URANIUM ISOTOPES DISTRIBUTION IN SOILS AT THE ROCKY-FLATS PLANT, COLORADO

Soils east of Rocky Flats Plant (RFP) near Golden, CO, are contaminated with U as a result of past waste-storage practices, accidental release of oils laden with U, and low-level airborne emissions. A spatial analysis of three U isotopes was conducted to determine the concentration and distribution pattern of U contamination. Soils were sampled from 118 plots of 1.01- or 4.05-ha by compositing 25 evenly spaced samples from the top 0.64 cm. Uranium-234 activity ranged from 25.9 to 92.8 Bq kg(-1), U-235 activity ranged from 0.1 to 25.1 Bq kg(-1), whereas U-238 activity ranged from 30.7 kg(-1) to 286 Bq kg(-1). Geostatistical techniques were used to model the spatial dependency and construct isopleth maps showing U isotope distributions. Spatial correlation was not observed for U-234. This implies that U-234 is randomly distributed in the soil environment east of RFP. Uranium-235 exhibited a spotty and localized concentration pattern with no clear relationship between known burial and spill sites, and the present distribution of U-235 in the soils. Proposed wind-dispersal mechanisms were not consistent with the spatial distribution of U isotopes. Most of the observed activities of U-234 and U-235 were well within the natural range of U isotopes in soils. The lack of similarity in spatial distribution between Pu-239 + 240 and U isotopes probably resulted from the higher solubility and leachability of U isotopes compared with Pu-239 + 249 in the soil system. Although U-238 exhibited a pattern of localized spatial distribution, most of its observed activity was well within the natural range of U-238 activity in soils.