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 in southwestern Illinoisʹ sinkhole plain were investigated 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. Previous work has shown that groundwater from approximately 18% of the wells in the sinkhole plain has NO3− concentrations in excess of the USEPAʹs drinking water standard of 10 mg N/l. 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 other than naturally occurring soil organic matter have contributed additional NO3− to groundwater in the shallow karst aquifer. This information, and the dominance of agriculture in the study area, suggest that agrichemical contributions may be significant. To test this hypothesis, water samples from 10 relatively large karst springs were collected during four different seasons and analyzed for inorganic constituents, dissolved organic carbon, atrazine, and δ15N and δ18O 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 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 characteristics were observed during the four consecutive seasons in which spring water samples were collected. Isotopic values for δ15N and δ18O of the NO3− ranged from 3.2‰ to 19.1‰ and from 7.2‰ to 18.7‰, respectively. The trend of δ15N and δ18O data for NO3− also indicated that a significant degree of denitrification is occurring in the shallow karst hydrologic system (within the soil zone, the epikarst and the shallow karst aquifer) prior to discharging to springs.