Runoff seasonal fluctuations on spatiotemporal accumulation and mobility of arsenic and thallium in mine stream sediments

Arsenic (As) and thallium (Tl) in mining-affected stream sediments exhibit distinct mobility patterns and ecological risks under varying hydrological conditions. This study investigates the spatiotemporal accumulation and mobility of arsenic and thallium in stream sediments under seasonal runoff variations. The results showed that runoff variations during the normal season (NS) and wet season (WS) significantly influence arsenic and thallium concentrations and mobility in stream sediments at the mining watershed. Elevated water levels during NS and WS increased As and Tl concentrations in sediments to 4.49-7.45 times and 0.52-1.09 times compared to dry season (DS), respectively. Furthermore, As predominantly accumulated in surface sediments (0-20 cm depth), whereas Tl showed greater enrichment in deeper layers (20-60 cm). Notably, active fractions of As and Tl in the upper sediment layer (10-20 cm) during NS and WS were 2-8% higher than those observed in the DS. Fine sediment particles (50-250 µm) serve as the key transport medium for As and Tl during NS and WS, demonstrating 6-21% higher active fractions compared to DS. Increased runoff velocity reduced channel contamination duration from 57.12 h in the DS to 12.96 h and 4.8 h in the NS and WS, respectively, with the highest potential risk occurring in stream curved zones. Runoff-induced hypoxic conditions elevated As and Tl concentrations in sediment porewater during NS and WS, which quinone-mediated organic matter transformation and acidophilic microbial community biogeochemistry were identified as primary regulators of As and Tl mobility. This study provides a critical foundation for As and Tl seasonal mitigation risk in mining-affected watershed sediments, which is optimized for control strategies at mining sites.