Department of Earth Sciences, University College London, London WC1E 6BT, UK.
Environ Sci Technol. 2011 Feb 15;45(4):1376-83. doi: 10.1021/es1032376. Epub 2011 Jan 26.
The consumption of groundwater polluted by arsenic (As) has a severe and adverse effect on human health, particularly where, as happens in parts of SE Asia, groundwater is supplied largely from fluvial/deltaic aquifers. The lateral distribution of the As-pollution in such aquifers is heterogeneous. The cause of the heterogeneity is obscure. The location and severity of the As-pollution is therefore difficult to predict, despite the importance of such predictions to the protection of consumer health, aquifer remediation, and aquifer development. To explain the heterogeneity, we mapped As-pollution in groundwater using 659 wells across 102 km(2) of West Bengal, and logged 43 boreholes, to reveal that the distribution of As-pollution is governed by subsurface sedimentology. Across 47 km(2) of contiguous palaeo-interfluve, we found that the shallow aquifer (<70 mbgl) is unpolluted by As (<10 μg/L) because it is capped by an impermeable palaeosol of red clay (the last glacial maximum palaeosol, or LGMP, of ref 1 ) at depths between 16 and 24 mbgl. The LGMP protects the aquifer from vertical recharge that might carry As-rich water or dissolved organic matter that might drive reduction of sedimentary iron oxides and so release As to groundwater. In 55 km(2) of flanking palaeo-channels, the palaeosol is absent, so invasion of the aquifer by As and dissolved organic matter can occur, so palaeo-channel groundwater is mostly polluted by As (>50 μg/L). The role of palaeosols and, in particular, the LGMP, has been overlooked as a control on groundwater flow and pollutant movement in deltaic and coastal aquifers worldwide. Models of pollutant infiltration in such environments must include the appreciation that, where the LGMP (or other palaeosols) are present, recharge moves downward in palaeo-channel regions that are separated by palaeo-interfluvial regions where vertical recharge to underlying aquifers cannot occur and where horizontal flow occurs above the LGMP and any aquifer it caps.
地下水砷污染的消耗对人类健康有严重和不利的影响，特别是在东南亚部分地区，地下水主要来自河流/三角洲含水层。这些含水层中砷污染的横向分布是不均匀的。这种不均匀性的原因尚不清楚。因此，尽管这些预测对于保护消费者健康、含水层修复和含水层开发至关重要，但砷污染的位置和严重程度很难预测。为了解释这种不均匀性，我们在西孟加拉邦的 102 平方公里范围内使用 659 口井绘制了地下水砷污染图，并记录了 43 个钻孔，结果表明砷污染的分布受地下沉积学控制。在 47 平方公里的连续古河间地，我们发现浅层含水层（<70 mbgl）没有受到砷的污染（<10μg/L），因为它被深度在 16 到 24 mbgl 之间的不透水古土壤（参考 1 的末次冰盛期古土壤，或 LGMP）所覆盖。LGMP 保护含水层不受可能携带富砷水或可能驱动沉积物氧化铁还原从而将砷释放到地下水中的溶解有机质的垂直补给。在 55 平方公里的古河道侧翼，古土壤不存在，因此砷和溶解有机质可能侵入含水层，因此古河道地下水大多受到砷污染（>50μg/L）。古土壤的作用，特别是 LGMP，作为三角洲和沿海含水层地下水流动和污染物运移的控制因素，在世界范围内一直被忽视。在这些环境中，污染物渗透模型必须包括对以下认识：在存在 LGMP（或其他古土壤）的情况下，补给在古河道区域向下移动，而古河道区域被古河间地分隔开来，在古河间地中，不能发生向下面含水层的垂直补给，水平流发生在 LGMP 及其覆盖的任何含水层之上。
