New research on iron-reducing bacteria.

Arsenic is a toxic, naturally occurring element that is sometimes found at high concentrations in well water. High arsenic concentrations can occur even in areas without a pollution source when the bedrock or soil that is in contact with the ground water releases arsenic as the water flows over the surfaces.

To see if bacteria affect arsenic concentration, ground water samples were taken from two high-arsenic areas in Maine, and water chemistry and two bacterial populations were measured. Wells with high total arsenic concentrations had a higher proportion of iron-reducing bacteria than wells with lower arsenic.

Iron-reducing bacteria, such as members of the genus Geobacter, grow in the absence of oxygen and can transform solid phase iron (Fe(III)) into Fe(II), which is soluble in water. Solid phase Fe(III) can bind to arsenic, immobilizing it on the surface of the solid. When bedrock or soil Fe(III) is transformed into Fe(II), any bound arsenic also would be released into the ground water. Arsenic (As) also has two commonly occurring forms in ground water: As(III) and As(V).

The prevalence of NP4, a microbe from the genus Sulfurospirillum that can transform As(V) to As(III), which is the more toxic form of As, also was measured. NP4 was more abundant in water samples with higher As(III) concentrations. Thus it appears that iron-reducing bacteria affect the overall arsenic concentration, and that arsenic-reducing bacteria (NP4) control its form, and thus toxicity, in these regions of Maine.

This work was completed at the University of Maine by Ph.D. candidate Jennifer Weldon under the supervision of Dr. Jean MacRae. The 15 ground water samples used in the study were examined using fluorescence in-situ hybridization (FISH). This technique allowed the target populations, Geobacter and NP4, to be distinguished from other bacteria. The number of each type of microorganism was compared to the total number of microorganisms in the sample. Geobacter ranged from 1 percent to 35 percent and NP4 from 0 percent to 17 percent of the total suspended bacterial population. Metals were measured by inductively coupled plasma atomic emission absorption (ICP-AES), and arsenic speciation was obtained by passing a sample through an ion exchange resin in the field to obtain As(III). Total arsenic concentrations ranged from less than 2 parts per billion (ppb) to 2,000 ppb, which is 200 times higher than the EPA's water quality limit of 10 ppb. As(III) ranged from <2 ppb to 1,100 ppb.

By learning more about the arsenic release mechanisms, high-risk areas may be targeted for testing and developing management options to minimize arsenic concentrations.
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Sidebar: New Arsenic Drinking Water Standard

The U.S. Environmental Protection Agency (EPA) is reminding water systems that a new, more protective national standard for arsenic in drinking water will take effect less than six months from now.

The EPA adopted the new standard in 2001 that requires all drinking water systems to reduce arsenic from the current standard of 50 parts per billion to 10 parts per billion. The main areas affected in the western United States include small water utilities that draw their drinking water from deep underground wells in California's foothills and Central Valley, and throughout Arizona and Nevada. The EPA estimates that nationwide, roughly 97 percent of the utilities that fail to meet the new standard are the smaller systems, which serve fewer than 10,000 people.

“We will continue working with our states and drinking water systems to find cost-effective ways to meet the new arsenic drinking water standard,” says Alexis Strauss, director of the water division for the EPA's Pacific Southwest Office in San Francisco. “This new standard will protect the health of over 13 million Americans, including many smaller communities in California, Arizona, Nevada and on tribal lands.”

The EPA estimates that the new standard will provide greater protection from cancer risks and other health problems. Ingesting too much arsenic can lead to lung, bladder, liver, skin and other cancers, as well as damage to the cardiovascular, pulmonary, immune, neurological and endocrine systems.

The EPA, state and local agencies are providing information and technical assistance to utilities that have yet to install treatment systems or take alternative approaches to reduce arsenic levels. Some of the treatment methods used to reduce arsenic levels include iron precipitation and filtration, ion exchange and adsorption to iron media.

For more information, contact the local office of your state's drinking water agency or visit the EPA's Web site: www.epa.gov/safewater.