FRESHWATER AND SALTWATER MIXING MECHANISMS AND ARSENIC DISTRIBUTION AT COASTAL AQUIFERS OF THE GANGES DELTA

Saltwater intrusion into groundwater can result in the formation of salt water wedge progressing inland and may induce geochemical changes in coastal aquifers. Hydrological and geochemical data collected from the Ganges Delta study area indicate that a significant degree of mixing of seawater and freshwater in coastal aquifers. The main hydrochemical facies of groundwater in the coastal aquifers is a Na-Cl-Ca-HCO₃ type. Groundwater is enriched in Na, K, Ca, Mg, and HCO₃^- relative to the conservative mixing line of seawater and dilute groundwater. To simulate saltwater intrusion in Ganges Delta, a numerical model was used that couples equations of flow and solute transport by advection and hydrodynamics, which account for permeability variation in space and considers flow driven by lateral density gradient. Hydrogeological modeling and historical water-table data show a general flow trend from north to south at velocity of a few meters to a few tens of meters per year across the coastal aquifers. The modeling results also show that the groundwater flow is dominated by a local rather than a regional system, with undulations in the water table controlled by local topographic highs and lows. Field data and modeling results show that high salinity groundwaters are present in shallow aquifers, suggesting that shallow aquifers may be contaminated by downward infiltration of saline surface water from tidal channels. Presence of less saline groundwater in deeper aquifers may be derived from lateral saltwater intrusion from the ocean. Although elevated groundwater arsenic concentrations (up to 553 ppb) are only found in shallow aquifers with low salinity in the study area, saltwater intrusion and competition processes may potentially cause arsenic desorption from sediments. Results of these monitoring and modeling efforts indicate that they would prove useful in predicting likely areas of deteriorating of water quality in increasing salinity and arsenic content in the coastal regions of the Ganges Delta.