Controls on Arsenic Concentrations in Groundwater near Lake

Chapter 12

Controls on Arsenic Concentrations in Groundwater near Lake Geneva, Wisconsin T.

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Root , J. M. Bahr , and M. B. Gotkowitz

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Downloaded by TUFTS UNIV on February 17, 2018 | https://pubs.acs.org Publication Date: October 3, 2005 | doi: 10.1021/bk-2005-0915.ch012

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Department of Geology and Geophysics, University of Wisconsin, Madison, WI 53706 Wisconsin Geological and Natural History Survey, Madison, WI 53705

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Geochemical and hydrochemical data from Lake Geneva, Wisconsin indicate that arsenic in groundwater may be the result of reductive dissolution of (hydr)oxide minerals. Geologic, hydrogeologic, and geochemical factors create reducing conditions that lead to arsenic mobilization in the deep Quaternary and upper Silurian aquifers, while groundwater in shallow Quaternary sediments is largely unaffected by arsenic. Groundwater chemistry data and the results of a pumping test suggest that there is little groundwater movement between the deeper, arsenic-impacted aquifers, and the shallow, non-impacted aquifer.

Introduction Approximately 15% of wells open to Quaternary glacial and shallow bedrock aquifers near Lake Geneva, Wisconsin, U.S.A. have As concentrations exceeding the U.S. Environmental Protection Agency standard of 10 μg/l. Because there is no evidence of anthropogenic sources of As in the region, the As is believed to be naturally occurring. However, prior to this study no geologic sources of As had been identified, and little was known about the hydrogeology and geochemistry of the As-impacted aquifers. © 2005 American Chemical Society

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

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Study Area Lake Geneva is located in southeastern Wisconsin (Figure 1). The surficial deposits in the study area consist of Quaternary glacial sediments that overlie Silurian dolomites (Figure 2). The glacial deposits are more than 150 m thick where glacial materials fill bedrock valleys (/). Beneath the Silurian dolomites are Ordovician and Cambrian sedimentary units and Precambrian basement. The principal aquifers in the study area are the Quaternary glacial, Silurian dolomite, and Cambrian-Ordovician sandstone aquifers (Figure 2). Stratified sand and gravel layers within the Quaternary aquifer are separated by lowpermeability fine grained tills, resulting in shallow and deep water-bearing zones within the aquifer. Regional groundwater flow is to the east. The Maquoketa Shale, a regional aquitard, separates the Silurian aquifer from the underlying Cambrian-Ordovician aquifer (/). The majority of wells in the study area obtain water from either the Quaternary or Silurian aquifers, and this study only addresses processes occurring in these units. There is significant spatial variation in As concentrations throughout the study area. Maximum As concentrations in water from wells open to the Silurian aquifer are around 100 μg/I, while maximum As concentrations in water from the deep Quaternary aquifer are around 50 μg/l. Water from wells open to the shallow Quaternary sediments has relatively low As concentrations.

Methods Two monitoring wells, MW-1 and M W - 2 , were completed approximately 10 m away from an existing As-impacted well, WS-3, which is open to the bottom of the Quaternary and top of the Silurian aquifers. Arsenic concentrations in groundwater samples taken from WS-3 ranged from 34 μg/l to 85 μ§/1 during a time span from 1996 to 2004. MW-1 was drilled using the rotosonic method, collecting 100 m of continuous, relatively undisturbed core through the Quaternary deposits and extending 8 m into the Silurian dolomite. MW-1 is screened at the base of the Quaternary deposits, above a clay-filled dissolution opening near the top of the bedrock. MW-2, drilled with air rotary drilling, is completed in the upper Quaternary deposits. Cross sections showing the stratigraphy of the study area are based on the core from MW-1 and existing well construction records. Approximately 5-cm thick slices were taken from the MW-1 core for wholerock geochemical analysis. At least one sample was taken from each distinct stratigraphie horizon. Within thick horizons, a sample was taken at least



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Figure 2. General geology and typical well construction in the study area. (I. deep sandstone well, 2. shallow Quaternary well, 3. deep Quaternary well, 4. shallow Silurian well.)



