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

Floris Loys Naus,Jasper Griffioen,Kazi Matin Ahmed,Koos Groen,Paul Schot

doi:10.5194/hess-23-1431-2019

Floris Loys Naus, Jasper Griffioen + Show 3 more

Open Access

https://doi.org/10.5194/hess-23-1431-2019

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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 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.

Highlights

In the Ganges–Brahmaputra–Meghna (GBM) river delta, home to 170 million people, availability of safe drinking water is problematic because of the very seasonal rainfall, the likelihood of arsenic occurrence in the shallow groundwater and the pollution of surface water bodies (Harvey et al, 2002; Ravenscroft et al, 2005; Chowdhury, 2010; Sharma et al, 2010; Bhuiyan et al, 2011)
The relevant Holocene sedimentary history of the researched upper 50 m of the subsurface starts in a landscape determined by conditions under the Last Glacial Maximum (LGM)
The LGM conditions in the study area are uncertain, as the area is near the edge of a possible palaeo-channel (Hoque et al, 2014; Goodbred et al, 2014), making it possible that the starting conditions for the Holocene sedimentation could be either a Pleistocene incised valley or a palaeosol

Summary

Introduction

In the Ganges–Brahmaputra–Meghna (GBM) river delta, home to 170 million people, availability of safe drinking water is problematic because of the very seasonal rainfall, the likelihood of arsenic occurrence in the shallow groundwater and the pollution of surface water bodies (Harvey et al, 2002; Ravenscroft et al, 2005; Chowdhury, 2010; Sharma et al, 2010; Bhuiyan et al, 2011). In the southwestern coastal region of Bangladesh, suitable drinking water options are even more limited, as here the groundwater is largely brackish to saline (Bahar and Reza, 2010; George, 2013; Fakhruddin and Rahman, 2014; Worland et al, 2015; Ayers et al, 2016). The people in this region are at high risk of preeclampsia, eclampsia and gestational hypertension from drinking groundwater, and at increased risk of ingesting pathogens from water from the traditional ponds Naus et al.: Groundwater salinity variation in Upazila Assasuni (southwestern Bangladesh)

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