Hydrochemical characterization and groundwater potential of the deep aquifer system in southwest coastal region of Bangladesh

Abstract

Maintaining the water quality of a coastal aquifer is very important for its sustainable development and management. The shallow aquifers in the southwest coast of Bangladesh are highly saline. This research was conducted to explore the hydrochemistry and potential of the deep aquifer using pumping test, hydrochemistry, multivariate statistics, and groundwater quality index. The groundwater is mostly of NaHCO3 type (85%), but small amount of NaCl (15%) type waters are also present. Principal component analysis indicates four factors (73.43% of the total variance) controlling the composition of groundwater: paleo-seawater, cation exchange, mineral dissolution, and oxidizing conditions. Zonation of hydrogeochemical processes within the deep aquifer are well established by hierarchical cluster analysis. Seven clusters have been distinguished from this analysis: cluster 1 (paleo-saltwater), clusters 2 and 4 (freshwater influence), clusters 3 and 7 (freshwater invasion and mineral dissolution), cluster 5 (no specific geographical location) and cluster 6 (mixed water). The groundwater quality index shows that 88% sampled water is of excellent to good and only 3% is unsuitable for drinking purposes. However, 53% of water has Na+ concentration above drinking standard. This parameter has very little contribution in groundwater quality index calculation, which we consider a shortage. Pumping tests revealed that transmissivity of deep aquifer has a median value of 888 m2/day and a hydraulic conductivity of around 15 m/d, indicating a good hydrodynamic potential. The overall water quality is good, except in the southwest corner of the study area. It is, therefore, recommended to exploit groundwater from the deep aquifer.