Geochemical controls on arsenic mobilization from legacy mining sediments: Implications for human exposure

Legacy mining districts constitute persistent secondary sources of arsenic (As), yet the geochemical mechanisms controlling its remobilization during hydrological perturbations remain insufficiently resolved. We investigated arsenic mobility in the Orbiel Valley (former Salsigne mining district, SW France), where riverbank sediments, flood deposits, and mining wastes contain high As concentrations (up to 176,000 μg g^-1). Kinetic batch leaching experiments (120 h) were performed using upstream pristine river water to compare arsenic release from contrasting sediments under controlled oxic conditions. Mineralogical characterization (XRD, SEM-EDS, XANES) and single-step chemical extractions were used to constrain likely As host pools and their reactivity. Arsenic release varied markedly among sediment types (0.01-11.9% of total As) and showed no correlation with bulk As concentration, highlighting the dominant role of mineralogical controls. Two contrasting mobilization regimes were inferred from the combined mineralogical and geochemical dataset. In Fe-rich sediments, As was primarily associated with secondary phases such as scorodite and Fe(III) oxyhydroxides (ferrihydrite and goethite), resulting in limited but measurable release under oxic conditions. In carbonate-rich downstream sediments, higher proportional As mobilization was observed, linked to carbonate dissolution, pH buffering, and possible contributions from Ca-associated arsenic pools. Flood-derived suspended sediments displayed rapid initial release kinetics, indicating high-flow events enhancing As export. Highly variable bioaccessible As was highlighted (1-88%), closely reflecting mineralogical speciation. Sediments enriched in labile Fe-bound or carbonate-associated As exhibited greater bioaccessible fractions than scorodite-dominated materials. These results demonstrate that As mobility in legacy mining catchments is governed by phase-dependent reactivity and event-driven remobilization, underscoring the need to integrate mineralogical speciation and hydrological dynamics into environmental risk assessment.