Groundwater’s seasonal variation and its impact on freshwater in a coastal mega city

Abstract

A comprehensive groundwater quality assessment is carried out in Khulna, which is the coastal mega city of Bangladesh adjacent to southwest Bengal delta. The study was conducted to identify the seasonal variation of ion composition in groundwater and insight into its impact on freshwater availability for the study area. Results of chemical analysis revealed that groundwater is alkaline, with dominant ion concentrations of Mg²⁺, Na⁺, Cl⁻ and HCO₃⁻. Cl⁻ concentration (254.70 mg/L in monsoon; 478 mg/L in winter; 531.75 mg/L in summer) exceed from the prescribed limits, i.e., 250 mg/L of WHO water quality standards. The Piper diagram indicated that groundwater occupies saline and mixed zone that are mostly affected by the seawater through ion exchange mechanism. The Endmember diagrams revealed that silicate weathering plays a major role while evaporite dissolution and carbonate weathering play the minor role for the evolution of groundwater. Saturation indexes reveal that halite saturation increases proportionally to Cl⁻ contents while calcite saturation shows most of the groundwater samples are undersaturated. WQI revealed that summer groundwater quality is poor for drinking purposes. Furthermore, the spatial distribution map of WQI indicated that the northeast and southwest parts of groundwater are vulnerable owing to landscape dynamics. The feedback and outlooks of seasonal ion concentration on freshwater indicated suitable water quality in monsoon and winter, whereas unsuitable, salty and hard water in the summer.