Though a worldwide problem, arsenic contamination of drinking water does not have a universal solution. Instead, recent work on arsenic-tainted wells shows that appropriate treatment varies, depending on the source of the contamination.
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 usually is associated
with sulfur- or iron-rich minerals, where it poses no threat to ground water,
explains lead researcher Madeline Gotkowitz, a hydrogeologist at the Wisconsin
Geological and Natural History Survey.
Once it is released from mineral form into ground water
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 ground water 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
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."
ground water arsenic affects some municipal water supply wells, but it
primarily is 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.
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
"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.