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 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.

Why Do People Remain Attached to Unsafe Drinking Water Options? Quantitative Evidence from Southwestern Bangladesh

“…However, the initial investment and maintenance costs are even higher than for the construction of manually operated DTWs. Additionally, it should be noted that fresh groundwater recharge is limited [9,14], which means that the use of deep groundwater would probably be unsustainable. Nevertheless, the mining of deep groundwater could be a useful solution for the short term.…”

“…Surface water resources are polluted [5,6], meteorological water resources are subject to distinct seasonality [7,8], and shallow groundwater is often contaminated with arsenic [9][10][11][12][13]. In southwestern Bangladesh, salinity in surface water and groundwater [9,[14][15][16] puts further pressure on the available drinking water options, leading to the consumption of bacterially contaminated pond water [17] and of shallow groundwater with elevated levels of arsenic and salinity [18,19].…”

Socio-hydrogeological Potential for Managed Aquifer Recharge in the Fresh-saline Aquifers of Southwestern Bangladesh

“…However, the initial investment and maintenance costs are even higher than for the construction of manually operated DTWs. Additionally, it should be noted that fresh groundwater recharge is limited Naus et al, 2019a), which means that the use of deep groundwater would probably be unsustainable. Nevertheless, the mining of deep groundwater could be a useful solution for the short term.…”

“…Surface water resources are polluted , meteorological water resources are subject to distinct seasonality , and shallow groundwater is often contaminated with arsenic BGS and DPHE, 2001;Gaus et al, 2003). In southwestern Bangladesh, salinity in surface water and groundwater Naus et al, 2019a;Naus et al, 2019b; puts further pressure on the available drinking water options, leading to the consumption of bacterially contaminated pond water and of shallow groundwater with elevated levels of arsenic and salinity .…”

“…It has been applied in different areas of the world to provide year-round drinking water access and to improve the drinking water quality (Dillon, 2005;Maliva et al, 2006;Sprenger et al 2017;Stefan and Ansems, 2018). In southwestern Bangladesh, the provision of safe drinking water is not guaranteed, leading to the consumption of bacterially contaminated pond water and of shallow groundwater with potentially high levels of arsenic and salinity Naus et al, 2019a;Naus et al, 2019b;. Overcoming consumption from these unsafe drinking water sources is directly linked to achieving sustainable Development Goal 6, to 'Ensure availability and sustainable management of water and sanitation for all' (UN, 2015).…”