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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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Discussion papers | Copyright
https://doi.org/10.5194/hess-2018-416
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 06 Sep 2018

Research article | 06 Sep 2018

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This discussion paper is a preprint. It is a manuscript under review for the journal Hydrology and Earth System Sciences (HESS).

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

Floris Loys Naus1, Paul Schot1, Koos Groen2, Kazi Matin Ahmed3, and Jasper Griffioen1,4 Floris Loys Naus et al.
  • 1Copernicus Institute, Environmental Sciences, Utrecht University, Utrecht, the Netherlands
  • 2Acacia water, Gouda, the Netherland
  • 3Department of Geology, Dhaka University, Dhaka, Bangladesh
  • 4TNO Geological Survey of the Netherlands, Utrecht, the Netherlands

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.

Floris Loys Naus et al.
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In this paper, we postulate a possible evolution of the groundwater salinity around a village in southwestern Bangladesh, based on high density fieldwork. We identified that the thickness of the surface clay layer, the surface elevation and the present-day land use determine whether fresh or saline groundwater has formed. The outcomes show how the large groundwater salinity variation in southwestern Bangladesh can be understood, which is valuable for the water management in the region.
In this paper, we postulate a possible evolution of the groundwater salinity around a village in...
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