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Abstract

Arsenic is widespread in groundwater of the Holocene alluvial aquifers in southern Bangladesh, yet its concentration is highly variable spatially and with depth. A conceptual model of arsenic in the aquifer is proposed, as a basis for addressing questions concerning sustainability of groundwater development. Patterns and profiles of arsenic distribution in the aquifer have been determined at Meherpur in western Bangladesh, over an area of 15 km2 and a depth range of 15–225 m. The hydrochemical and hydraulic environments of arsenic occurrence have been established. The conceptual model incorporates the conditions of arsenic release to groundwater, the depth distribution of the arsenic source, likely sedimentological controls on the lateral discontinuity of the arsenic source, and the hydraulic regime imposed by pumping from the hydrogeologically leaky, multi-layered aquifer. Reducing conditions, conducive to arsenic release from sedimentary iron oxyhydroxides, are widespread. The arsenic source occurs at a distinct horizon at a depth of about 20 m, but is laterally discontinuous. The catchments of shallow, hand-pumped tubewells (HTWs) are limited in extent by vertical leakage. Arsenic concentration in water pumped from tubewells depends on the depth separation between the HTW screen and the arsenic source, the overlap between the HTW catchment and the arsenic source layer, and the duration of pumping. Implications are drawn for treatment, tubewell location and design, monitoring, and predictive modelling.

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