In situ high-resolution insights into the dynamics of arsenic (As) species and heavy metals across the sediment-water interface in a deep karst reservoir.

Arsenic (As) and heavy metal contamination in aquatic systems pose critical environmental challenges, particularly in reservoirs. This study utilized dual-sided high-resolution diffusive gradients in thin films (DGT) probes on-site to investigate the spatial distribution and mobility of As species and heavy metals (Cd, Cr, Cu, Ni, Pb, Sb, and Zn) in the Hongfeng Reservoir, a deep karst reservoir in southwest China. Results revealed that As mobility was primarily governed by redox-sensitive processes, including the reduction of As(V) to As(III) and the reductive dissolution of Fe/Mn oxides. As(III) dominated porewater under reducing conditions, while As(V) was prevalent in overlying water under oxidative environments. Sulfate reduction significantly influenced As mobility, and competitive adsorption with P enhanced As release in eutrophic conditions. Heavy metals exhibited distinct spatial profiles and inter-element correlations, shaped by redox variability. Flux analysis identified sediments as sources for As, Fe, Mn, P, and S, and as sinks for most heavy metals. As(III) fluxes in the North Central reflected strong reducing conditions, while As(V) fluxes in the South Central highlighted localized oxidative processes. These findings offer valuable insights into geochemical processes in karst reservoirs, aiding in the understanding of contaminant dynamics and providing guidance for managing sediment pollution and protecting water quality.