I. Choudhury

Arsenic migration to deep groundwater in Bangladesh influenced by adsorption and water demand

Drinking shallow groundwater with naturally elevated concentrations of arsenic is causing widespread disease in many parts of South and Southeast Asia. In the Bengal Basin, growing reliance on deep (>150 m) groundwater has lowered exposure. In the most affected districts of Bangladesh, shallow groundwater concentrations average 100 to 370 μg L−1, while deep groundwater is typically < 10 μg L−1. Groundwater flow simulations have suggested that, even when deep pumping is restricted to domestic use, deep groundwater in some areas of the Bengal Basin is at risk of contamination. However, these simulations have neglected the impedance of As migration by adsorption to aquifer sediments. Here we quantify for the first time As sorption on deeper sediments in situ by replicating the intrusion of shallow groundwater through injection of 1,000 L of deep groundwater modified with 200 μg L−1 of As into a deeper aquifer. Arsenic concentrations in the injected water were reduced by 70% due to adsorption within a single day. Basin-scale modelling indicates that while As adsorption extends the sustainable use of deep groundwater, some areas remain vulnerable; these areas can be prioritized for management and monitoring.

Vulnerability of low-arsenic aquifers to municipal pumping in Bangladesh

Sandy aquifers deposited >12,000 years ago, some as shallow as 30 m, have provided a reliable supply of low-arsenic (As) drinking water in rural Bangladesh. This study concerns the potential risk of contaminating these aquifers in areas surrounding the city of Dhaka where hydraulic heads in aquifers >150 m deep have dropped by 70 m in a few decades due to municipal pumping. Water levels measured continuously from 2012 to 2014 in 12 deep (>150m), 3 intermediate (90-150 m) and 6 shallow (<90 m) community wells, 1 shallow private well, and 1 river piezometer show that the resulting drawdown cone extends 15-35 km east of Dhaka. Water levels in 4 low-As community wells within the 62-147 m depth range closest to Dhaka were inaccessible by suction for up to a third of the year. Lateral hydraulic gradients in the deep aquifer system ranged from 1.7×10-4 to 3.7×10-4 indicating flow towards Dhaka throughout 2012-2014. Vertical recharge on the edge of the drawdown cone was estimated at 0.21±0.06 m/yr. The data suggest that continued municipal pumping in Dhaka could eventually contaminate some relatively shallow community wells.

Stimulation of Microbially Mediated Arsenic Release in Bangladesh Aquifers by Young Carbon Indicated by Radiocarbon Analysis of Sedimentary Bacterial Lipids.

The sources of reduced carbon driving the microbially mediated release of arsenic to shallow groundwater in Bangladesh remain poorly understood. Using radiocarbon analysis of phospholipid fatty acids (PLFAs) and potential carbon pools, the abundance and carbon sources of the active, sediment-associated, in situ bacterial communities inhabiting shallow aquifers (<30 m) at two sites in Araihazar, Bangladesh, were investigated. At both sites, sedimentary organic carbon (SOC) Δ(14)C signatures of -631 ± 54‰ (n = 12) were significantly depleted relative to dissolved inorganic carbon (DIC) of +24 ± 30‰ and dissolved organic carbon (DOC) of -230 ± 100‰. Sediment-associated PLFA Δ(14)C signatures (n = 10) at Site F (-167‰ to +20‰) and Site B (-163‰ to +21‰) were highly consistent and indicated utilization of carbon sources younger than the SOC, likely from the DOC pool. Sediment-associated PLFA Δ(14)C signatures were consistent with previously determined Δ(14)C signatures of microbial DNA sampled from groundwater at Site F indicating that the carbon source for these two components of the subsurface microbial community is consistent and is temporally stable over the two years between studies. These results demonstrate that the utilization of relatively young carbon sources by the subsurface microbial community occurs at sites with varying hydrology. Further they indicate that these young carbon sources drive the metabolism of the more abundant sediment-associated microbial communities that are presumably more capable of Fe reduction and associated release of As. This implies that an introduction of younger carbon to as of yet unaffected sediments (such as those comprising the deeper Pleistocene aquifer) could stimulate microbial communities and result in arsenic release.

Evidence for Elevated Levels of Arsenic in Public Wells of Bangladesh Due To Improper Installation

One of the mainstays of mitigation to reduce the exposure of the rural population of Bangladesh to arsenic (As) from private, mostly <90-m deep wells over the past 15 years has been the installation of over 300,000 deeper community wells. A comprehensive testing campaign previously conducted across a 180 km2 of area of Bangladesh identified 9 out of total of 927 wells >90 m deep that contained >50 µg/L arsenic. We show here that for five of these nine wells, conductivity profiles obtained after spiking the well bore with salt indicate a shallow leak that could explain the high As in the well water. In two of the five leaky wells, the presence of additional screens at the depth of the leak was documented with a downhole camera. The downhole camera did not detect anomalies in the construction of the remaining three leaky wells or in the four wells that did not leak. The four wells that did not leak were all >150-m deep and located in two villages separated by less than 500 m. Excluding these two villages and a handful of leaky wells, the results indicate an aquifer that is consistently low in As over a sizeable area at depths >90 m. Isolated cases of public wells that are elevated in As that have been reported elsewhere in Bangladesh may therefore reflect improper installation rather than actual contamination of the deep aquifer.

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

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.

Human and livestock waste as a reduced carbon source contributing to the release of arsenic to shallow Bangladesh groundwater

Recent studies have demonstrated that the supply of relatively young organic carbon stimulates the release of arsenic to groundwater in Bangladesh. This study explores the potential role of human and livestock waste as a significant source of this carbon in a densely populated rural area with limited sanitation. Profiles of aquifer sediment samples were analyzed for phytosterols and coprostanol to assess the relative contributions of plant-derived and human/livestock waste-derived organic carbon at two well-characterized sites in Araihazar. Coprostanol concentrations increased with depth from non-detection (<10m at Site B and <23m at Site F) to maxima of 1.3 and 0.5ng/g in aquifer sands recovered from 17m (Site B) and 26m (Site F), respectively. The commonly used sewage contamination index ([5β-coprostanol]/([5α-cholestanol]+[5β-coprostanol])) exceeds 0.7 between 12 and 19m at Site B and between 24 and 26m at Site F, indicating input of human/livestock waste to these depths. Urine/fecal input within the same depth range is supported by groundwater Cl/Br mass ratios >1000 compared to Cl/Br <500 at depths >50m. Installed tube wells in the areas study sites may act as a conduit for DOC and specifically human/livestock waste into the aquifer during flood events. The depth range of maximum input of human/livestock waste indicated by these independent markers coincides with the highest dissolved Fe (10-20mg/L) and As (200-400μg/L) concentrations in groundwater at both sites. The new findings suggest that the oxidation of human/livestock waste coupled to the reductive dissolution of iron-(oxy)-hydroxides and/or arsenate may enhance groundwater contamination with As.