Recharge of low‐arsenic aquifers tapped by community wells in Araihazar, Bangladesh, inferred from environmental isotopes

Abstract

More than 100,000 community wells have been installed in the 150–300 m depth range throughout Bangladesh over the past decade to provide low-arsenic drinking water (<10 μg/L As), but little is known about how aquifers tapped by these wells are recharged. Within a 25 km2 area of Bangladesh east of Dhaka, groundwater from 65 low-As wells in the 35–240 m depth range was sampled for tritium (3H), oxygen and hydrogen isotopes of water (18O/16O and 2H/1H), carbon isotope ratios in dissolved inorganic carbon (DIC, 14C/12C and 13C/12C), noble gases, and a suite of dissolved constituents, including major cations, anions, and trace elements. At shallow depths (<90 m), 24 out of 42 wells contain detectable 3H of up to 6 TU, indicating the presence of groundwater recharged within 60 years. Radiocarbon (14C) ages in DIC range from modern to 10 kyr. In the 90–240 m depth range, however, only 5 wells shallower than 150 m contain detectable 3H (<0.3 TU) and 14C ages of DIC cluster around 10 kyr. The radiogenic helium (4He) content in groundwater increases linearly across the entire range of 14C ages at a rate of 2.5×10−12 ccSTP 4He g−1 yr−1. Within the samples from depths >90 m, systematic relationships between 18O/16O, 2H/1H, 13C/12C and 14C/12C, and variations in noble gas temperatures, suggest that changes in monsoon intensity and vegetation cover occurred at the onset of the Holocene, when the sampled water was recharged. Thus, the deeper low-As aquifers remain relatively isolated from the shallow, high-As aquifer.

Security of Deep Groundwater in the Coastal Bengal Basin Revealed by Tracers

“…Stable isotope ratios (δ 18 O versus δ 2 H) for groundwater samples at all depths suggest that there has been limited modification due to evaporation (Figure S3d) and that paleowaters have been recharged within the prevailing Indian monsoon regime (δ 18 O fall between −7‰ and −2‰). This finding is consistent with other recent studies (Desbarats et al, ; Majumder et al, ; Mihajlov et al, ). Consistency in δ 13 C values with depth and 14 C (δ 13 C range between −20‰ and −5‰) also may suggest a relatively uniform source of DIC within the groundwater system over the last 20 ka (Aggarwal et al, ).…”

“…Overall, within the top 100 m there is a sharp age‐depth gradient followed by a gradual increase in age with depth between 100 and 300 mbgl, which is consistent with other recent studies (Hoque & Burgess, ; Mihajlov et al, ). There is one groundwater sample that gives age estimates exceeding 10 ka throughout the 0–360 m profile (Figure ).…”

“…Indeed, a comparison of 14 C activities between pumped and monitoring sites, within the 100–250 mbgl depth interval, reveals greater activities at pumped sites (mean of 66 pmc and 33 pmc, respectively) and a significant difference was found between these groups ( P < 0.001, rank sum test, Figure a). In contrast, Mihajlov et al () found no evidence for a significant contribution of recent recharge from handpump sites >150 mbgl in Araihaza. Screen intake lengths.…”

“…Regional‐scale modeling (Burgess et al, ; Hoque et al, ; Michael & Voss, , ) and field studies (McArthur et al, ; Mihajlov et al, ; Mukherjee et al, ; Ravenscroft et al, ) have explored the vulnerability of the deep groundwater in the coastal Bengal Aquifer System (BAS) to contamination yet key knowledge gaps remain. These include the relative importance of site‐specific, borehole‐ scale problems due to casing breaks at abstraction well sites (I. Choudhury et al, ) and aquifer‐scale vulnerabilities (Hoque et al, ) to contamination from shallow groundwater and the long‐term impacts of intensive pumping at depth (Khan et al, ; Knappett et al, ; Michael & Khan, ; Zahid et al, ).…”

Global Isotope Hydrogeology―Review

“…Pertaining to this specific example, an approach better suited for isolating and estimating late‐Pleistocene temperatures is the comparison of late‐Holocene versus late‐Pleistocene groundwater noble gas concentrations (Mazor, ). The noble‐gas‐based paleotemperature reconstruction approach has been applied to estimate past temperatures in dozens of aquifer systems around the world (e.g., Andrews & Lee, ; Mazor & Verhagen, ; Heaton et al, ; Phillips et al, ; Andrews et al, ; Stute & Deák, ; Fontes et al, ; Stute, Clark, et al, ; Stute, Forster, et al, ; Clark et al, , ; Beyerle et al, ; Stute & Talma, ; Edmunds et al, ; Elliot et al, ; Weyhenmeyer et al, ; Condesso de Melo et al, ; Huneau et al, ; Beyerle et al, ; Guendouz et al, ; Lehmann et al, ; Kulongoski et al, ; McMahon et al, ; Plummer et al, ; Zuber et al, ; Kloppmann et al, ; Edmunds & Smedley, ; Edmunds et al, ; Kennedy & Genereux, ; Klump et al, ; Kreuzer et al, ; Osenbrück et al, ; Kulongoski et al, ; Blaser et al, ; Morrissey et al, ; von Rohden et al, ; Wieser et al, ; Varsányi et al, ; Corcho Alvarado et al, ; Szocs et al, ; Abouelmagd et al, ; Seltzer et al, ; Mihajlov et al, ; Saadi et al, ; Darling et al, ). More frequent intercomparisons of noble gas concentrations and fossil groundwater δ 18 O values may help to decouple influences of late‐Pleistocene to late‐Holocene warming from deglacial shifts to other hydroclimatic processes.…”