Sources of salinity and arsenic in groundwater in southwest Bangladesh

Abstract

BackgroundHigh salinity and arsenic (As) concentrations in groundwater are widespread problems in the tidal deltaplain of southwest Bangladesh. To identify the sources of dissolved salts and As, groundwater samples from the regional shallow Holocene aquifer were collected from tubewells during the dry (May) and wet (October) seasons in 2012–2013. Thirteen drill cores were logged and 27 radiocarbon ages measured on wood fragments to characterize subsurface stratigraphy.ResultsDrill cuttings, exposures in pits and regional studies reveal a >5 m thick surface mud cap overlying a ~30 m thick upper unit of interbedded mud and fine sand layers, and a coarser lower unit up to 60 m thick dominated by clean sands, all with significant horizontal variation in bed continuity and thickness. This thick lower unit accreted at rates of ~2 cm/year through the early Holocene, with local subsidence or compaction rates of 1–3 mm/year. Most tubewells are screened at depths of 15–52 m in sediments deposited 8000–9000 YBP. Compositions of groundwater samples from tubewells show high spatial variability, suggesting limited mixing and low and spatially variable recharge rates and flow velocities. Groundwaters are Na–Cl type and predominantly sulfate-reducing, with specific conductivity (SpC) from 3 to 29 mS/cm, high dissolved organic carbon (DOC) 11–57 mg/L and As 2–258 ug/L, and low sulfur (S) 2–33 mg/L.ConclusionsGroundwater compositions can be explained by burial of tidal channel water and subsequent reaction with dissolved organic matter, resulting in anoxia, hydrous ferric oxide (HFO) reduction, As mobilization, and sulfate (SO4) reduction and removal in the shallow aquifer. Introduction of labile organic carbon in the wet season as rice paddy fertilizer may also cause HFO reduction and As mobilization. Variable modern recharge occurred in areas where the clay cap pinches out or is breached by tidal channels, which would explain previously measured 14C groundwater ages being less than depositional ages. Of samples collected from the shallow aquifer, Bangladesh Government guidelines are exceeded in 46 % for As and 100 % for salinity.

Seasonal Variability of Forces Controlling Sedimentation in the Sundarbans National Forest, Bangladesh

“…Interestingly, the deposition rates determined by this and other recent studies (e.g., Rogers et al, 2013;Auerbach et al, 2015;Bomer et al, in review) are similar to the local deposition rates observed across the entire Holocene (0.7-1.0 cm/year; Ayers et al, 2016). Further to the west, where sedimentation has been reduced as a result of the eastward migration of the Ganges River over the past 4000 years, the longterm deposition rates are closer to 0.1-0.2 cm/year (Flood et al, 2015).…”

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.…”

“…In the Ganges-Brahmaputra-Meghna delta, with a population of over 170 million, unsafe drinking water is used due to drinking water resources being severely stressed. 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].…”

Variations in hydrostratigraphy and groundwater quality between major geomorphic units of the Western Ganges Delta plain, SW Bangladesh

“…The northern part shows single aquifer system and aquifer complexity increases towards the south where it ends up as a multi-layer aquifer system. Several studies in Bangladesh (Ravenscroft 2003;Ahmed et al 2004;Shamsudduha and Uddin 2007;Shamsudduha et al 2011;Ayers et al 2016) and West Bengal (Acharyya et al 2000;Bhattacharya et al 1997;Goodbred et al 2003;Shamsudduha and Uddin 2007;Biswas et al 2014;Ghosal et al 2015) have stated the existence of multi-aquifer system in the Ganges Delta plain. Generalized classification schemes define two aquifer units: shallow aquifers (occurring within a depth of 150 m) and deep aquifers (occurring at depths below 150 m) in Bangladesh (Ahmed et al 2004).…”