DETERMINATION OF THE SOURCES OF NITRATE CONTAMINATION 
IN KARST SPRINGS USING ISOTOPIC AND CHEMICAL INDICATORS

Samuel V. Panno, Keith C. Hackley, Hue Hwa Hwang, and Walton R. Kelly

Illinois State Geological Survey and Illinois State Water Survey
 

Abstract

The sources of NO₃^- in groundwater of the shallow karst aquifer in southwestern Illinois' sinkhole plain were examined using chemical and isotopic techniques. The groundwater in this aquifer is an important source of potable water for about half of the residents of the sinkhole plain area. Groundwater from many of the wells in the sinkhole plain exceeds the U.S. Environmental Protection Agency's drinking-water standard of 10 mg-N/L. Relative to background levels, the NO₃^- concentration in water from many wells and probably all springs in the study area is anomalously high, suggesting that sources other than naturally occurring soil organic matter have contributed additional NO₃^- to groundwater in the shallow karst aquifer. The dominance of agriculture in the study area suggested that agrichemical contributions may be significant. To test this hypothesis, water samples from 10 relatively large karst springs were collected during three different seasons and analyzed for inorganic constituents, and d¹⁵N and  d¹⁸O of the NO₃^- ions. The isotopic data were most definitive and suggested that the sources of NO₃^- in spring water are dominated by N-fertilizer and, to a lesser extent, animal waste. Differences in the isotopic composition of NO₃^- were observed during the three consecutive seasons in which spring water samples were collected. Isotopic values for  d¹⁵N and  d¹⁸O of the NO₃^- ranged from 3.2 to 19.1^o/_oo and 7.2 to 18.7^o/_oo, respectively. The trend of  d¹⁵N and  d¹⁸O data for NO₃^- also indicated that there is some denitrification occurring in the shallow karst hydrologic system (within the soil and karst aquifer) prior to discharging at springs.