Geological Structure of an Arsenic‐Contaminated Aquifer at Sonargaon, Bangladesh

Continuous sediment core samples and groundwater were collected in the northern part of Sonargaon, central Bangladesh, to document the hydrogeological constraints on As‐contaminated aquifers. The study area spans the alluvial plain of the Old Brahmaputra River and a Pleistocene terrace, the Madhupur Tract. The Quaternary sequence comprises Plio‐Pleistocene sand, Upper Pleistocene mud, and Holocene sand units. Highly As‐contaminated groundwater (50–1000 μg/L) is found in the upper aquifer, corresponding to the Holocene sand unit that underlies the alluvial plain, and it plausibly appears to be closely related to the distribution of lenses of silt to fine sand. As‐free (<1 μg/L) groundwater occurs only within sediments coarser than medium sand. Highly As‐contaminated groundwater is characterized by low concentrations of Cl− and SO\,²⁻₄ and high concentrations of NH\,⁺₄ , suggesting that the As is released in association with reduction of water recharged during rainy season. The restricted occurrence of strongly As‐contaminated (>100 μg/L) groundwater is associated with (1) the intercalation of silt to fine‐sand lenses in the Holocene sandy aquifer, (2) the stagnant condition of the aquifer along the buried valley, and (3) the vertical infiltration of groundwater in close proximity to installed tube wells.