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Abstract. In the southwestern coastal region of Bangladesh, options for drinking waterare limited by groundwater salinity. To protect and improve the drinkingwater supply, the large variation in groundwater salinity needs to be betterunderstood. This study identifies the palaeo and present-day hydrologicalprocesses and their geographical or geological controls that determinevariation in groundwater salinity in Upazila Assasuni in southwesternBangladesh. Our approach involved three steps: a geological reconstruction,based on the literature; fieldwork to collect high-density hydrological andlithological data; and data processing to link the collected data to thegeological reconstruction in order to infer the evolution of the groundwatersalinity in the study area. Groundwater freshening and salinization patternswere deduced using PHREEQC cation exchange simulations and isotope data wereused to derive relevant hydrological processes and water sources. We foundthat the factor steering the relative importance of palaeo and present-dayhydrogeological conditions was the thickness of the Holocene surface claylayer. The groundwater in aquifers under thick surface clay layers iscontrolled by the palaeohydrological conditions prevailing when the aquiferswere buried. The groundwater in aquifers under thin surface clay layers isaffected by present-day processes, which vary depending on present-daysurface elevation. Slightly higher-lying areas are recharged by rain andrainfed ponds and therefore have fresh groundwater at shallow depth. Incontrast, the lower-lying areas with a thin surface clay layer havebrackish–saline groundwater at shallow depth because of flooding bymarine-influenced water, subsequent infiltration and salinization. Recently,aquaculture ponds in areas with a thin surface clay layer have increased thesalinity in the underlying shallow aquifers. We hypothesize that tounderstand and predict shallow groundwater salinity variation in southwesternBangladesh, the relative elevation and land use can be used as a firstestimate in areas with a thin surface clay layer, while knowledge ofpalaeohydrogeological conditions is needed in areas with a thick surface claylayer.

hydrology and earth system sciences

Groundwater salinity variation in Upazila Assasuni (southwestern Bangladesh), as steered by surface clay layer thickness, relative elevation and present-day land use