@article {2705, title = {Origin of Groundwater Arsenic in a Rural Pleistocene Aquifer in Bangladesh Depressurized by Distal Municipal Pumping}, journal = {Water Resources Research}, volume = {56}, year = {2020}, month = {Jan-07-2020}, abstract = {Across South Asia, millions of villagers have reduced their exposure to high-arsenic (As) groundwater by switching to low-As wells. Isotopic tracers and flow modeling are used in this study to understand the groundwater flow system of a semi-confined aquifer of Pleistocene (>10 kyr) age in Bangladesh that is generally low in As but has been perturbed by massive pumping at a distance of about 25 km for the municipal water supply of Dhaka. A 10- to 15-m-thick clay aquitard caps much of the intermediate aquifer (>40- to 90-m depth) in the 3-km2 study area, with some interruptions by younger channel sand deposits indicative of river scouring. Hydraulic heads in the intermediate aquifer below the clay-capped areas are 1{\textendash}2 m lower than in the high-As shallow aquifer above the clay layer. In contrast, similar heads in the shallow and intermediate aquifer are observed where the clay layer is missing. The head distribution suggests a pattern of downward flow through interruptions in the aquitard and lateral advection from the sandy areas to the confined portion of the aquifer. The interpreted flow system is consistent with 3H-3He ages, stable isotope data, and groundwater flow modeling. Lateral flow could explain an association of elevated As with high methane concentrations within layers of gray sand below certain clay-capped portions of the Pleistocene aquifer. An influx of dissolved organic carbon from the clay layer itself leading to a reduction of initially orange sands has also likely contributed to the rise of As.}, keywords = {Arsenic, Dhaka Bangladesh, groundwater pumping, modeling}, issn = {0043-1397}, doi = {10.1029/2020WR027178}, url = {https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2020WR027178}, author = {Mozumder, M. R. H. and Michael, H. A. and Mihajlov, I. and Khan, M. R. and Knappett, P. S. K. and Bostick, B. C. and Mailloux, B. J. and Ahmed, K. M. and Choudhury, I. and Koffman, T. and Ellis, T. and Whaley-Martin, K. and San Pedro, R. and Slater, G. and Stute, M. and Schlosser, P. and Geen, A.} } @article {2767, title = {The Impact of Aquifer Flushing on Groundwater Arsenic Across a 35-km Transect Perpendicular to the Upper Brahmaputra River in Assam, India}, journal = {Water Resources Research}, volume = {54}, year = {2018}, month = {Jun-10-2019}, pages = {8160 - 8173}, abstract = {Well testing in the floodplain of the Brahmaputra River in Golaghat and Jorhat districts of Assam, India, shows that groundwater arsenic (As) concentrations increase with distance from the river. To establish the origin of this pattern, an additional 900 wells <60-m deep were tested for As and nine sites were drilled along a 35-km transect perpendicular to the river. The field data show no relation between groundwater As concentrations ranging from <1 to 660 μg/L along the transect and (a) As concentrations of <1{\textendash}5 mg/kg in cuttings of aquifer sand recovered while drilling or (b) the degree of reduction of iron oxides in these cuttings. The drilling indicates, however, a marked increase in the thickness of a clay layer capping the aquifer starting from <1{\textendash}5 m near the river to over 60 m at the most distant site toward the base of the Naga foothills. Organic radiocarbon ages of 18{\textendash}46 kyr obtained from all but one of 13 clay samples indicate pre-Holocene deposition of the underlying sands across the entire transect. Radiocarbon ages of dissolved inorganic carbon of 0.2, 4.7, and 17.8 kyr were measured in groundwater from three monitoring wells installed to 30{\textendash}60-m depth at distances of 10, 20, and 40 km from the river, respectively. A conceptual groundwater flow model consistent with monitored heads and groundwater ages suggests that thick clay layers capping the aquifer further from the river inhibited flushing of the aquifer and, as a result, preserved higher As levels in groundwater.}, keywords = {aquifer flushing, Arsenic, groundwater, South Asia}, issn = {0043-1397}, doi = {10.1029/2017WR022485}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1029/2017WR022485}, author = {Choudhury, R. and Nath, B. and Khan, M. R. and Mahanta, C. and Ellis, T. and Geen, A.} }