Groundwaters and aquifer sediments from Chundpur and its surroundings, along the downstream Meghna River at the meeting with Padma River, Bangladesh, were analyzed to document the mechanism of As contamination. The As was released in association with infiltration of surface water into the shallow groundwater aquifer without strong reduction. In situ chemical weathering and the following desorption at increasing pH would be the primary and secondary processes to release As into the groundwater of the studied area.