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

Abstract

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 was 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.

Factors Determining the Natural Fresh‐Salt Groundwater Distribution in Deltas

“…This is a result of both the salinity and the hydraulic gradient of the river systems that stimulate flushing with fresher water, contrary to the constant sea level and salinity of the sea. The relatively small effect of salt water originating from rivers is also observed in case studies, as a local phenomenon constrained to areas in proximity to (paleo)‐channels in the upper ∼50 m of the groundwater system (Ayers et al., 2016; Faye et al., 2005; Naus et al., 2019).…”

“…We are not aware of any reported large‐scale, free‐convective systems on a delta scale, which raises doubts on the realism of an aquifer with a representative conductivity K v ,aqf between 0.001 and 0.1 m/d. Instead, chaotic salinity patterns are now often ascribed to heterogeneity in lithology and salt sources (e.g., Delsman et al., 2014; Michael et al., 2016; Naus et al., 2019; van Engelen et al., 2019). Given that many deltas have experienced a Holocene transgression (Larsen et al., 2017), the lack of free‐convection distribution in real‐world cases provides an additional constraint on K v ,aqf and thus indirectly on K h ,aqf .…”

A Search for Freshwater in the Saline Aquifer of Coastal Bangladesh

“…Studies, including this investigation, point repetitively to variable groundwater salinities in this region (Ayers et al 2016;Bahar and Reza 2010;Naus et al 2019;Worland et al 2015). We were unable to explain the regions of freshwater with GW-SW interactions.…”

A global dataset of surface water and groundwater salinity measurements from 1980–2019

“…In addition, using observed salinity data in modelling approaches can contribute to better process understanding and in reducing model prediction uncertainty, enabling better water quality projections under global change 13 , 14 . Although the number of studies sharing salinity datasets are increasing 15 – 18 , few assessments extend to the global scale, and even less target both the surface and groundwater systems. In addition, salinity data is often scattered and non-harmonized, both in terms of reported parameters, units, and spatio-temporal resolution.…”