Comparative uptake of U and Th by native plants at a U production site

During a 3- to 4-year period, concentrations of 238U, 234U, 230Th, 232Th and 228Th were determined in soils and native vegetation at various sites around a typical U mining and milling operation in Wyoming. Plant/soil concentration ratios (CR) for U and Th isotopes were estimated for (1) exposed, weathered tailings, (2) the edge of a tailings impoundment, (3) an area downwind from exposed tailings, (4) a reclamation area and (5) several background, native range locations. The 238U/234U concentration ratio of 0.9 to 1.1 in soil and vegetation indicated near-radioactive equilibrium of both radionuclides at all locations. Mean concentrations of the U and Th isotopes in background soil ranged from 44 to 52 mBq g-1. Concentrations of 238U and 230Th in soil and vegetation were elevated above background at all sites disturbed by mining and milling activities. Uranium concentrations in tailings and invading vegetation were an order of magnitude greater than in the background locations, whereas 230Th concentrations were elevated above background by some two orders of magnitude. No demonstrable differences in radionuclide concentrations between plant groups and collection years were found. The observed CR values for 238U and 230Th of 0.81 and 0.69 for vegetation growing on exposed tailings were elevated above native range by factors of 9.0 and 3.6, respectively, and generally higher than other published values. Exceptionally high CR values for 230Th (1.9-2.9) observed near the tailings impoundment demonstrate that under certain conditions, vegetation can accumulate 230Th to a much greater extent than previously reported. Vegetation concentrations were lower for 232Th relative to 230Th and 228Th at locations where they are present at similar soil concentrations.