Concentrations and spatial distribution of plutonium in the terrestrial environment of the Marshall Islands

Measurements of plutonium in the terrestrial environment of the Marshall Is lands are one indicator of the degree of environmental contamination from n uclear weapons testing. Moreover, the spatial pattern of environmental plut onium concentrations is indicative of the pattern of total radionuclide dep osition from all of the nuclear tests. Measurements of plutonium (Pu239+240 ) to discern the spatial pattern of deposition and the degree of contaminat ion were made in soil samples collected from 1990 through 1993 at all 29 of the atolls which form the archipelago of the Marshall Islands. Measured co ncentrations ranged over nearly five orders of magnitude though the differe nce between the highest observed values and the estimated global fallout co ntribution was about four orders of magnitude. A strong gradient of increas ing concentration of plutonium in soil was noted with increasing latitude b etween 9 and 11.5 degrees N. The pattern of contamination and latitudinal i ncrease was similar to that for Cs-137. Global fallout deposition of pluton ium was estimated from two different data sets. Measurements of environment al plutonium in nearly 650 soil samples from the Marshall Islands are summa rized and compared to estimates of the global fallout deposition. The ratio of Pu239+240 in the soil to Cs-137 in the soil over a 1000 km distance was also examined to determine if there was evidence for fractionation, i.e. d ifferential deposition. It was found that the plutonium/cesium (Pu/Cs) rati o varied considerably with distance from the Bikini test site, decreasing w ith increasing distance. The ratio of Pu/Cs was found to decrease about 133 times over the 1000 km distance. (C) 1999 Elsevier Science B.V. All rights reserved.