Polizzotto Matthew L, Kocar Benjamin D, Benner Shawn G, Sampson Michael, Fendorf Scott
School of Earth Sciences, Stanford University, Stanford, California 94305, USA.
Nature. 2008 Jul 24;454(7203):505-8. doi: 10.1038/nature07093.
Tens of millions of people in south and southeast Asia routinely consume ground water that has unsafe arsenic levels. Arsenic is naturally derived from eroded Himalayan sediments, and is believed to enter solution following reductive release from solid phases under anaerobic conditions. However, the processes governing aqueous concentrations and locations of arsenic release to pore water remain unresolved, limiting our ability to predict arsenic concentrations spatially (between wells) and temporally (future concentrations) and to assess the impact of human activities on the arsenic problem. This uncertainty is partly attributed to a poor understanding of groundwater flow paths altered by extensive irrigation pumping in the Ganges-Brahmaputra delta, where most research has focused. Here, using hydrologic and (bio)geochemical measurements, we show that on the minimally disturbed Mekong delta of Cambodia, arsenic is released from near-surface, river-derived sediments and transported, on a centennial timescale, through the underlying aquifer back to the river. Owing to similarities in geologic deposition, aquifer source rock and regional hydrologic gradients, our results represent a model for understanding pre-disturbance conditions for other major deltas in Asia. Furthermore, the observation of strong hydrologic influence on arsenic behaviour indicates that release and transport of arsenic are sensitive to continuing and impending anthropogenic disturbances. In particular, groundwater pumping for irrigation, changes in agricultural practices, sediment excavation, levee construction and upstream dam installations will alter the hydraulic regime and/or arsenic source material and, by extension, influence groundwater arsenic concentrations and the future of this health problem.
南亚和东南亚数千万人经常饮用砷含量不安全的地下水。砷天然来源于喜马拉雅山被侵蚀的沉积物，据信在厌氧条件下从固相还原释放后进入溶液。然而，控制砷在孔隙水中的水溶液浓度和释放位置的过程仍未得到解决，这限制了我们在空间上（井与井之间）和时间上（未来浓度）预测砷浓度以及评估人类活动对砷问题影响的能力。这种不确定性部分归因于对恒河 - 布拉马普特拉河三角洲因广泛灌溉抽水而改变的地下水流路径了解不足，而大多数研究都集中在该地区。在这里，通过水文和（生物）地球化学测量，我们表明，在柬埔寨受干扰最小的湄公河三角洲，砷从近地表、源自河流的沉积物中释放出来，并在百年时间尺度上通过下伏含水层输送回河流。由于地质沉积、含水层源岩和区域水文梯度的相似性，我们的研究结果代表了一个理解亚洲其他主要三角洲扰动前状况的模型。此外，对砷行为的强烈水文影响的观察表明，砷的释放和输送对持续和即将到来的人为干扰很敏感。特别是，用于灌溉的地下水抽取、农业实践的变化、沉积物挖掘、堤坝建设和上游大坝安装将改变水力状况和/或砷源物质，进而影响地下水中的砷浓度以及这个健康问题的未来。
