The evolution of the groundwater quality in the alluvial aquifers of the south-western part of Bengal Basin, Bangladesh

The present study was conducted in the south-western part of Bengal Basin, Bangladesh in order to evaluate the geochemical evolution and processes controlling the hydrogeochemical behaviour of the groundwater system of this region. The multivariate statistical methods of hierarchical cluster analysis (HCA) and principal component analysis (PCA) were applied to a dataset consisting of 174 samples in combination with graphical analysis to classify the samples according to plausible hydrogeochemical processes in the study area. PCA and HCA yielded similar results and classified samples into three major facies or groups (A-C). PCA has extracted three major components based on the likely processes that influence the evolution of the chemical composition of groundwater. Group A has two compositional variations. The first one is dominated by Na+-Ca2+-Mg2+-HCO3--Cl- and is likely to be associated with recently infiltrated water that has been modified by interactions in the vadose zone and in shallow aquifers. Another one is Na+-Cl--HCO3- type, entrapped paleo-saline water that occurred in deep aquifers. Group B has a Na+-Cl--HCO3- compositional type and with variable amounts of HCO3-, NO3- and SO42- enrichment in the coastal shallow aquifers. Group C has a chemical composition dominated by sodium and chloride ions with variable salinities. Concentrations of trace metals (Fe2+ and Mn2+) are higher in the shallow aquifer than in deeper groundwater. This research demonstrates that the different groundwater groups have either specific natural or anthropogenic water controls.