Determination of the sources of nitrate contamination in karst springs using isotopic and chemical indicators

The sources of nitrate (NO3-) in groundwater of the shallow karst aquifer i n southwestern Illinois' sinkhole plain were investigated using chemical an d isotopic techniques. The groundwater in this aquifer is an important sour ce of potable water for about half of the residents of the sinkhole plain a rea. Previous work has shown that groundwater from approximately 18% of the wells in the sinkhole plain has NO3- concentrations in excess of the USEPA 's drinking water standard of 10 mg N/1 Relative to background levels, the NO3- concentrations in water from 52% of the wells, and probably all of the springs in the study area, are anomalously high, suggesting that sources o ther than naturally occurring soil organic matter have contributed addition al NO3- to groundwater in the shallow karst aquifer. This information, and the dominance of agriculture in the study area, suggest that agrichemical c ontributions may be significant. To test this hypothesis, water samples fro m 10 relatively large karst springs were collected during four different se asons and analyzed for inorganic constituents, dissolved organic carbon, at razine, and delta (15) N and delta O-18 of the NO3- ions. The isotopic data were most definitive and suggested that the sources of NO3- in spring wate r are dominated by N-fertilizer with some possible influence of atmospheric NO3- and, to a much lesser extent, human and/or animal waste. Differences in the isotopic composition of NO3- and some of the chemical characteristic s were observed during the four consecutive seasons in which spring water s amples were collected. Isotopic values for delta N-15 and delta O-18 of the NO3- ranged from 3.2%o to 19.1%o and from 7.2%o to 18.7%o respectively. Th e trend of delta N-15 and delta O-18 data for NO3- also indicated that a si gnificant degree of denitrification is occurring in the shallow karst hydro logic system (within the soil zone, the epikarst and the shallow karst aqui fer) prior to discharging to springs. (C) 2001 Elsevier Science B.V. All ri ghts reserved.