164 every 8 feet. The samples were sent to A L S Chemex (formerly Bondar-Clegg, Inc.) for pulverization, aqua regia digestion, and subsequent analysis by inductively coupled plasma-optical emission spectrometry (ICP-OES). Loss-oningnition (2), done by the University of Wisconsin Soil and Plant Analysis Laboratory (SPAL), was used to estimate the percent organic matter (OM) in core samples. Some groundwater chemistry data for the study area are available from previous investigations (J). Additional groundwater samples were obtained from residential supply wells. Temperature, pH, redox potential, conductivity, and dissolved oxygen measurements were made in the field. Arsenic species were separated in the field using As speciation cartridges from MetalSoft, Inc. (4). Samples for determination of major cations, dissolved metals, and dissolved As were filtered to 0.45 μπι and preserved with OPTIMA nitric acid. Filtered (0.45 μηι), non-preserved samples were collected for determination of major anions. Non-filtered, non-preserved samples were collected for dissolved organic carbon (DOC) analysis. The inflection point titration method was used to determine alkalinity (5). Other major anions were determined by ion chromatography at SPAL. Major cations, metals, and As were analyzed by ICP-OES. D O C concentrations were determined using a high-temperature combustion analyzer. Only samples with complete major ion analyses and charge imbalances less than 10% were included in the data analysis. Mineral saturation indices were calculated using The Geochemist's Workbench (6). A 19-hour pumping test provided information about the groundwater flow system in the study area. During this test, approximately 20,000 gallons (73 well volumes) of water were discharged from WS-3. Water chemistry was monitored in the pumped well, and water levels were monitored at MW-1 and MW-2.

Results Solid-phase Geochemistry Low As concentrations were found in samples of the yellowish brown and brown tills ( 20 ug/l

Figure 7. Saturation indices for primary minerals in groundwater samples from the study area.

Comparison of groundwater As concentrations with well construction records revealed a correlation between As and well completion depth. A l l lowAs wells are completed in the shallow sand and gravel above the dark grayish brown till (Figure 5F and well 2 in Figure 2). The moderate and high-As wells are all open beneath the dark grayish brown till in the deep Quaternary and shallow Silurian aquifers (Figure 5F and wells 3 and 4 in Figure 2).


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Discussion


Possible Arsenic Sources Arsenic-rich pyrite is one of the most common As-bearing minerals (7). Oxidation of As-rich sulfide minerals is one possible mechanism for releasing As to groundwater (8). Arsenic is also found in association with (hydr)oxide minerals, both as sorbed species and incorporated into the mineral structure. Reductive dissolution of (hydr)oxides is another possible mechanism of As mobilization (8). Arsenic may also sorb to clay minerals, calcite, and organic matter (8, 9). Desorption of As may occur via reductive mechanisms or competition with other species for sites on mineral surfaces (8). Based on the apparent negative correlation between As and S in the MW-1 core samples (Figure 3), solid-phase As is not associated with sulfide minerals. Additionally, the negative correlation between As and Eh (Figure 5C), and the fact that many of the high-As groundwater samples smell strongly of reduced S, indicate that sulfate reduction (rather than sulfide oxidation) occurs in the Asimpacted aquifers. Although limited in extent, zones of sulfide mineralization occur in both the Silurian dolomite and Maquoketa shale, which subcrop in the region. Therefore oxidation of sulfide minerals in the bedrock may occur upgradient from the study area. However, the negative correlation between As and S 0 ' (Figure 5A) in groundwater suggests that sulfide dissolution is not the cause of the observed aqueous As concentrations. The similarities in the profiles of As, Fe, and M n in the core (Figure 3) suggest that solid-phase As might be associated with (hydr)oxide minerals. The presence of (hydr)oxides has not been confirmed by X-ray diffraction (XRD); however, poorly crystalline (hydr)oxides present at a few weight percent are not detectable using X R D . The fact that As concentrations are higher in more strongly reducing waters (Figure 5C) also indicates that As is being mobilized via reductive dissolution of (hydr)oxides. Reduction of (hydr)oxides is often coupled to microbial oxidation of O M (12). Thus, the positive correlations between As and O M in the core (Figure 3) and groundwater (Figure 6) also support the reductive dissolution hypothesis. The Fe(OH) saturation indices (Figure 7) indicate that dissoluton of ferric hydroxide is thermodynamically favorable in high-As groundwaters. A positive correlation between As and H C 0 " has been reported for groundwater in Bangladesh and West Bengal, where As is thought to be mobilized by reductive dissolution of Fe-(hydr)oxides (12, 13). The H C 0 " is produced by the oxidation of O M . We do not observe a correlation between As and H C 0 " in groundwater samples from our study area (Figure 5B). It is not 2

