Saltwater Intrusion and Trace Element Contaminations at the Coastal Aquifers of the Ganges Delta

Abstract

The coastal area of Bangladesh has a low topography lying at the confluence of the Ganges and Brahmaputra river systems in the Bengal Basin. The distribution of aquifer sediments in the subsurface is very complex where aquifer and aquitard beds show crossing one another even within a short distance. Commonly, three aquifer layers are encountered within 350 m depth. Hydrochemical analyses effectively distinguish between groundwater in the three aquifers, and also distinguish saline groundwater from seawater. Trends in major ion chemistry in the coastal aquifer system, particularly Na/Cl and Ca/Cl rations, indicate some degree of ion exchange accompanying seawater intrusion. Mineral dissolution and major ion chemistry clearly identify solute flux trends in the coastal aquifer system. Lateral saltwater intrusion models were constructed for the sea level rise scenarios which show a saltwater wedge consistent with the shape predicted by the Ghyben-Herzberg relation. Sensitivity analysis for these models shows that saltwater intrusion can be limited by an increase in the hydraulic gradients of fresh groundwater in the southern (downgradient) direction or by the presence of a confining clay layer in the coastal region.Vertical saltwater infiltration models show that small tidal channels have a local effect with a infiltration of saline surface water into the shallow layers, whereas larger tidal channels affect a larger area which can reach the deeper and main aquifer.Trace element concentrations were compared to Cl- concentrations which showed either no or to a moderate positive correlation. Although the pH or salinity effect may cause additional mobilization of trace element in this area, geochemical data show that the elevated concentrations of Fe, Mn, Sr and As can be correlated with high pH and HCO3. These geochemical correlations suggest that elevated metal concentrations may be caused by bacterial iron and manganese reduction. Furthermore, bacterial sulfate reduction may also affect either major and minor trace element mobilization or removal, but the pH effect could become problematic in the future should seawater intrusion continue to drive up pH and salinity in the aquifers.