A new study shows that for most US ground waters high in arsenic one arsenic species (either As(III) or As(V)) is dominant. Interestingly, As(III) was the dominant form in 92% of wells in the midwest region in comparison to only 21% in the east region, 7% in the west region, and 9% in the farwest region.
Background:Arsenic is a common, naturally occurring element in ground water in many countries and its concentration varies widely and regionally. Moreover, ground water is a major source of drinking water and the occurrence of arsenic in drinking water has been associated with adverse heath effects in humans. While municipal water suppliers in the US are required to meet the E.P.A.’s safety standard of 10 p.p.b. for arsenic in drinking water, no such regulation exists for private wells. Researcher estimate, that nationwide about 13 million people get drinking water from private wells with arsenic levels above the federal standard.
The variability of the arsenic concentration in ground waters is attributed to the arsenic content of the aquifer materials and the varying desorption/dissolution processes that release the arsenic from the solid phase into the liquid phase. As a result of such processes, arsenic exists in ground water as oxyanions having two oxidation states, As(III) and As(V). Because of the difference in toxicity and removability of As(III) and As(V), arsenic speciation is important in the selection and design of an arsenic treatment system. Identifying the arsenic species is also helpful in explaining and understanding the origin, behavior and characteristics of arsenic in the environment.
However, arsenic speciation analysis is more complex than just arsenic trace element determination and requires either an appropriate preservation method that stabilizes both As(III) and As(V) or make use of field separation methods that are somewhat complex and costly. Thus, very few studies have incorporated arsenic speciation.
The new study:A U.S. Environmental protection Agency (EPA) arsenic treatment research program provided a unique opportunity to determine the concentration of the naturally occurring arsenic species in 65 well waters scattered across the USA with many of them being speciated monthly for up to three years. Because of the lack of a univerase preservation method for arsenic species in ground waters containing different amounts of iron and manganese, the researchers used an on-site procedure to separate As(V) and As(III). This well established method makes use of an anion exchange resin whereby the uncharged As(III) passes through the resin while the charged As(V) is retained on the resin.
Fig.:
Percent arsenic III in well waters by regions of the USA.
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Speciation test data obtained by this method showed that 31 wells had predominantly As(V), 29 had predominantly As(III) and five had a mixture of both. A general pattern was found where As(III) was the dominant species in midwest ground waters where anoxic conditions and elevated iron concentrations prevailed and the well waters in the east, west and farwest had either As(III) or As(V) as the dominant species. The monthly (12–36) speciation tests results at many of these sites also found no major changes in the ratio of arsenic species over time even for those few well waters that showed significant variability in total arsenic concentration.
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Related information British geological survey: Arsenic contamination of groundwater National Resources Defense Council: Arsenic in drinking water USEPA: Arsenic in drinking water USGS: Arsenic in groundwater of the United States Water Management Association of Ohio: Arsenic in Ohio groundwater WHO: Arsenic in drinking water
Related EVISA Resources Link database: Toxicity of arsenic species Link database: Environmental arsenic pollution Brief summary: Speciation and Toxicity
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