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clear whether this lack of correlation indicates that elevated As concentrations are not linked to microbial respiration or i f weathering reactions involving H C 0 " overwhelm any microbial H C 0 " contribution linked to reductive dissolution. Mobilization of As via reductive dissolution of Fe-(hydr)oxides is expected to yield a positive correlation between aqueous As and Fe . However, under reducing conditions Fe * may be removed from solution via the formation of Fesulfide minerals or siderite. Thus, the lack of correlation between As and F e in the groundwater samples (Figure 5E) is not sufficient to rule out the reductive dissolution hypothesis. Most of the groundwater samples are saturated with respect to siderite (Figure 7). However, no siderite has been identified in X R D analysis of core samples. Because sediments in the study area have high clay and carbonate contents, these materials are possible As sources. Both clays and calcite typically contain much less weight percent As than (hydr)oxide minerals (8). However, the relatively high As concentration in the clay-filled dissolution opening in the MW-1 core (Figure 3) suggests that the clay may be an important source of As for some wells. Relatively high As concentrations are found in the organic-rich horizon in the MW-1 core (Figure 3), but this horizon is not likely a significant regional source of As. The organic material was likely deposited in small wetlands situated in topographic lows that are not laterally extensive. Additionally, the organic-rich horizon at MW-1 is less than 20 cm thick and is above the water table (Figure 3). 3

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Controls on Arsenic Concentrations The As-depth relationship (Figure 5F) indicates that geologic, hydrogeologic, and geochemical factors create conditions that lead to As mobilization in the deep Quaternary and shallow Silurian aquifers (wells 3 and 4 in Figure 2) but not in the shallow Quaternary aquifer (well 2 in Figure 2). The lower solid-phase As concentrations in the shallower parts of the system (Figure 3) may be at least partially responsible for the observed As-depth relationship. However, the groundwater chemistry data (Figures 4-7) indicate that geochemical conditions in the shallow part of the system are also different than those in the deeper parts of the system. The higher TDS, HC0 ", and S 0 " (Figure 5) in water from shallow wells are likely the result of unsaturated zone and/or shallow groundwater processes. is typically elevated in unsaturated zones and shallow groundwater systems leading to enhanced mineral weathering that results in increased concentrations of H C O 3 " and TDS. As noted above, these weathering reactions may overwhelm 2

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any H C 0 " signal resulting from microbial oxidation of O M . The S 0 ' is likely generated by oxidation of sulfide minerals near the water table, where dissolved oxygen concentrations are higher. Sulfide minerals were probably entrained in till as glaciers overrode mineralized zones in underlying bedrock units. Dissolution of gypsum is another documented source of S0 * in glacial aquifers (14), but we are unaware of previous studies identifying gypsum in glacial deposits in Wisconsin. The increasing (Na + K ) / ( C a + M g ) in moderate-As samples (Figure 4) is probably the result of N a and K on the surface of clays exchanging for C a and M g in solution. The dark grayish brown till and clays in the dissolution opening (Figure 3) are the most clay-rich units in the core and the most probable locations for cation exchange to occur. The consistent (Na + K ) / ( C a + M g ) ratio (Figure 4) among high-As samples indicates that these samples are not influenced by cation exchange. These samples may be influenced more by water-rock interactions in the Silurian dolomite rather than in the clay in the Quaternary aquifer. The reducing conditions in the deep Quaternary/shallow Silurian play a key role in promoting (hydr)oxide dissolution and As mobility. The reducing conditions are likely brought on by microbial oxidation of O M , which consumes electron acceptors. More oxidizing conditions may persist in the shallow Quaternary due to the lower O M concentrations in those aquifer sediments. The shallow parts of the aquifer are also likely to receive younger, more oxidizing recharge. In addition to contributing directly to the relatively oxidizing conditions, this supply of electron acceptors would enhance rates of O M consumption, and O M would be depleted more quickly in the shallow sediments than in the deeper sediments. Thus the distribution of O M in the aquifer sediments and the redox state of the groundwater may be greatly influenced by the groundwater flow regime. The lack of response in water level in the shallow Quaternary well (MW-2) during the pumping test indicates that there is little hydraulic connection between the shallow Quaternary aquifer (low-As) and the deeper Quaternary and shallow Silurian (high-As) aquifers. The groundwater chemistry data lend further credence to the hypothesis that shallow, low-As wells are on a different groundwater flow path than deeper, As-impacted wells. Vertical groundwater gradients are downward; yet, shallower, low-As wells have higher TDS than deeper, high-As wells (Figure 5D). Thus, the relatively high TDS in the low-As wells is more likely the result of water-rock interaction within the shallow flow system than water-rock interaction along a flow path between the deep and shallow systems. These observations suggest that the dark grayish brown till is an aquitard separating the upper, non-As-impacted system from the deeper, Asimpacted system. If the upper system is recharged locally with relatively young and oxygenated water and the lower system lies along a regional flow path further from the recharge area, the hydrogeology of the study area may 3

