Ground water contaminated by arsenic poses a risk to the health of millions of people, especially in the densely populated river deltas of Southeast Asia. To date, no method had been available for identifying high-risk areas without conducting costly sampling campaigns. Now, researchers have developed a model that allows vulnerable areas to be pinpointed using existing data on geology and soil properties. It also has enabled them to detect high-risk areas in regions where ground water studies had not previously been carried out, such as in Myanmar and on Sumatra.

Scientists from Eawag, a Swiss-based aquatic research institute, have described a method that allows high-risk areas to be identified without the need for ground water analysis. The research team, led by geologist Lenny Winkel and environmental chemist Michael Berg, compiled existing geological data from Bangladesh, Myanmar, Thailand, Cambodia, Vietnam and Sumatra to produce a uniformly classified map.

The data related only to surface sediments and soil properties; surprisingly, this combination of data permits sufficiently accurate conclusions to be drawn concerning chemical and physical conditions in ground water.

The scientists then studied the statistical relations between 30 surface parameters, including geological, hydrological and climate data, and arsenic concentrations, finally incorporating the eight most relevant variables into a logistic regression model. In particular, young river deposits with organic rich sediments proved to be indicators of ground water arsenic contamination. This finding is apparent from the maps in which the probabilities calculated for elevated arsenic concentrations are presented in a graphical form.

Worldwide, more than 100 million people are exposed to excessive amounts of arsenic in drinking water. Arsenic occurs as a natural component of underground rocks worldwide, with small quantities being dissolved in ground water as a result of weathering. The inorganic salts of arsenic are tasteless and odourless, but highly toxic to humans. If ingested over long periods, even low concentrations can damage health, including hyper-pigmentation of the skin, disorders of liver and kidney function, and various types of cancer.

Problems arise from the fact that arsenic concentrations can vary widely at the local level, and that, in many areas, people are completely unaware of the risk because their well water or ground water never has been tested for arsenic. Arsenic concentrations below 10 µg/L are deemed to be safe, and therefore, are recommended by the World Health Organization as a guideline value for arsenic in drinking water. In the deltas of the Red River and the Mekong, Eawag detected arsenic concentrations exceeding 100 µg/L in one in five of the samples analyzed, with maximum values as high as 3,000 µg/L. In Myanmar’s Irrawaddy delta, another study found arsenic concentrations of more than 50 µg/L in two-thirds of the sampled wells.

The latest findings from Southeast Asia are part of the Water Resource Quality (WRQ) project, a research program studying the occurrence of geogenic contaminants in ground water worldwide. Methods also are being developed to allow the affected populations to treat contaminated water using appropriate technologies. To date, the work has been carried out on a very coarse scale, but now has been successfully refined, thanks to the project in Southeast Asia. The new model is of particular interest for regions where no ground water measurement data are yet available.

Accordingly, the Swiss aquatic research team applied the model to the Indonesian island of Sumatra, where an area covering 38,610 square miles on the eastern coast was found to be at high risk for arsenic contamination. The researchers subsequently used about 100 ground water samples to verify the probabilities predicted by the model for a region on the border between a low- and a high-risk area. Once again, the results of analyses were found to agree with the predictions: 94 percent of the wells in the low-risk area showed arsenic concentrations below 10 µg/L. The maps also indicate an increased risk of elevated arsenic concentrations in ground water in the Irrawaddy delta and along the Chao Phraya river north of Bangkok, Thailand – both areas where the risk had not previously been recognized.