GEOCHEMICAL MODELLING OF SALTWATER INTRUSION AT THE COASTAL AQUIFERS OF THE BENGAL BASIN, BANGLADESH

Bangladesh is a low‐lying country situated at the confluence of the Ganges and Brahmaputra river systems in the Bengal Basin. The coastal region of Bangladesh is subject to high rates of sedimentation where river and saline storm surge flooding during the rainy season. Distribution of aquifer sediments in the subsurface is found very complex and aquifer and aquitard beds can exchange even within a short distance. Clay or silty-clay aquitards are not continuous and their appearance may affect hydraulic connectivity between shallow and deep aquifers. Commonly, 3 to 4 aquifer layers are encountered which have been separated by aquitards, and some degree of abstraction of groundwater from any aquifer depth may not affect the others. This study deals with assessing salinity distribution and the geochemical characteristics of groundwater as a result of mixing of saltwater with freshwater in the relatively flat tidal delta areas of Bangladesh.

High salinity groundwaters (up to 3000 mg/L) are found in shallow aquifers, suggesting that shallow aquifers may be contaminated by downward infiltration of saline surface water from streams and rivers. Presence of less saline groundwater in deeper aquifers may be derived from lateral saltwater intrusion. Major ions Na⁺ and K⁺ exhibits conservative behavior at low salinity and non-conservative removal from groundwater at high salinity, suggesting ion-exchange processes involving clay minerals under varying salinity. Ca²⁺ and Mg²⁺ displays both non-conservative addition and removal over a wide range of salinity, which may be influenced by a combination of ion exchange (i.e., clay-bound Ca²⁺ and Mg²⁺ are displaced by Na⁺ and K⁺) and carbonate diagenesis (carbonate precipitation or de-domitilization). Groundwater arsenic concentrations do not show significant correlation with Fe or Mn, suggesting that its mobilization may not be controlled by bacterial iron or manganese reduction alone.