Groundwater salinization and freshening processes in coastal aquifers from southwest Bangladesh

Highlights: • In the upper aquifer, infiltration of redissolved evaporite salts by monsoon heavy rainfall leads to salinization • Cl⁻/Br⁻ ratio and d-excess in the NaCl waters of the upper aquifer indicate evaporate dissolution • Freshening is well advanced in the deep aquifer, resulting in NaHCO₃ water type due to cation exchange • Poor correlation of δ¹⁸O with Cl⁻ and low Cl⁻/Br⁻ ratio indicate that the deep aquifer has not undergone evaporation The hydrogeochemical processes in the multilayer aquifers of southwest (SW) coastal area of Bangladesh provide important information for quantifying hydrochemical differences between different aquifers. Hydrogeochemical processes affecting groundwater chemistry in this coastal area have been evaluated by interpreting conventional plots, ionic delta, HFE-diagram, stable isotopes and geochemical modelling. The median TDS distribution of the aquifers has an increasing trend from below 1000 mg/l in the deep aquifer (DA) to 2622 mg/l in lower shallow aquifer (LSA) and 7012 mg/l in upper shallow aquifer (USA). Na⁺ is the dominant cation in all the aquifers. HCO₃⁻ is the dominant anion in DA with high median concentration (495 mg/l), which is more than double than that in LSA (214 mg/l) and USA (159 mg/l). The groundwater in the DA is freshened NaHCO₃+ type due to cation exchange process that enhances second stage of calcite dissolution. Few NaCl+ type DA waters found in the SW corner of the study area indicate the presence of connate water confined in the inter-basin during the Holocene transgression. The stable isotopes indicate that the DA waters have been deposited during warmer periods. In contrast, both the LSA and USA are dominated by NaCl- type water; some CaCl- type waters have also been found in these aquifers, indicating salinization. The salinization processes consist of infiltration of redissolved evaporite salts, reverse cation exchange, and mixing with marine influenced flood water. Thus, the knowledge of different processes controlling freshening and salinization will help properly manage and preserve the environmental characteristics in the aquifer systems of SW Bangladesh as well as similar complex coastal geological settings.