Multi-compartment assessment of thallium in a typical agricultural watershed in China: Occurrence, criteria...

Thallium (Tl) contamination in the Huashan Stream Basin, Pinghe County, has sparked significant concern following its detection in a critical drinking water source. This study performed a multi-compartment assessment, spanning river water, groundwater, soil, sediment, and vegetation, to characterize spatial distribution patterns and quantify ecological risks. Our findings revealed that the average Tl concentrations were 1.35 ± 0.602 mg/kg in soil and 0.135 ± 0.116 μg/L in surface water, while Tl levels in vegetation ranged from 0.000390 to 0.241 mg/kg. Spatial analysis identified a legacy mining district in the northwest as the primary point source, while the intensive use of Tl-enriched fertilizers in pomelo orchards constitutes a critical diffuse pathway. Using species sensitivity distribution (SSD) modeling, we derived a long-term aquatic predicted no-effect concentration (PNEC) of 0.285 μg/L and a background-adjusted terrestrial PNEC of 0.921 mg/kg. Probabilistic risk assessment (PRA) via joint probability curves (JPC) revealed a moderate ecological risk for soil, with an overall risk probability (ORP) of 29.17 %, whereas the ORP for aquatic compartments remained negligible, ranging from 0.27 % to 0.89 %. Furthermore, exceedance risk probability (ERP) analysis indicated that soil Tl posed severe risks at ERP₅ and ERP₂₀ levels, whereas aquatic risks were negligible. Our findings suggest that integrating geogenic background values into probabilistic frameworks is essential to avoid overestimating risk. To mitigate Tl ecological risks, we recommend controlling soil acidification to reduce bioavailability and restricting Tl-containing fertilizers. This study highlights the necessity of integrated management to balance agricultural productivity with ecological safety.