Geochemical distributions of natural radionuclides in surface soils and sediments impacted by lead-zinc mining activity

This study investigated the distribution features of uranium-238 (²³⁸U), radium-226 (²²⁶Ra), thorium-232 (²³²Th), and potassium-40 (⁴⁰K) and evaluated the associated environmental radiological hazards of the topsoil and river sediments in the Jinding lead-zinc (Pb-Zn) mine catchment from Southwest China. The activity concentrations of ²³⁸U, ²²⁶Ra, ²³²Th, and ⁴⁰K ranged from 24.0 ± 2.29–60.3 ± 5.26 Bq.kg^-1, from 32.5 ± 3.95–69.8 ± 3.39 Bq.kg^-1, from 15.3 ± 2.24–58.3 ± 4.92 Bq.kg^-1, and from 203 ± 10.2–1140 ± 27.4 Bq.kg^-1, respectively. The highest activity concentrations for all these radionuclides were primarily found in the mining areas and decreased with increasing distance from the mining sites. The radiological hazard indices, including radium equivalent activity, absorbed gamma dose rate in the air, outdoor annual effective dose equivalent, annual gonadal dose equivalent, and excess lifetime cancer, revealed that the highest values were observed in the mining area and downstream, specifically in the vicinity of the ore body. These elevated values exceeded the global mean value but remained below the threshold value, suggesting that routine protection measures for Pb-Zn miners during production activities are sufficient. The correlation analysis and cluster analysis revealed strong associations between radionuclides such as ²³⁸U, ²²⁶Ra, and ²³²Th, indicating a common source of these radionuclides. The activity ratios of ²²⁶Ra/²³⁸U, ²²⁶Ra/²³²Th, and ²³⁸U/⁴⁰K varied with distance, suggesting the influence of geological processes and lithological composition on their transport and accumulation. In the mining catchment areas, the variations in these activity ratios increased indicated the impact of limestone material dilution on the levels of ²³²Th, ⁴⁰K, and ²³⁸U in the upstream region. Moreover, the presence of sulfide minerals in the mining soils contributed to the enrichment of ²²⁶Ra and the removal of ²³⁸U caused those activity ratios decreased in the mining areas. Therefore, in the Jinding Pb[sbnd]Zn deposit, the patterns of mining activities and surface runoff processes in the catchment area favored the accumulation of ²³²Th and ²²⁶Ra over ⁴⁰K and ²³⁸U. This study provides the first case study on the geochemical distributions of natural radionuclides in a typical Mississippi Valley-type Pb[sbnd]Zn mining area and offers fundamental information on radionuclide migration and baseline radiometric data for Pb[sbnd]Zn deposits worldwide.