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

Measurements of plutonium in the terrestrial environment of the Marshall Islands are one indicator of the degree of environmental contamination from nuclear weapons testing. Moreover, the spatial pattern of environmental plutonium concentrations is indicative of the pattern of total radionuclide deposition from all of the nuclear tests. Measurements of plutonium (239+240Pu) to discern the spatial pattern of deposition and the degree of contamination 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 concentrations ranged over nearly five orders of magnitude though the difference between the highest observed values and the estimated global fallout contribution was about four orders of magnitude. A strong gradient of increasing concentration of plutonium in soil was noted with increasing latitude between 9 and 11.5oN. The pattern of contamination and latitudinal increase was similar to that for 137Cs. Global fallout deposition of plutonium was estimated from two different data sets. Measurements of environmental plutonium in nearly 650 soil samples from the Marshall Islands are summarized and compared to estimates of the global fallout deposition. The ratio of 239+240Pu in the soil to 137Cs in the soil over a 1000 km distance was also examined to determine if there was evidence for fractionation, i.e. differential deposition. It was found that the plutonium/cesium (Pu/Cs) ratio varied considerably with distance from the Bikini test site, decreasing with increasing distance. The ratio of Pu/Cs was found to decrease about 133 times over the 1000 km distance.