Distribution of uranium-238 in environmental samples from a residential area impacted by mining and milling activities

The northern region of Karnes County, Texas, USA, has been the site of extensive mining/milling of uranium for over 30 years. A previous study in our laboratory indicates that residents living near these facilities have increased chromosomal aberrations and a reduced DNA repair capacity. In this study, the long-lived radionuclides uranium-238 (²³⁸U) and thorium-232 (²³²Th) were measured in order to evaluate the extent of contamination from mining/milling facilities. ²³²Th was quantified simultaneously and served as a reference. Soil samples (n = 70) were collected from the yards of previously studied households and adjacent areas near former mining and mining/milling sites at the surface and 30 cm subsurface. Additionally, samples from drinking water wells (n = 6) were collected from selected households. Sites located over 14 km from the study area with no known history of mining/milling served as the control. In the control area, ²³⁸U concentrations in soil were consistent between surface (0.13-0.26 mg/kg) and subsurface (0.13-0.23 mg/kg) samples. Near mining/milling sites, ²³⁸U in surface soil (0.34-15.71 mg/kg) was found to be consistently and statistically higher (p < 0.05) than corresponding subsurface samples (0.17- 2.11 mg/kg). Near mining-only areas, ²³⁸U in surface soil (1.11-1.97 mg/kg), however, was not significantly increased over subsurface soil (0.96- 1.68 mg/kg). As expected, ²³⁸U was much higher overall in the mining/milling and mining-only areas compared to the control sites. No trends were detected in the distribution of ²³²Th. The concentration of ²³⁸U was up to six times higher in a drinking water well near a former mining/milling operation, indicating possible leaching into the groundwater, while ²³²Th concentrations were low and uniform. Furthermore, lead isotope ratio analysis indicates contamination from the interstate shipping of ore by rail to and from a mining/milling facility. These data indicate contamination of the environment by the mining/milling activities in a residential area.