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