Linking land subsidence to deep arsenic release in the Mekong Delta aquifer system

据估计，生活在亚洲洪泛区的1亿人暴露在来自喜马拉雅沉积物的地下水中的砷中。砷是一种毒素，通过直接摄入受污染的地下水，与癌症和其他各种严重疾病有关。越来越多地开采这些受污染的含水层增加了面临这些健康风险的人数，并使更多的人口面临消费用砷污染地下水灌溉的农产品的危险。尽管人们普遍认识到砷的危害，但对地下水开采与砷污染之间关系的了解仍然有限，特别是在深层含水层中，深层含水层越来越多地提供了抽取地下水的大部分。在这项工作中，我们将重点放在湄公河三角洲，在那里我们获得了一个全面、独特、未经分析的数据集，其中包括来自越南南部的42,000个溶解砷测量值，显示了在广泛用于供水的深层含水层（200米）中广泛存在（1000平方公里）的污染。一种假设是，深层抽水导致浅层溶解的砷或砷动员溶质向深处移动。然而，初步分析并不支持这一机制，因为在湄公河三角洲观察到广泛的深层砷污染，存在厚粘土沉积物，作为相对垂直的水流屏障。我们假设了一种以前未被认识到的深层砷源机制，其中含砷的水从粘土中被释放出来，当上覆和下伏的深层含水层被开采时，粘土中的砷被压缩。这项工作结合了地下水砷观测的空间统计建模、三维含水层流动和压实模拟，以及利用卫星雷达图像（InSAR）远程测量地面沉降。我们的目标是探索深层地下水砷可能是由于数百万年前沉积的粘土层中含有砷的孔隙水释放的概念。这项研究对科学和社会都有重要意义。首先，我们以前未认识到的污染机制可能是理解砷在含水层系统中发生以及深层地下水开采相关健康风险的基础。我们的调查将对东南亚受砷影响的流域的水资源开发和人类健康产生影响，在这些地区，一些计划开采深层含水层的地区可能会在不知不觉中使人们暴露于深源砷。这项工作对世界各地沉积盆地中含有可能含有砷和其他污染物的互层可压缩粘土的类似砷影响含水层系统有影响。其次，在方法方面，沉降与砷释放之间的联系具有很大的潜力，可以作为一种侦察工具，特别是在过度抽水对地面沉降的影响尚未认识到的欠发达地区。利用卫星雷达探测陆地变形可以作为一种手段，确定可能发生粘土压实和由此产生的砷释放的地区。

An estimated 100 million people living in floodplains in Asia are exposed to arsenic in groundwater that is derived from Himalayan sediments. Arsenic is a toxin linked to cancer and a variety of other serious ailments through direct ingestion of contaminated groundwater. Growing exploitation of these contaminated aquifers increases the number of people facing these health risks, and exposes still greater populations to the hazard of consuming agricultural products irrigated with arsenic-contaminated groundwater. Despite widespread awareness of the arsenic hazard, understanding of the relationship between groundwater exploitation and arsenic contamination remains limited, particularly in deep aquifers, which are increasingly providing a larger portion of total pumped groundwater.In this work, we focus on the Mekong Delta, where we have obtained a comprehensive, unique, unanalyzed dataset consisting of 42,000 dissolved arsenic measurements from southern Vietnam showing widespread contamination (1000 sq km) in deep aquifers (200m) that are used extensively for water supply. One hypothesis is that deep pumping has induced shallow dissolved arsenic or arsenic-mobilizing solutes to move deeper. However, preliminary analysis does not support this mechanism in the Mekong Delta given the observed widespread deep arsenic contamination in the presence of thick clay deposits that serve as relative vertical flow barriers. We hypothesize a previously unrecognized deep arsenic source mechanism in which water containing arsenic is expelled from storage in clays that compact when overlying and underlying deep aquifers are exploited. This work combines spatial statistical modeling of groundwater arsenic observations, 3D aquifer flow and compaction simulation, and remote measurement of land subsidence using satellite radar imagery (InSAR). Our goal is to explore the notion that deep groundwater arsenic may be due to the release of pore-waters containing arsenic trapped in clay beds deposited millions of years ago. This research has important implications to science and society. First, our formerly unrecognized contamination mechanism may be fundamental to understanding arsenic occurrence in aquifer systems and the associated health risks of deep groundwater exploitation. Our investigation will have implications for water resources development and human health in the arsenic-affected basins of Southeast Asia where some regions of planned deep aquifer exploitation may unknowingly expose people to deep-source arsenic. This work has consequences for analogous arsenic-affected aquifer systems in sedimentary basins around the world containing interbedded compressible clays that may harbor arsenic and other contaminants. Second, in terms of methods, the link between subsidence and arsenic release has significant potential as a reconnaissance tool, particularly in underdeveloped regions where the impacts of excessive pumping on land subsidence have not been recognized. The use of satellite radar to detect land deformation can serve as a means to identify areas where clay compaction and consequent arsenic release may be occurring.