SPATIAL-ANALYSIS OF PLUTONIUM-239+240 AND AM-241 IN SOILS AROUND ROCKY-FLATS, COLORADO

Plutonium and americium contamination of soils around Rocky Flats, Colorado resulted from past outdoor storage practices. Pour previous studies produced four different Pu isopleth maps. Spatial estimation techniques were not used in the construction of these maps and were also based on an extremely small number of soil samples. The purpose of this study was to elucidate the magnitude of Pu-239 + 240 and Am-241 dispersion in the soil environment east of Pocky Flats using robust spatial estimation techniques. Soils were sampled from 118 plots of 1.01 and 4.05 ha by compositing 25 evenly spaced samples in each plot from the top 0.64 cm. Plutonium-239 + 240 activity ranged from 1.85 to 53 560 Bq/kg with a mean of 1924 Bq/kg and a standard deviation of 6327 Bq/kg. Americium-241 activity ranged from 0.18 to 9990 Bq/kg with a mean of 321 Bq/kg and a standard deviation of 1143 Bq/kg. Geostatistical techniques were used to model the spatial dependency and construct isopleth maps showing Pu-239 + 240 and Am-241 distribution. The isopleth configuration was consistent with the hypothesis that the dominant dispersal mechanism of Pu-239 + 240 was wind dispersion from west to east. The Pu-239 + 240 isopleth map proposed by this study differed significantly in the direction and distance of dispersal from the previously published maps. This isopleth map as well as the Am-241 map should be used as the primary data for future risk assessment associated with public exposure to Pu-239 + 240 and Am-241.