EVOLUTION OF SALINE AND ARSENIC CONTAMINATED GROUNDWATER IN SOUTHWEST BANGLADESH

Compositions of groundwater samples in a poldered area of southwest Bangladesh were used to model the sources and chemical processes that control salinity and arsenic concentration. Physical parameters, such as electrical conductivity, pH, and major ion concentrations, such as Ca, Mg, Na, K, Cl, HCO₃, CO₃, and SO₄, from polder shallow aquifer tubewell samples were taken into consideration. Compositions vary spatially, but with limited seasonal variation. Reaction path models in The Geochemist Workbench were used to test the hypothesis that the groundwater is a mixture of connate seawater and meteoric water that has experienced geochemical modification due to redox reactions. Dissolved organic carbon was titrated into hypothetical seawater-meteoric water mixtures in the presence of Hydrous Ferric Oxyhydroxide (HFO) to induce HFO reductive dissolution, sulfate reduction, and sulfide precipitation. Arsenic adsorption on HFO was modeled using two-layer surface complexation. These reactions can explain how ancient seawater-meteoric water mixtures evolved to modern groundwater compositions with high arsenic concentrations. With the majority of polder residents reliant on groundwater resources, this study provides a better understanding of the hydrogeochemical processes that control groundwater salinity and arsenic levels.