Adding a naturally occurring mineral to water contaminated by arsenic could be a quick and cheap means of removing the toxic chemical, say researchers in the United States.

Water containing high concentrations of arsenic threatens the health of tens of millions of people, mainly in Bangladesh and the Indian state of West Bengal. The problem exists because certain bacteria trigger the release of arsenic from earth into groundwater (see 'Microbes to blame' for arsenic threat to millions).

Researchers at the University of Illinois, led by Craig Bethke, say that changing the chemical composition of water could encourage different types of microbe to dominate — and halt the release of arsenic.

The researchers analysed 21 wells in central Illinois where arsenic levels were thought to have been uniform because of glacial activity. They were surprised to find that levels of the chemical actually varied from well to well.

Even levels of arsenic in wells near each other varied so much that one might have dangerous amounts, while its neighbour’s were undetectable.

The researchers also found that when arsenic levels were low, levels of the mineral sulphate were high, and vice versa. This, they say, could make identifying contaminated wells quicker and easier.

"Unlike detecting the presence of arsenic — which generally requires a sensitive laboratory analysis — testing for sulphate is simple and straightforward," says Bethke.

What's more, when sulphate levels are high, bacteria consuming the sulphate produce sulphide as a by-product. Sulphide causes any arsenic present to form a solid, leaving little in solution. This means that adding sulphate to contaminated wells could help clear them of arsenic by precipitating the toxin out of the water.

"If we are correct that these bacteria control arsenic levels, and if augmenting sulphate can stimulate the bacteria, we may have identified an inexpensive means of remediation, but of course this needs to be demonstrated in the field," says Bethke.

Sulphate salts, such as gypsum — calcium sulphate — readily dissolve in water, are widely available and cheap, costing less than US$1 for 10 kilograms. The price would put it well within the reach of authorities in the worst-affected countries.

"The bacteria are already present, so all you have to do is stimulate them," says Matthew Kirk, lead author of the research, published in the November 2004 edition of Geology.

According to Bethke, waters in Bangladesh show the same patterns as in the Illinois study.

"Most groundwaters in Bangladesh are poor in sulphate, but the ones with even modest amounts generally contain little arsenic," he told SciDev.Net. "We suspect that the arsenic problem is as widespread as it is there because of the paucity of sulphate." 

Recent research has suggested that sequencing the entire genomes of bacteria that cause arsenic mobilisation could lead to improved means of detecting and decontaminating arsenic-rich waters (see Gene test reveals arsenic-releasing microbes in water).

"Genomic research, in spite of its cost, has considerable intrinsic scientific value," says Bethke. "It may turn out that the behaviour of arsenic in other aquifers, perhaps acidic ones, differs from that in our study, where water is alkaline, so sophisticated methods like sequencing an entire genome are needed to study the problem."

"Looking at water chemistry, which is readily observed using inexpensive techniques, may nonetheless prove to be of more immediate practical significance than genomic techniques, especially in the developing world," he adds.

Adding sulphate to drinking water need not bring any additional health risks, says Bethke. The World Health Organisation guidelines for drinking water says levels of 400 mg of sulphate per litre are safe, whereas most concentrations of sulphate in the Illinois study were less than 50 mg/L.

"If this form of remediation proves effective, it might be no more complicated than preparing a calcium sulphate solution and pumping it into the aquifer," says Bethke. "In this case, it would require no advanced technology or expensive equipment."

Another advantage to such a method is that because arsenic would be immobilised within the aquifer, there would be no arsenic-bearing waste to dispose of.

The next step, says Bethke, "is to test the scheme, on the laboratory bench or in the field".