Spatial evolution and geochemical controls of U- and Th-series radionuclides in acid mine drainage affected waters of the Odiel Basin, SW Spain.

Acid mine drainage (AMD) affects the mobility of naturally occurring radioactive materials (NORM), but its influence on the distribution of U- and Th-series radionuclides in polymetallic mining districts and downstream estuaries remains poorly understood. Here, we characterize the spatial evolution and isotopic signatures of U- and Th-series radionuclides in the dissolved fraction along the Oraque-Meca-Odiel river-estuary system (SW Spain). This system shows a strong geochemical gradient, from highly acidic AMD headwaters (locally pH < 3; high sulfate and metal concentrations) through weakly acidic reaches (pH ≈ 4.5) to near-neutral estuarine waters (pH ≈ 7). 238U and 232Th activity concentrations were strongly enriched in AMD-impacted headwaters, reaching values in the order of 103 and 102 mBq L-1, respectively, with 238U exceeding global riverine background by more than two orders of magnitude. PHREEQC modeling indicated that modeled aqueous U(VI) was dominated by free uranyl and uranyl-sulfate species under strongly acidic conditions. In contrast, dissolved 232Th decreased more strongly than U as pH increased, producing a clear UTh geochemical decoupling. This pattern is consistent with the generally higher particle reactivity of Th reported in previous studies and suggests transfer from the dissolved phase during downstream evolution. 234U/238U ratios were generally >1, indicating isotopic disequilibrium and preferential 234U mobilization. The 230Th/232Th activity ratios remained close to unity, indicating limited isotopic fractionation within the dissolved Th pool. These results show that the spatial variations of dissolved U and Th across the investigated river-estuary system coincided with hydrochemical gradients and changing source contributions, while the selected confluences showed nominal element-specific departures from the adopted sulfate-based mixing predictions.