Uptake, vertical distribution, migration, and phytoremediation of thorium progeny at four radioactive waste sites

The release rates and transformation processes that influence the mobility, biological uptake, and transfer of radionuclides is essential for the ecological and human health assessment. This study examined concentrations of 232Thorium (232Th) in both soil and vegetation at a closed military training site, Kirtland Air Force Base (KAFB), New Mexico. Brazilian sludge was intentionally introduced into the topsoil of four sites in the early 1960s to simulate nuclear weapons accidents. Soil (60) and vegetation (120) samples were collected from 1996–2000 and analyzed for radionuclides and progeny. High-resolution gamma-ray spectroscopy was used to determine radionuclide activities. The results indicate that the thorium progeny were the predominate contamination in soil and vegetation. Concentration ratios (CR) were calculated based on actinium levels. Vertical soil characterization and migration of radionuclides were investigated at four radioactively contaminated sites to determine the vertical downward migration of radionuclides in a semi-arid environment. The majority of the radioactivity was found in the top 15cm of soil, with retention ranging from 69–88%. The migration rates of actinium were calculated with the diffusion compartment model for all sites, and ranged from 0.009 to 0.1 cm/yr increasing with depth. The migration rates calculated with the physical models were similar to those using the diffusion compartment model and did not increase with depth (0.061, 0.076, 0.045 and 0.075 cm/yr, respectively). It is demonstrated that the physical and chemical properties governing transport processes of water and solutes in soil provide the best approach for a valid radionuclide transport model.; The third study examined the concentrations of 232Th progeny in two plant species to determine uptake differences between species at a radioactively contaminated site and control. The soil at the site was intentionally contaminated with 232Th in the early 1960s to simulate the conditions resulting from a nuclear weapons accident. Soil (n = 70) and vegetation (n = 167) samples were collected, dried, ashed, and analyzed by gamma-ray spectroscopy. The results of the study show a significant variation between the radionuclide concentrations in site and control grasses and tumbleweeds and that phytoremediation is a viable and cost effective method for radionuclides from large contaminated areas of soil.