Groundwater salinity dynamics and seasonal water quality trends in coastal Bangladesh: a case study from Khulna

Groundwater is crucial for drinking, domestic, and agricultural use in Bangladesh, but Khulna faces challenges in ensuring the availability of safe drinking water due to increasing groundwater salinization from saltwater encroachment. This study monitored groundwater salinity and other water quality parameters (pH, TDS, Cl−, HCO3−, SO42−, Na+, K+, Ca2+, Mg2+, Fe, NO3−) in 14 tube wells across Khulna over a year. Seasonal water quality and geochemical processes were assessed through the weighted arithmetic water quality index (WQI) and graphical analyses employing Piper and Gibbs diagrams. Results showed higher concentrations of the mentioned parameters at depths < 500 ft during the post-monsoon season and lower levels during the monsoon season. A significant inverse relationship (p < 0.001) was found between tube well depth and salinity. During the pre-monsoon season, WQI classified 50% of samples as ‘Good’, 21% as ‘Poor’, and 29% as ‘Excellent’. All samples were ‘Good’ during the monsoon and post-monsoon seasons. Piper diagrams indicated that water at < 500 ft was predominantly NaCl type, while deeper wells had Mg-HCO3− and mixed types across all the seasons. Gibbs diagrams revealed pre-monsoon samples were evaporation-dominated, indicating saline groundwater, while monsoon and post-monsoon samples were evaporation-precipitation dominated. These findings highlight seasonal variations in groundwater quality and salinity in Khulna, driven by geochemical processes. The results can inform policymakers, aiding in the development of effective groundwater management and conservation strategies to ensure a sustainable supply of safe drinking water in the region.