Arsenic Mobilization in Clay–Aquifer Systems: Biogeochemical and Hydrological Controls

Increasing evidence indicates that microbial metabolism and mineral transformations within low-permeability clay layers, together with solute exchange between clay layers and adjacent aquifers, can jointly regulate arsenic (As) release, immobilization, and remobilization. However, how coupled C, N, S, and Fe cycling in clay layers regulates As oxidation state, solid–liquid partitioning, and transport, and how these effects extend across clay–aquifer interfaces, has not been critical narratively synthesized. This review focuses on subsurface clay layers and summarizes their hydrogeochemical properties, microbial communities, and vertical redox zonation. It examines how microbially mediated C–N–S–Fe processes influence As release, transformation, and immobilization, and further discusses the export of reactive constituents from clay layers, biogeochemical reactions at clay–aquifer interfaces, As enrichment in adjacent aquifers, and the effects of groundwater pumping on As transport. Available studies suggest that As behavior is condition-dependent and reflects interactions among microbial metabolism, electron-donor and electron-acceptor availability, mineral transformations, and interfacial transport, which together determine whether As is retained, released, or remobilized. Management implications and future research priorities are also considered. The resulting cross-scale framework links biogeochemical processes within clay layers with clay–aquifer exchange and aquifer responses, providing a basis for long-term groundwater As risk assessment, monitoring, and management.