Application of Nitrogen and Oxygen Isotopes to Identify Sources of Nitrate, 2002

The goal of this project was to establish the effectiveness and applicability of using nitrogen and oxygen isotope analyses on dissolved nitrate in groundwater as a useful technique to help identify sources and fate of nitrate found in the aquifers of Illinois. The initial focus of the project was to characterize the isotopic shift that occurs during denitrification. Three sites were investigated. The first and primary site was located in rural Mason County, Illinois at a research tree nursery facility, where the surrounding land use is completely agricultural. The second site is located near the banks of the Illinois River in Marshall County, Illinois, where there is the possibility of multiple sources of nitrate. The third site was in Shelby County, Illinois near the Kaskaskia River where a municipal well field contains elevated nitrate levels and there has not been any previous investigation as to the source. Groundwater samples from all three sites had isotopic values that followed a linear relationship typical of denitrification processes. The results for the Mason and Shelby County sites showed isotopic values for the nitrate that were characteristic of primarily originating from nitrogenfertilizer applications and the oxidation of soil organic matter. At the Mason site there is a redoxcline at a depth of 40 feet which suggests there may be significant amount of denitrification occurring. However, the isotopic composition and chloride concentrations indicate that very little denitrification is actually occurring across the redoxcline. The Marshall County site had some isotope values that suggested there may be some contribution of nitrate from septic systems, however, much of the nitrate appeared to be from up gradient agricultural sources.Ope