Though a worldwide problem, arsenic contamination of drinking water does not have a universal solution.
Instead, recent work by University of Wisconsin-Madison researchers on arsenic-tainted wells shows that appropriate treatment varies depending on the source of the contamination.
Naturally occurring arsenic in rocks is usually associated with sulfur- or iron-rich minerals, where it poses no threat to groundwater, explains lead researcher Madeline Gotkowitz, a hydrogeologist at the Wisconsin Geological and Natural History Survey.
Once it is released from mineral form into groundwater through geochemical or biological processes, however, chronic exposure to arsenic has been linked to skin lesions and increased risk of several cancers. The issue has gained international prominence in Southeast Asia but affects populations around the world.
"It's stunning how many people worldwide are affected by toxic levels of arsenic," Gotkowitz says. "There are thousands upon thousands of people who become ill from having their drinking water contaminated with arsenic."
Though on a smaller scale, arsenic-tinged groundwater is a problem in parts of the United States as well, including regions in the Northwest, East and Midwest.
Management practices in Wisconsin have been complicated by two competing sources of soluble arsenic, Gotkowitz says. Arsenic associated with sulfide minerals in rock can be released by the weathering effects of oxygen-rich environments.
Alternately, arsenic bound to iron oxides can be released by iron-reducing bacteria, which thrive in low-oxygen conditions. "There is different geochemistry in different [areas]," Gotkowitz says. "That makes it a harder nut to crack. ... People might have a similar symptom - arsenic in their water - but there are different solutions because the geologic environment is quite different."
In Wisconsin, groundwater arsenic affects some municipal water supply wells, but it is primarily an issue for rural communities and others where residents often rely upon shallow private wells.
"Large areas of Outagamie and Winnebago counties have high arsenic levels in one of the shallower aquifers," Gotkowitz says. "Upwards of 10,000 private homes are affected by having arsenic above the standard [acceptable level]."
Wells are routinely disinfected with chlorine bleach to control pathogenic and other bacteria. However, such treatment raises questions in regions with arsenic problems.
While bleach should kill off arsenic-producing bacteria, it also creates a high-oxygen environment that some worry could enhance release of additional arsenic from the rocks.
Gotkowitz and UW-Madison geologists Eric Roden and Evgenya Shelobolina evaluated the impact of chlorination on bacteria and arsenic levels in Wisconsin wells.
The results were presented at the American Geophysical Union meeting in San Francisco today (Dec. 10).
In wells with arsenic levels only moderately above the accepted standard, the scientists found that the presence of iron-reducing bacteria was associated with higher arsenic concentrations. Disinfection of these wells with chlorine adequately removed bacteria and reduced arsenic levels in the short term.
In addition, chlorination did not increase arsenic release from the surrounding rocks, showing that oxidation of the rocks is not an important source of arsenic here.
Similar effects were seen in areas with a relatively high water table, where aquifers are exposed to less oxygen.
The results suggest that disinfection is an effective way to control pathogenic bacteria and may also limit arsenic release in wells under these conditions.
"It's not like there's going to be an easy solution, but there are some basic indicators," Gotkowitz says. Under low-oxygen conditions or where water levels are high, "you might want to try to control those types of bacteria as a way to improve well water quality."
Chlorine treatment may not be appropriate in all environments, however. For example, she says, the oxidizing properties of bleach may pose more of a concern in arsenic-affected regions with lower water tables, while wells drawing from aquifers highly contaminated with arsenic are unlikely to benefit from localized treatment.