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 (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.