A detailed study of the upper Mekong delta of Cambodia has revealed that arsenic released from near-surface wetland sediments contaminates ground water. The work is a clear demonstration of how a groundwater flow system can control arsenic levels in southeast Asia, and provides a potential framework for predicting future groundwater quality in the region.
Tens of millions of people in Asia routinely consume water that has dangerously high arsenic levels. The arsenic is derived naturally from eroded Himalayan sediments that wash down to low-lying regions, but the processes controlling the aqueous concentrations and the location of arsenic release to pore water have been unclear.
Scott Ferdorf of Stanford University and colleagues in the United States and Cambodia used hydrologic and biogeochemical measurements to build a model of arsenic release and transport through the Mekong delta. This natural cycle that occurs on a centennial timescale can be influenced by land-use changes such as irrigation pumping, agricultural intensification and urbanization.
The team’s results, which are reported in Nature this week, represent a model for understanding pre-disturbance conditions for other major deltas in Asia, and indicate that release and transport of arsenic are sensitive to ongoing and impending anthropogenic disturbances.
In a related ‘News and Views’ article, Charles Harvey of the Massachusetts Institute of Technology notes that if similar processes are responsible for arsenic contamination observed in other aquifers throughout south Asia, wells could be placed so as to avoid drawing contaminated groundwater.
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