No 'One-size-fits-all' Arsenic Remedy
Recent work by University of Wisconsin-Madison (UW-Madison) researchers on arsenic-tainted wells has found 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, explained 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.
Management practices in Wisconsin have been complicated by two competing sources of soluble arsenic, Gotkowitz said.
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 said. "That makes it a harder nut to crack."
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.
Wells are routinely disinfected with chlorine bleach to control pathogenic and other bacteria. However, such treatment raises questions in regions with arsenic problems, the researchers said. 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 evaluated the impact of chlorination on bacteria and arsenic levels in Wisconsin wells. 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.
Chlorine treatment may not be appropriate in all environments, however. For example, 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, Gotkowitz said.