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Conclusions Depth profiles of As, Fe, Mn, and O M concentrations in core collected from the study area show similar patterns, and As appears more mobile at lower elevations where conditions are relatively reducing and S 0 " concentrations are negligible. These observations support the hypothesis that As is associated with (hydr)oxide minerals and is mobilized via reductive dissolution of those (hydr)oxides. Geologic, hydrogeologic, and geochemical factors create conditions where As is mobile in the deep Quaternary and upper Silurian aquifers but not in the shallow Quaternary aquifer. Given our present understanding of the problem, the only viable mitigation measure for existing As-impacted wells is treatment to remove As from the water. New wells should be completed in the shallow flow system, if possible. 2


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Acknowledgements This work has been supported by the U.S. Geological Survey, the University of Wisconsin System, the Wisconsin Department of Natural Resources, the Wisconsin Geological and Natural History Survey, the U.S. Department of Energy, and the Geological Society of America. We would also like to thank the staff at Woods Elementary School in Lake Geneva, Katie Thornburg, and Nita Sahai.

References 1.

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Borman, R.G. Ground-Water Resources and Geology of Walworth County, Wisconsin; Wisconsin Geological and Natural History Survey Information Circular 34: Madison, WI, 1976. Ben-Dor, E.; Banin, A . Commun. in Soil Sci. Plant Anal., 1989, 20, 16751695. Gotkowitz, M. Report on the Preliminary Investigation of Arsenic in Groundwater near Lake Geneva, Wisconsin; Wisconsin Geological and Natural History Survey Open File Report 2000-02: Madison, WI, 2000. Meng, X.; Wang, W. Speciation of Arsenic By Disposable Cartridges. In Third International Conference on Arsenic Exposure and Health Effects: Society of Environmental Geochemistry and Health, University of Colorado at Denver: Denver, CO, 1998.


174 5.

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7.

Wilde, F.D.; Radtke, D.B. Alkalinity and Acid Neutralizing Capacity. In National Field Manual for the Collection of Water-Quality Data; Techniques of Water-Resources Investigations, Book 9, Chap. A6.6; U.S.G.S: Accessed on-line at http://pubs.water.usgs.gov/twri9Al (10/4/03). Bethke, C. M. The Geochemist's Workbench Release 3.0, A Users' Guide To Rxn, Act2, Tact, React, And Gtplot; Craig Bethke: Urbana-Champaign, IL, 1998. Nordstrom, D. K . Overview O f Arsenic Occurrences And Processes Controlling Arsenic Mobility In Groundwater. In Institute on Lake Superior Geology Proceedings, 47 Annual Meeting: Madison, WI, 2001, 47-1, 73. Smedley, P.L.; Kinniburgh, D.G. Appl. Geochem. 2 0 0 2 , 17, 517-568. Stollenwerk, K.G. Geochemical Processes Controlling Transport of Arsenic in Groundwater: A Review of Adsorption. In Arsenic in Groundwater, Geochemistry and Occurrence; Welch, A.H.; Stollenwerk, K.G.; Ed.; Kluwer Academic Publishers: Boston, M A , 2003, pp 67-100. Attig, J. Personal Communication; Wisconsin Geological and Natural History Survey: Madison, WI. Eaton, T.T.; Bradbury, K . R . Hydraulic Properties and Porewater Geochemistry in the Maquoketa Shale, Southeastern Wisconsin. In Geological Society of America Annual Meeting Abstracts with Programs: 1998. Nickson, R. T.; McArthur, J. M.; Ravenscroft, P.; Burgess, W. G.; Ahmed, Appl. Geochem. 2 0 0 0 , 15, 406-413. Zheng, Y.; Stute, M.; van Geen, Α.; Gavrieli, I.; Dhar, R.; Simpson, H.J.; Schlosser, P.; Ahmed, K.M. Appl. Geochem. 2 0 0 4 , 19, 210-214. Swanson, K.D. Chemical Evolution of Groundwater in Clay Till in a Prairie Wetlands Setting in the Cottonwood Lake Area Stutsman County, North Dakota. M.S. Thesis, University of Wisconsin, Madison, WI, 1990. th


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Controls on Arsenic Concentrations in Groundwater near Lake

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