APPLICATION OF NITROGEN AND OXYGEN ISOTOPES TO IDENTIFY DENITRIFICATION IN A SHALLOW AQUIFER WITH A VARIABLE INFLUX OF NITRATE

The application of the nitrate-oxygen and nitrate-nitrogen isotope ratios is a useful technique to help identify sources and fate of nitrate found in aquifers with variable nitrate inputs. An initial step in using this technique 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 d¹⁸O values versus increasing d¹⁵N values with a slope of 0.5, indicating the enrichment for d¹⁵N relative to d¹⁸O 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 d¹⁵N and d¹⁸O 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. Each sampling event represents only a snapshot of a dynamic system which involves variable inputs of nitrate over time and changes in water level and recharge to the aquifer. 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. An increase in sulfate and decrease in d³⁴S values with depth suggests that oxidation of sulfide minerals such as pyrite probably plays an important roll in the reduction of nitrate in this aquifer.