Arsenic risk in the Mekong Delta of Cambodia

Since the awful discovery in the 1980s that millions of people in the delta plains of the northern Indian subcontinent were at risk of chronic arsenic poisoning if they drank water drawn from wells in alluvium, that hazard has been found to exist in other alluvial areas close to sea level. The arsenic is of natural origin and is released when iron hydroxide, the most common sediment colorant and powerful medium for adsorption of many elements including arsenic, breaks down. Iron hydroxide is destabilised in strongly reducing environments, when its component Fe3+ gains an electron to become soluble Fe2+. The most common source of reducing conditions is vegetation buried in alluvial sediments. In Bangladesh and West Bengal, India, the problem is peat layers buried by rapid sedimentation since about 7 thousand years ago that filled channels cut by rivers when sea level was much lower during the ast glacial maximum. The risky areas in the Mekong Delta are more complex (Papacostas, N.C. et al. 2008. Geomorphic controls on groundwater arsenic distribution in the Mekong River Delta, Cambodia. Geology, v. 36, p. 891-894). Areas at risk are strongly focused by recent landforms associated with channel migration, rather than extending across entire flood plains as in Bangladesh. Features such as meander scrolls, point bars and islands that have grown to be incorporated in older floodplains show the highest arsenic concentration in groundwater. These accumulate organic debris in large amounts, whose decay releases arsenic from iron hydroxide veneers on sand grains. Older features of the same kinds show less arsenic contamination in their groundwater, suggesting that eventually either the reductants become exhausted or available arsenic is flushed out. So, careful mapping and dating of fluviatile geomorphology may be a means of screening for arsenic risk in the Mekong and other low-lying delta plains.

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