Abstract. In the southwestern coastal region of Bangladesh, options for drinking water
are limited by groundwater salinity. To protect and improve the drinking
water supply, the large variation in groundwater salinity needs to be better
understood. This study identifies the palaeo and present-day hydrological
processes and their geographical or geological controls that determine
variation in groundwater salinity in Upazila Assasuni in southwestern
Bangladesh. Our approach involved three steps: a geological reconstruction,
based on the literature; fieldwork to collect high-density hydrological and
lithological data; and data processing to link the collected data to the
geological reconstruction in order to infer the evolution of the groundwater
salinity in the study area. Groundwater freshening and salinization patterns
were deduced using PHREEQC cation exchange simulations and isotope data were
used to derive relevant hydrological processes and water sources. We found
that the factor steering the relative importance of palaeo and present-day
hydrogeological conditions was the thickness of the Holocene surface clay
layer. The groundwater in aquifers under thick surface clay layers is
controlled by the palaeohydrological conditions prevailing when the aquifers
were buried. The groundwater in aquifers under thin surface clay layers is
affected by present-day processes, which vary depending on present-day
surface elevation. Slightly higher-lying areas are recharged by rain and
rainfed ponds and therefore have fresh groundwater at shallow depth. In
contrast, the lower-lying areas with a thin surface clay layer have
brackish–saline groundwater at shallow depth because of flooding by
marine-influenced water, subsequent infiltration and salinization. Recently,
aquaculture ponds in areas with a thin surface clay layer have increased the
salinity in the underlying shallow aquifers. We hypothesize that to
understand and predict shallow groundwater salinity variation in southwestern
Bangladesh, the relative elevation and land use can be used as a first
estimate in areas with a thin surface clay layer, while knowledge of
palaeohydrogeological conditions is needed in areas with a thick surface clay
layer.
Groundwater salinity variation in Upazila Assasuni (southwestern Bangladesh), as steered by surface clay layer thickness, relative elevation and present-day land use