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Circular 570 2007

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January 2007

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K.C. Hackley, S.V. Panno, H.-H. Hwang, and W.R. Kelly Circular 570 2007 Illinois Department of Natural Resources ILLINOIS STATE GEOLOGICAL SURVEY William W. Shilts, Chief 615 E. Peabody Drive Champaign, Illinois 61820-6964 217-333-4747 www.isgs.uiuc.edu Abstract About half the residents living in the area of southwestern Illinois known as the sinkhole plain obtain their potable water from the region's shallow karst aquifer. Previous work has shown that the groundwater from approximately 18% of the wells in the sinkhole plain has nitrate concentrations in excess of the U.S. Environmental Protection Agency's drinking water standard of 10 mg of N/L of water. The nitrate concentrations in water samples collected from approximately 50% of the wells and from all of the springs in the sinkhole plain area are greater than background concentrations, suggesting that sources other than naturally occurring organic matter in soil have contributed additional nitrate to groundwater in the shallow karst aquifer. This investigation characterized the geochemistry of the groundwater to determine which source of nitrogen in the sinkhole plain is the major contributor to the anomalous concentration of nitrate observed in the shallow karst aquifer. Considering the dominance of agriculture and the expansion of urban development in the study area, sources of excessive nitrate and groundwater contamination include agrichemical, livestock, and sewage waste. Water samples from 10 karst springs and 17 wells were collected during different seasons and analyzed for chemical, isotopic, and bacterial characteristics. The samples from each spring were a representative composite of the shallow water recharging the associated watershed. Samples from the wells reflect individual points within the watersheds and were more susceptible to influ- ence from local environments, including anthropogenic activities. Chemical characteristics and the isotopic composition of some of the dissolved constituents varied seasonally in the samples of spring water, attesting to the rapid infiltration of surface and soil water into the karst aquifer. Bacteria concentrations in the springs and most of the wells were greater than those allowed by county and state regulations for drinking water. Nitrate concentrations in the springs covered a fairly narrow range, from 1.7 to 7.5 mg of N/L. In the wells, nitrate concentrations varied greatly, ranging from less than the detection limit (0.2 mg of N/L) to 81 mg of N/L. The isotopic data for the dissolved nitrate (NO 3 -) from the springs and wells were useful in distinguishing NO 3​ -sources. The nitrogen and oxygen isotope composition of the NO 3​ -ranged from 2.2 to 25.9 per mil (%) and 5.1 and 21.9% respectively. These isotopic results suggest that the nitrate sources in spring water were dominated by fertilizer nitrogen and soil organic nitrogen that mixed with nitrate having en enriched 18O signature. The isotopic results for the wells indicate that the largest NO 3 -concentrations (between 13 and 80 mg of N/L) originated primarily from septic and livestock wastes. The isotopic results for most of the wells with NO 3 -concentrations between 2 and 12 mg of N/L indicate that nitrogen fertilizer was the dominant NO 3 -source. The combined chemical, bacterial, and isotopic analyses of springs and individual wells provided independent evidence concerning the major susceptibility of the karst aquifer to surface contamination and helped to differentiate the sources of NO 3 -in the groundwater. Although many livestock facilities and septic systems were present in individual watersheds, the typical isotopic character- istics of NO - originating from such point sources were overwhelmed by the constant input of nonpoint source nitrogen for the composite samples of spring water. However, results from groundwater samples from several residential wells did show the impact of point sources in NO 3 -contamination on a local scale. Open Access See Extended description for more information.

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K26-01516

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