Advances in Arsenic Research - American Chemical Society

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


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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 (0.25 mg L" ) arsenic usually was not (Figure 4). Others have found arsenic and sulfate to be mutually exclusive (8, 24). Similarly, for N P O C concentrations 3

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greater than 2 mg L" , sulfate was undetectable, which is consistent with sulfate reduction in areas where N O M was abundant. Although no sulfide analyses were performed, all groundwater samples had at least 0.1 mg L* Fe, so any sulfide produced would probably have precipitated as FeS. Arsenic(III) sorbs to FeS and FeS (25) and arsenic sorption/coprecipitation accompanies bacterial sulfate reduction (26). Therefore, sulfate reduction may affect the arsenic distribution in the Mahomet and Glasford Aquifers. Dissolved arsenic may have been released in areas where sulfate became depleted but re-sorbed in areas where there was active sulfate reduction. 1


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Figure 2. Dissolved arsenic concentrations in Tazewell County wells. The Mahomet Aquifer underlies the entire county except for the outlined areas.

The relationship between manganese, arsenic, and N P O C was similar to that between sulfate, arsenic, and NPOC. Arsenic was undetectable for almost all samples with more than 0.1 mg L" manganese and detectable for lower manganese concentrations. Manganese concentrations were below 0.1 mg L" for almost all samples with over 2 mg L" N P O C and over 0.1 mg L" for lower N P O C concentrations (Figure 4). A similar relationship between manganese and arsenic was found for groundwater in West Bengal (8). It was proposed that M n 0 may be present where the manganese concentrations are highest and, because M n 0 reduction is thermodynamically favored, iron oxide reduction is incomplete and arsenic remains sorbed to the remaining iron oxide. In the Mahomet Aquifer, M n 0 may be depleted where the NPOC concentrations are high. 1

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Figure 3. Dissolved arsenic as a function of iron (a), non-purgeable organic carbon (b), alkalinity (c), and ammonia-N (d). Tazewell County.

There were no apparent differences in arsenic concentrations among the different depth classes in both Champaign and Tazewell Counties. Also, the samples with the highest arsenic concentrations had the lowest chloride concentrations, while most samples with high chloride concentrations had low arsenic concentrations (Figure 5). These results differ from those of Warner (5), who found that for DeWitt, Logan, and McLean Counties in the central part of the aquifer, arsenic was correlated with chloride and generally increased with depth. Clearly, the bedrock is not a significant arsenic source in the eastern and western areas of the aquifer, although it may be a source in the central area. In most samples As(III) was the main arsenic species (Figure 6). For total arsenic concentrations less than 8 μg L" As(III) was usually the only detectable species, while for samples with over 8 μg L ' As(III) usually accounted for more than 90% of the dissolved arsenic. The arsenic speciation was consistent with the redox conditions in the aquifer, i.e., no detectable DO or nitrate and moderately high iron. The speciation results generally agree with those for the central part of the aquifer (5). 1

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Figure 4. Dissolved arsenic as a function of sulfate (a) and manganese (c). Sulfate (b) and manganese (d) as a function of non-purgeable organic carbon. Sulfate concentration units mg L' as SO/~. 1

The arsenic Eh-pH diagram (Figure 7) was constructed for 14EC, the typical aquifer temperature, using a recent thermodynamic data compilation (27). The ranges of p H and potential include the measured p H and ORP values. Most of the measurements lie in a band near the line for equal concentrations of As(III) and As(V) regardless of the dissolved arsenic concentration. This agrees qualitatively with the measured arsenic speciation because both species were detected in many samples with over 8 μg L" . This may, in turn, indicate that arsenic speciation was somehow related to the redox couples the ORP probe responded to. However, only one point is near the theoretical potential for an As(V):As(III) ratio of 1:10, even though the results shown in Figure 6 suggest that most of the points should lie near this line i f the system were in redox equilibrium. Most measured potentials were in the range for which the As(V) concentrations would be expected to be up to 100 times greater than the As(III) concentrations. Other researchers have also found that their pH-ORP data plot completely in the As(V) field (17, 28-32). Smedley (33) reported groundwater analyses for which As(III) made up 3-39% of the .total arsenic, the ORP values were 221-469 mV, and the p H values were between 5.4 and 7.2. Although both arsenic species were detectable, many of these data would be off the top of the scale of Figure 7. Clearly, arsenic speciation is not always quantitatively related to measured ORP values. 1


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Advances in Arsenic Research - American Chemical Society

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