Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA.
Nature. 2013 Sep 12;501(7466):204-7. doi: 10.1038/nature12444.
Groundwater drawn daily from shallow alluvial sands by millions of wells over large areas of south and southeast Asia exposes an estimated population of over a hundred million people to toxic levels of arsenic. Holocene aquifers are the source of widespread arsenic poisoning across the region. In contrast, Pleistocene sands deposited in this region more than 12,000 years ago mostly do not host groundwater with high levels of arsenic. Pleistocene aquifers are increasingly used as a safe source of drinking water and it is therefore important to understand under what conditions low levels of arsenic can be maintained. Here we reconstruct the initial phase of contamination of a Pleistocene aquifer near Hanoi, Vietnam. We demonstrate that changes in groundwater flow conditions and the redox state of the aquifer sands induced by groundwater pumping caused the lateral intrusion of arsenic contamination more than 120 metres from a Holocene aquifer into a previously uncontaminated Pleistocene aquifer. We also find that arsenic adsorbs onto the aquifer sands and that there is a 16-20-fold retardation in the extent of the contamination relative to the reconstructed lateral movement of groundwater over the same period. Our findings suggest that arsenic contamination of Pleistocene aquifers in south and southeast Asia as a consequence of increasing levels of groundwater pumping may have been delayed by the retardation of arsenic transport.
地下水每天从南亚和东南亚广大地区的浅层冲积砂中抽取，估计有超过一亿人接触到有毒水平的砷。全新世含水层是该地区广泛砷中毒的根源。相比之下，在该地区 12000 多年前沉积的更新世砂中，大多数并不含有高砷水平的地下水。更新世含水层正越来越多地被用作安全的饮用水源，因此，了解在什么条件下可以维持低水平的砷含量非常重要。在这里，我们重建了越南河内附近更新世含水层污染的初始阶段。我们证明，地下水抽取引起的地下水流动条件和含水层砂的氧化还原状态的变化，导致砷污染从全新世含水层侧向侵入，侵入距离超过 120 米，进入以前未受污染的更新世含水层。我们还发现，砷吸附在含水层砂上，并且在同一时期相对于重建的地下水侧向运动，污染的扩展有 16-20 倍的延迟。我们的研究结果表明，由于地下水抽取量的增加，南亚和东南亚更新世含水层的砷污染可能因砷运移的延迟而被推迟。
