Estimating Aquifer Sensitivity to Nitrate Contamination Using Geochemical Information |
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| Authors: | Michael D Trojan Moira E Campion Jennifers Maloney James M Stockinger Erin P Eid |
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| Institution: | Michael D. Trojan;is a hydrologist with the Minnesota Pollution Control Agency (520 Lafayette Rd. N., St. Paul, MN 55155–4194; 297–5219;fax 297–7709;). Since receiving an M.S. in hydrology and Ph.D. in soil physics from the University of Minnesota, he has worked for the State of Minnesota in ground water mapping, regulatory, and monitoring programs. Moira Campion;is a hydrologist with the Minnesota Department of Natural Resources (500 Lafayette Road N., St. Paul, MN55155), working in ground water geophysics and mapping programs. She has a B.S. in geology from the University of Minnesota (Diduth) and an M.S. in geology from the University of North Dakota. Jennifer S. Maloney;is a hydrologist with the Minnesota Pollution Control Agency's Ground Water Monitoring and Assessment Program (520 Lafayette Rd., St. Paul, MN 55155–4194). She receiv a B.S. in geology from Macalester College. James M. Stockinger;is a hydrologist with the Minnesota Pollution Control Agency's Ground Water Monitoring and Assessment Program (520 Lafayette Rd., St. Paul, MN 55155–4194). He has a B.S. in conservation from the University of Wisconsin (River Falls). Erin P. EM;has a B.S. in geology from the University of Wisconsin (River Falls). |
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| Abstract: | Methods for predicting aquifer sensitivity to contamination typically ignore geochemical factors that affect the occurrence of contaminants such as nitrate. Use of geochemical information offers a simple and accurate method for estimating aquifer sensitivity to nitrate contamination. We developed a classification method in which nitrate-sensitive aquifers have dissolved oxygen concentrations > 1.0 mg/L, Eh values >250 mV, and either reduced iron concentrations < 0.1 mg/L or total iron concentrations < 0.7 mg/L. We tested the method in four Minnesota aquifer systems having different geochemical and hydrologic conditions. A surficial sand aquifer in central Minnesota exhibited geochemical zonation, with a rapid shift from aerobic to anaerobic conditions 5 m below the water table. A fractured bedrock aquifer in east-central Minnesota remained aerobic to depths of 50 m, except in areas where anaerobic ground water discharged upward from an underlying aquifer. A bedrock aquifer in southeast Minnesota exhibited aerobic conditions when overlain by surficial deposits lacking shale, whereas anaerobic conditions occurred under deposits that contained shale. Surficial sand aquifers in northwest Minnesota contained high concentrations of sulfate and were anaerobic throughout their extent. Nitrate-nitrogen was detected at concentrations exceeding 1 mg/L in 135 of 149 samples classified as sensitive. Nitrate was not detected in any of the 109 samples classified as not sensitive. We observed differences between our estimates of sensitivity and existing sensitivity maps, which are based on methods that do not consider aquifer geochemistry. Because dissolved oxygen, reduced iron, and Eh are readily measured in the field, use of geochemistry provides a quick and accurate way of assessing aquifer sensitivity to nitrate contamination. |
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