Land use modified recharge and hydrogeological controls on arsenic dynamics in a shallow Mekong Delta aquifer

Arsenic (As) contamination of shallow deltaic aquifers is strongly influenced by sedimentary redox processes, yet the roles of agriculturally modified recharge and local hydrogeology remain poorly resolved. This study investigated groundwater As dynamics along a river-inland transect in a known As hotspot of the Vietnamese Mekong Delta (VMD). Five shallow monitoring wells spanning river-adjacent, seasonal-cropland, and perennial-orchard settings were installed and sampled monthly from March 2023 to November 2024. Time-series hydrochemistry and groundwater levels were integrated with sediment characterization, stable isotopes, conservative tracers, As speciation, dissolved gases and SF₆ evidence. Groundwater was strongly reducing, with mean dissolved As increasing from 289 ± 15 μg L^-1 at the river-proximal well (KA-W1) to 656 ± 49 μg L^-1 at KA-W2 and 826 ± 43 μg L^-1 at KA-W3, reaching 960 ± 61 μg L^-1 at KA-W4 before decreasing to 802 ± 41 μg L^-1 at KA-W5. Approximately 89% of dissolved As occurred as As(III), consistent with reductive mobilization from Fe(III) (oxyhydr)oxides in organic-rich Holocene sediments. However, As did not vary systematically with river distance, Fe(II), or methanogenic advancement. KA-W1 showed strong river influence and comparatively dilute groundwater, whereas KA-W3 exhibited greater hydrogeochemical isolation and advanced anaerobic evolution. At KA-W4, located adjacent to intensively managed seasonal cropland, increasing Cl^-, major cations, NH₄⁺, CO₂, Fe(II), and As(III), together with stable-isotope and Cl/Br evidence, were consistent with land-use-modified, solute-enriched recharge. Fe-As decoupling further indicated roles for dilution, mixing, and mineralogical repartitioning. Overall, groundwater As dynamics reflect the interaction of geogenic reductive mobilization with hydrostratigraphic connectivity, recharge chemistry, hydrological renewal, and post-release partitioning. Agricultural management therefore acts as a context-dependent modifier of recharge rather than a direct source of As.