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.

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Arsenic contamination of groundwater has been identified in Cambodia, where some 100,000 family-based wells are used for drinking water needs. We conducted a comprehensive groundwater survey in the Mekong River floodplain, comprising an area of 3700 km(2) (131 samples, 30 parameters). Seasonal fluctuations were also studied. Arsenic ranged from 1 to 1340 microg L(-1) (average 163 microg L(-1)), with 48% exceeding 10 microg L(-1). Elevated manganese levels (57% >0.4 mg L(-1)) are posing an additional health threat to the 1.2 million people living in this area. With 350 people km(-2) potentially exposed to chronic arsenic poisoning, the magnitude is similar to that of Bangladesh (200 km(-2)). Elevated arsenic levels are sharply restricted to the Bassac and Mekong River banks and the alluvium braided by these rivers (Kandal Province). Arsenic in this province averaged 233 microg L(-1) (median 100 microg L(-1)), while concentrations to the west and east of the rivers were <10 microg L(-1). Arsenic release from Holocene sediments between the rivers is most likely caused by reductive dissolution of metal oxides. Regions exhibiting low and elevated arsenic levels are co-incident with the present low relief topography featuring gently increasing elevation to the west and east of a shallow valley-understood as a relict of pre-Holocene topography. The full georeferenced database of groundwater analysis is provided as Supporting Information.

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