Application of Nitrogen and Oxygen Isotopes to Identify Sources of Nitrate, 2001

The goal of this project is to establish the effectiveness and applicability of the nitrate-oxygen isotope ratio as a useful technique to help identify sources and fate of nitrate found in the aquifers of Illinois. The initial focus of the project is to characterize the isotopic shift that occurs during denitrification. Samples were collected from multilevel wells in a surficial sand aquifer at a study site in Mason County, Illinois, where the denitrification of a nitrate plume was known to occur. In one of the wells, the nitrate concentration dropped from over 13 mg/L (as N) to less than 1 mg/L across a thin zone at a depth between 38 feet and 44.5 feet. The samples had isotopic values that followed a linear relationship of increasing 18O values versus increasing 15N values with a slope of 0.5, indicating the enrichment for 15N relative to 18O is equal to 2 during denitrification. The nitrate samples from the upgradient well nearest the corn fields adjacent to the study site had isotopic values which are typically observed for nitrate originating from nitrogen fertilizers. The 15N and 18O data of the dissolved nitrate in the samples indicate that there was denitrification occurring along the flow path, even though the concentration of nitrate was higher at the downgradient well. This discrepancy is due to variable inputs of nitrate over time, changes in water level and recharge to the aquifer, and observing only a snapshot of a dynamic chemical system. Using the Rayleigh equation and the calculated fractionation factors, it was determined that the high nitrate portion of the plume had already undergone 40% denitrification before it reached the distinct denitrification zone between 38 feet and 44.5 feet. A large increase in sulfate below 38 feet may indicate that the dissolved oxygen and the nitrate are oxidizing sulfide minerals such as pyrite in the aquifer.Ope