Identifying multiple deep aquifers in the Bengal Basin: Implications for resource management

Abstract

In the Bengal Basin of Bangladesh and West Bengal (India), where arsenic (As) and salinity adversely affect groundwater in shallow aquifers (<150 m deep), consumers increasingly turn to "the deep aquifer" as a source of low-As and low-salinity water.We show that "the deep aquifer," which has traditionally been regarded as a single entity, can be divided into at least eight deep aquifer units each with distinctive chemical, isotopic, hydraulic, and piezometric characteristics. Except in the rapidly subsiding Sylhet Basin, the deep aquifers are largely As-safe (<50 μg/L and mostly <10 μg/L) but differ in terms of iron (Fe), manganese (Mn), salinity, stable isotopic composition, groundwater age, and in their piezometric response to pumping. We use isotopic data to identify recharge from five periods, of which two are quantitatively dominant: (a) from the Last Glacial Maximum, isotopically heavy water (δ 18 O >−3.5‰) with low As, Cl, and Fe concentrations occupies hydraulically trapped positions in the South-Central, Southeast, Southwest, and Madhupur aquifer units and (b) terminal Pleistocene to Early Holocene recharge of isotopically lighter water (δ 18 O <−3.5‰) that has high Fe and slightly elevated Cl concentrations and was recharged along palaeochannels that facilitated deep circulation up to the mid-Holocene. Piezometric responses to development change in style towards the coast. In the north, water levels at all depths tend to be coincident and transition through a seasonally divergent pattern to become parallel in the south with a near constant downward gradient. The delineation of the deep aquifer units provides a practical framework for the assessment and management of groundwater resources for water supply.

Groundwater Constituents and Trace Elements in the Basement Aquifers of Africa and Sedimentary Aquifers of Asia: Medical Hydrogeology of Drinking Water Minerals and Toxicants

“…Fine-grained sedimentary deposits such as clay or silts are often interbedded within the sand layers in sedimentary aquifers, known as aquitards, providing hydraulic basis for hierarchical flow systems (Hoque et al 2017). Often the deeper flow systems (made of regional groundwater bodies of relatively homogeneous water quality) contains low amounts of As and Na, and can provide superior drinking water (Burgess et al 2010;Hoque et al 2017;Ravenscroft et al 2018).…”

Replumbing the Bengal Basin: A Shift in Recharge Driven by Groundwater Irrigation Revealed by Stable Water Isotopes

“…Groundwater 291 below 100 m is, on average older than 4000 years and below 200 m is, on average, older than 292 10,000 years (Figure 1B). In the Bengal basin, precipitation isotopes in the Pleistocene were 293 isotopically heavier than in Holocene precipitation (53). Thus, the observed pattern in the depth-294 average groundwater isotope ratios is most likely due to changes in the precipitation endmember 295 values; the groundwater isotope ratios become progressively heavier with increasing depth as the 296 component of older Pleistocene precipitation water increases.…”

“…In other words, more recently recharged water (i.e., shallower depths) have higher 282 contribution from evaporated sources (ponds, rice fields and local rivers during the dry season) 283 and deeper water have lower contribution from evaporated sources. It is very unlikely that these 284 patterns in the top 100 meters of groundwater represent changes in precipitation isotope ratios 285 because most of this water recharged in the last millennium, much more recently than the last 286 changes in rainfall isotope ratios that occurred about 10,000 years ago (45,53). 287…”

Origin of Groundwater Arsenic in a Rural Pleistocene Aquifer in Bangladesh Depressurized by Distal Municipal Pumping

“…Studying the geochemical and hydrologic processes that regulate groundwater As within this intermediate zone over time is therefore particularly relevant to private well installations to reduce exposure (Jamil et al, 2019). In contrast to these private initiatives, government policy so far has relied primarily on nationwide installation of deep tubewells (Ravenscroft et al, 2009(Ravenscroft et al, , 2013(Ravenscroft et al, , 2014(Ravenscroft et al, , 2018 that are too costly for most households.…”

“…Studying the geochemical and hydrologic processes that regulate groundwater As within this intermediate zone over time is therefore particularly relevant to private well installations to reduce exposure (Jamil et al, 2019). In contrast to these private initiatives, government policy so far has relied primarily on nationwide installation of deep tubewells (Ravenscroft et al, 2009(Ravenscroft et al, , 2013(Ravenscroft et al, , 2014(Ravenscroft et al, , 2018 that are too costly for most households. Elevated levels of As (>100 μg/L) in shallow groundwater across the Ganges-Brahmaputra Delta have been widely attributed to microbially mediated reductive dissolution of iron(oxy)hydroxides (Ahmed et al, 1998(Ahmed et al, , 2004Berg et al, 2001;Bhattacharya et al, 1997;BGS/DPHE, 2001;Islam et al, 2004;McArthur et al, 2001;Nickson et al, 1998;Oremland & Stolz, 2003, 2005Swartz et al, 2004;van Geen et al, 2004).…”