<p style="text-align: center;">by Scott Ferndorf, School of Earth Science, Stanford University</p>
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<img alt="" src="http://www.slu.se/Global/externwebben/forskarskolor/focus-soils-water/FoSW_logo.jpg" /></p>
We are presently witnessing the largest poisoning in history within South and Southeast Asia. As a result of the high population density and regular flooding, surface water (rivers, lakes, and ponds) across the low-lying areas of Asia are ladened with high levels of microbial pathogens. In a quest to obtain pathogen-free water, a large-scale initiative to use groundwater for drinking was initiated in the early 1970s. However, arsenic levels often more than 50 times the maximum concentration recommended by the World Health Organizations were unknowingly present in groundwater. Presently, more than 200 million people are at risk of exposure to dangerous levels of geogenic arsenic in drinking water across Asia. Arsenic within rocks of the Himalayas is liberated to the sediment load of the major river systems and ultimately deposited within the massive deltas of Asia, particularly the South and Southeast regions. Upon burial, arsenic is released to the aqueous phase through a complex web of microbially driven geochemical processes, and its distribution determined by a coupling of biogeochemical reactions and groundwater hydrology. Generally, arsenic binds strongly to soil/sediment solids under aerated conditions; under anaerobic condition, aqueous concentrations increase, a phenomenon attributed to microbial reduction of iron or arsenic. Owing to heterogeneity within soils and sediments, processes controlling the fate and transport of arsenic vary at the micron-scale, and, as a consequence, aqueous concentrations of arsenic are distributed unevenly in the subsurface. While such heterogeneity makes predicting groundwater arsenic concentrations difficult both spatially and temporally, it provides an opportunity to potentially extract water safe (or safer) for human consumption. Here the fate controlling processes of arsenic and predictive and corrective approaches to the groundwater contaminations are described.