Legacy Mining Shapes Toxic Element Accumulation and Source-oriented Ecological and Health Risks in the Soil-sediment System of a Mountainous Reservoir Basin， North China

Legacy mining has left a persistent imprint on toxic element accumulation and risk in mountainous reservoir basins， yet its relative importance compared with diffuse agricultural inputs remains insufficiently resolved at the soil-sediment system scale. Here， we investigated the accumulation patterns， source categories， and ecological and human health risks of toxic elements (Cr， As， Cu， Pb， Zn， Ni， Cd， and Hg) in the soil-sediment system of the Miyun Reservoir Basin， North China. By integrating newly collected topsoil samples with published soil and sediment data， we combined geo-accumulation and enrichment assessments with self-organizing map， positive matrix factorization， and source-oriented risk analysis to identify priority pollutants， quantify source contributions， and determine high-risk areas. Hg， Cr， Cd， and Cu were identified as the priority pollutants and exhibited marked spatial heterogeneity across the basin. Four source categories were resolved， including geogenic background overprinted by legacy mining (52.71%)， agricultural inputs (5.51%)， fossil fuel combustion (18.10%)， and mixed geogenic-agricultural-traffic sources (23.68%). Ecological risk was dominated by Hg and Cd. Non-carcinogenic risks were generally acceptable， whereas carcinogenic risks， driven mainly by Cr and As， exceeded the negligible threshold， particularly for children in the northern sub-basins. Source-oriented risk analysis further revealed that legacy mining-related sources were the dominant contributors to carcinogenic risk. These findings demonstrate the long-term hazard imprint of historical mining on the soil-sediment system of mountainous reservoir basins and provide a basis for source-prioritized risk management in water-source watersheds.