Mozumder M R H, Michael H A, Mihajlov I, Khan M R, Knappett P S K, Bostick B C, Mailloux B J, Ahmed K M, Choudhury I, Koffman T, Ellis T, Whaley-Martin K, San Pedro R, Slater G, Stute M, Schlosser P, van Geen A
Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY, USA.
Now at Gradient, Boston, MA, USA.
Water Resour Res. 2020 Jul;56(7). doi: 10.1029/2020wr027178. Epub 2020 May 6.
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-km 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 1-2 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 H-He 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.
在南亚，数百万村民通过改用低砷水井，减少了对高砷地下水的接触。本研究中使用同位素示踪剂和水流模型，来了解孟加拉国一个更新世（>10 kyr）半承压含水层的地下水流系统，该含水层通常砷含量较低，但因在距达卡市供水约25公里处大量抽水而受到干扰。在3公里的研究区域内，一层10至15米厚的黏土隔水层覆盖了大部分中间含水层（深度>40至90米），有一些地方被较新的河道砂沉积物打断，表明河流冲刷情况。黏土覆盖区域下方中间含水层的水头比黏土层上方高砷浅层含水层的水头低1至2米。相比之下，在黏土层缺失的地方，浅层和中间含水层的水头相似。水头分布表明存在一种水流模式，即通过隔水层的间断处向下流动，以及从砂质区域向含水层承压部分的侧向平流。解释的水流系统与H-He年龄、稳定同位素数据和地下水流模型一致。侧向水流可以解释在更新世含水层某些黏土覆盖部分下方的灰砂层中，砷含量升高与高甲烷浓度之间的关联。来自黏土层本身的溶解有机碳流入导致最初橙色砂的减少，这也可能促成了砷含量的升高。
