Another arsenic hot spot
High arsenic concentrations in shallow Cambodian groundwater wells could pose health risks for more than a million people.
A brewing health crisis in Cambodia may be following the same pattern of arsenic poisoning that has devastated parts of Bangladesh and West Bengal (India). Poor surface-water quality recently led Cambodians to rely more heavily on shallow groundwater wells for drinking water, but recent reports indicate elevated levels of naturally occurring arsenic in some of these wells. In the first comprehensive groundwater survey of the area, published today on ES&T’s Research ASAP website (DOI: 10.1021/es062056k), scientists map out the magnitude of the contamination problem, which includes arsenic as well as manganese, in Cambodia. Those involved in the work are calling for immediate mitigation efforts.
Michael Berg, an environmental chemist at the Swiss Federal Institute of Aquatic Science and Technology, and colleagues sampled 131 household wells across 3700 square kilometers (km2) of the Mekong River floodplain in Cambodia. Arsenic concentrations ranged from 1–1340 micrograms per liter (µg/L); 48% exceeded the 10 µg/L drinking-water guideline recommended by the World Health Organization (WHO). More than a third exceeded the 50 µg/L drinking-water limit set in many developing countries.
“The big surprise is that the areas of arsenic contamination are sharply restricted, and we find perfectly safe water in some areas,” Berg says. “This is an extremely good message,” he notes, adding that “it’s not the case anywhere else that you have such clear boundaries of arsenic-safe and arsenic-polluted water.”
Berg and his colleagues show that the elevated levels are confined to the floodplain between the Bassac and Mekong rivers in Kandal province and their adjacent riverbanks. Arsenic concentrations there averaged 233 µg/L, whereas concentrations to the east and west of the rivers were below 10 µg/L.
While surveying samples for such parameters as water hardness, pH, and metal concentrations, the scientists also found another potential health threat: manganese. More than half of the samples taken from western areas, which were relatively arsenic free, contained manganese levels above the WHO guideline value of 0.4 milligrams per liter. Although arsenic’s human health effects are clear, only recently have studies indicated an association between manganese and neurotoxic effects in children, Berg notes. Only 18% of the wells sampled provided chemically safe drinking water.
“This is a very important piece of work, because so far we haven’t been paying much attention to groundwater contamination” outside of the Bengal Delta, says Dipankar Chakraborti, research director at Jadavpur University’s School of Environmental Studies (India). He maintains, however, that it’s too early to tell whether the situation is as severe as in Bangladesh, where the total arsenic-affected region is about 80,000 km2 and some 43 million people have been drinking arsenic-laden water for several decades. As in Vietnam, Cambodians only began installing groundwater wells about 10 years ago, Berg acknowledges. The symptoms of chronic arsenic exposure generally don’t appear until after 5–10 years of continuous exposure, and they become more pronounced over time.
These findings are “part of an emerging picture of naturally occurring arsenic and manganese problems in shallow groundwater in many Asian countries,” agrees Alan Welch, a geologist with the U.S. Geological Survey. “A number of other areas with similar hydrogeologic settings haven’t really been documented with respect to arsenic and manganese,” but the findings linking Cambodia’s topography with high and low arsenic concentrations offer a guideline for where other surveys might be focused, Welch adds.
“We got to wondering why the contaminated areas were so sharply restricted,” Berg explains, and “that’s why we had a look at the topography.” Digitized maps showed a correlation with the lowest depressions, indicating that the arsenic problem may be related to alluvial sediments from the Holocene age that filled in an old valley between the Bassac and Mekong rivers. “It’s a feature that came out quite clearly and should be considered in other studies to see whether similar patterns can be seen,” Berg says.
Meanwhile, Berg and his colleagues are continuing their survey in the upper branches of the Mekong River. “There will be more [arsenic-affected] areas to come,” Berg predicts. “Everyone is now awakened to the problem and looking for it.”
