296-4 BEHAVIOR OF GEOGENIC ARSENIC IN SHALLOW AQUIFERS UNDER THE INFLUENCE OF DYNAMICALLY FLUCTUATING RIVERS

Session: Innovations in Research of Groundwater-Surface Water Interactions over Multiple Spatio-Temporal Scales (Posters)

Presenting Author:

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Abstract:

Millions across South and Southeast Asia, notably in Bangladesh, Vietnam, and Cambodia are exposed to high natural arsenic (As) concentrations in groundwater. While many studies proposed As mechanisms release along riverbanks, a cross-site comparison across multiple riverbank sites is missing.

This study compares six riverbank aquifers-three in Bangladesh (MgR-1, MgR-2, MgR-3), two in Vietnam (RR-1, RR-2), and one in Cambodia (MkR)- to identify As sources and sinks along groundwater flowpaths using spatial trends and multivariate analysis; based on new and existing data.

Although flow direction varies seasonally, for MkR, MgR-1, MgR-2, and MgR-3 sites, net groundwater flows from aquifer to river, but away from the river for RR-1 and RR-2 sites. Spatial plots from the sites show that As concentrations are highest in MkR (5 to 1212 µg/L) and lowest in RR-1 (1 to 198 µg/L). Across all sites, Fe concentrations mimic the As pattern. The sites MkR, RR-1 and RR-2 have high Ca and Mg concentrations (12 to 81 mg/L) from carbonate dissolution or silicate weathering. Only MkR has high Na concentration (8 to 210 mg/L). The sites MkR, RR-1 and RR-2 have high HCO₃⁻ (120 to 772 mg/L). Multivariate analyses revealed consistent collinearity of chemical variables across sites. The sites RR-1 and RR-2 have strong As-Fe-NH₄⁺ clustering- consistent with reductive dissolution of Fe oxides. The site MkR features As-Fe-NH₄-pH on the same dimension. The site MgR-3 shows As-NH₄⁺ grouping, and separate Cl-SO₄ and Ca-HCO₃-K groupings. The site MgR-2 has clear As-pH-SO₄ and Na-Cl clusters. The sites MgR-1, MgR-2 have strong Ca-HCO₃, Mg-HCO₃ and Ca-Mg clusters while MgR-3 has diffused controls. For Bangladesh sites, carbonate weathering is dominant while for others, products of reductive dissolution of Fe oxide are more evident that control the As distribution patterns across these sites. Lithological cross-sections reveal that sediment architecture can potentially be responsible for As release. Geochemical modeling will be further used to investigate how mineral saturation states influence As speciation and mobility in these sites.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-11200

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