Plutonium and americium in soils of Bikini Atoll

A study has been made to determine the concentrations of plutonium and americium in surface soils and in soil profiles on Bikini Atoll. The soils consist of calcareous materials and a thin layer of organic matter, which has produced a shallow, organically rich horizon suitable for certain plant growth. During the testing period from 1946 through 1958 Bikini Atoll was the site of 23 nuclear detonations, which contaminated the islands of the atoll with radioactive fall-out, including the transuranium elements. Plutonium and americium measurements of surface soil samples collected on 6 of the 26 islands of the atoll show that sup(239,240)Pu values vary from 0.5 to 360 pCi/g and 241Am values from 1.2 to 45 pCi/g. The vertical distribution of plutonium in soil varies from area to area. Although about 98% of the plutonium is retained in the top 25 cm in one core profile, the remaining 2% is detectable as deep as 100 cm. The suspension and resuspension of plutonium and plutonium-bearing particles by rainwater (150-175 cm/a) seems to be the principal mode of plutonium transport in the soil. Plutonium is found associated with the algal crust of the atoll soils. The present concentrations of sup(239,240)Pu and their distribution at Bikini are not likely to change significantly, whereas 241Am concentrations will increase by 241Pu decay and will approach a maximum value in about 50 years. (author)