than 137Cs associated with continental soils of North America or Europe. Soil-to-plant 137Cs median concentration ratios (CR) (kBq·kg-1 dry weight plant/kBq·kg-1 dry weight soil) for tropical fruits and vegetables range between 0.8 and 36, much larger than the range of 0.005 to 0.5 reported for vegetation in temperate zones. Conversely, 90Sr median CRs range from 0.006 to 1.0 at the atoll versus a range from 0.02 to 3.0 for continental silica-based soils. Thus, the relative uptake of 137Cs and 90Sr by plants in carbonate soils is reversed from that observed in silica-based soils. The CRs for 239+240Pu and 241Am are very similar to those observed in continental soils. Values range from 10-6 to 10-4 for both 239+240Pu and 241Am. No significant difference is observed between the two in coral soil. The uptake of 137Cs by plants is enhanced because of the absence of mineral binding sites and the low concentration of potassium in the coral soil. Cesium-137 is bound to the organic fraction of the soil, whereas 90Sr, 239+240Pu and 241Am are primarily bound to soil particles. Assessment of plant uptake for 137Cs and 90Sr into locally grown food crops was a major contributing factor in: (1) reliably predicting the radiological dose for returning residents and (2) developing a strategy to limit the availability and uptake of 137Cs into locally grown food crops.