Evaluation of Hydrogeochemical Processes in Groundwater Using Geochemical and Geostatistical Approaches in the Upper Bengal Basin

Abstract

Overexploitation of groundwater for irrigation and radical changes of river morphology in the Bengal basin has posed difficulties to sustainable management of this resource. Forty groundwater samples were collected from tube wells of the pre-monsoon and postmonsoon seasons in 2020, and the water parameters were analyzed. The hydrogeochemical studies, bivariate plots, and multivariate techniques were used to evaluate the rock-water interactions, influencing factors, and contamination pathways. The principal component analysis (PCA) was used to extract several directions in the data space and understand the different geochemical processes. Q-mode hierarchical cluster analysis coupled with the post hoc ANOVA test of variance was also used to divide the sampling sites based on the geochemical water facies. The PHREEQC-3v software was used to measure the partial pressure of CO2 in groundwater and elucidate the chemical reactions controlling the water chemistry. Near-neutral pH (7.4) and high EC (813.2 μS/cm), TDS (507.35 mg/L), and total hardness (383.45 mg/L) characterize the groundwaters of the study area. The research revealed that the order of abundance of cations was Ca2+>Mg2+>Na+>K+ and of anions was HCO3−>>Cl->SO42−>NO3->PO43-. The PCA revealed that the chemical properties of the groundwater are derived from rock-water interactions. Hierarchical cluster analysis showed that two distinct groundwater zones were affected by neighboring river flow and irrigation return flow. Several diagrams suggested that the water was mainly of Ca-HCO3 type originating from chemical weathering of rock-forming minerals with advanced water-rock interaction. The analyzed groundwater was supersaturated with calcite and partially saturated with dolomite. As a result, the chemical features of groundwater in the study area were largely dependent on the water-rock interaction, local lithological conditions, and neighboring river morphology. This study can be helpful for the improvement of water resource management, especially for drinking and irrigation purposes.