Geochemical, mineralogical and particle-size controls on origin, transport and storage of sediment-associated metal(loid)s in an acid mine drainage-affected river

Acid mine drainage (AMD) continues to contaminate river systems worldwide, yet the interactions of metal(loid)s with sediments and the controls on their storage and remobilisation remain poorly constrained. This study examines the geochemical and mineralogical factors governing sediment-associated As, Cu, and Zn along an AMD-impacted river system (Carnon River, UK). To assess the interaction of metal(loid)s in sediments, their mineral hosts, and distribution over different particle sizes were investigated. Sediment concentrations reached 8210 mg/kg As, 5040 mg/kg Cu, 2760 mg/kg Zn, and 137,000 mg/kg Fe, substantially above guideline values and persisting downstream of the initial AMD input. Sulfide and sulfate minerals hosted Zn and Fe oxides were the primary hosts for As and Cu. Arsenic was strongly correlated with Fe, suggesting association with secondary Fe-oxide. Elevated downstream concentrations of such ecotoxic metal(loid)s within sediments indicated continuous input from both AMD precipitation and remobilised historic waste and limited natural attenuation. This integrated approach reveals a bimodal distribution: sorption/co-precipitation with secondary Fe-oxide or coarse sulfide sources that sustained contamination decades after mining ceased. These findings demonstrate how sediment mineralogy and particle size dictate contaminant mobility and long-term storage in AMD-affected catchments.