Advances in Arsenic Research - American Chemical Society

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Chapter 11

Arsenic Distribution and Speciation in the Mahomet and Glasford Aquifers, Illinois 1

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Downloaded by UNIV OF LIVERPOOL on May 29, 2018 | https://pubs.acs.org Publication Date: October 3, 2005 | doi: 10.1021/bk-2005-0915.ch011

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Thomas R. Holm , Walton R. Kelly , Steven D. Wilson , George S. Roadcap , Jonathan L. Talbott , and John S. Scott 2

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Illinois State Water Survey, Champaign, IL 61820 Illinois Waste Management and Research Center, Champaign, IL 61820

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The distribution and geochemistry of arsenic in two large aquifers in central Illinois was characterized. The areal distribution was complex; wells with high arsenic concentrations were often located less than 1km from wells with low or undetectable arsenic. High arsenic concentrations were associated with high concentrations of iron, organic carbon, bicarbonate, and ammonia, which is consistent with iron oxide reduction as the arsenic source. There was no clear pattern of arsenic concentration vs. depth and no correlation with chloride, so saline groundwater from the bedrock was not a significant source. High arsenic concentrations were also associated with low sulfate concentrations which may indicate sulfate reduction and sorption of arsenic to sulfide minerals. In most samples As(III) made up over 90% of the total arsenic, which is consistent with the redox conditions.

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© 2005 American Chemical Society

O'Day et al.; Advances in Arsenic Research ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

149 The Mahomet Aquifer underlies a large area of east-central Illinois and serves as the water source for several communities and thousands of private homes and farms. The aquifer is a sand and gravel deposit that is up to 50m thick and partially fills the Mahomet buried bedrock valley (/). It is isolated from the surface by up to 70m of glacial till. The bedrock consists mostly of shale and carbonates. The Glasford Aquifer, which overlies parts of the Mahomet Aquifer, is a discontinuous sand and gravel deposit that is up to 20m thick and is separated from both the Mahomet Aquifer below and the land surface above by up to 20m of clayey till. Groundwater from some wells in both aquifers has arsenic concentrations over 10 μg L" and a few wells have up to 100 μg L" (2, J). Arsenic concentrations in the bedrock shale, sand and gravel, and till are 5-55, 3-5, and 7-8 μg g" , respectively (5). 1

Downloaded by UNIV OF LIVERPOOL on May 29, 2018 | https://pubs.acs.org Publication Date: October 3, 2005 | doi: 10.1021/bk-2005-0915.ch011

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The likely source of arsenic to groundwater in south Asia is release to solution following iron oxide reduction (4, 5). Iron oxide reduction may also be a source of arsenic in the Mahomet Aquifer. Iron oxide coatings have been found on the Mahomet and Glasford sands (Δ Δ

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Figure 6. Arsenic(HI) as a percentage of dissolved arsenic in all samples with detectable arsenic.

O'Day et al.; Advances in Arsenic Research ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

157 No samples had any detectable methylated arsenic species, which is consistent with some published studies (34-37). Other authors (38-41) have found one or more methylated species at low levels (