Geochemical behaviour and health risks of trace elements in karst water systems: Role of allogenic water in mitigating contamination.

Karst water systems, vital to global freshwater security, are susceptible to trace elements (<1 mg/L) and major anion contamination, particularly in regions with high geological metal backgrounds and dynamic water recharge. Yet the influence of externally sourced (allogenic) water on mitigating such contamination in karst water systems is poorly understood. This study investigates the geochemical behaviour and health risks of trace elements in allogenic water (Aw), surface water recharged by Aw (ASw), groundwater recharged by Aw (AGw), and karst groundwater (Kw) in Guangxi, China. While most trace metal levels met Chinese drinking water standards (GB 5749-2022), Fe exceeded 0.3 mg/L in 34.5 % of samples, and Al and Mn surpassed permissible limits in 13.8 %. Monte Carlo simulations revealed Mn as the dominant non-carcinogenic risk via dermal exposure, with As and Cd posing ingestion risks, especially for children. Multivariate analysis revealed that redox and oxide/clay weathering primarily drive Al, Fe, Mn, Pb, Cd, and As mobility, while carbonate weathering influences Ca, Cr, Ni, and Cd. Agricultural runoff and dilution further shape water chemistry. Allogenic recharge mitigates contamination in AGw through dilution, redox shifts, and adsorption. These findings underscore the need to manage redox-sensitive processes and prioritize Mn, As, and Cd for long-term health protection in karst areas.