Correlation of δ15N and δ18O in NO3- during denitrification in groundwater

Stable isotopes of nitrogen and oxygen are important tools for investigating the occurrence of denitrification in groundwater. Previous investigators have noted a linear relationship between delta N-15 and delta O-18 field data collected from shallow groundwater systems, but the reason for such a linear correlation is not clear. In this study, we represent denitrification reactions using parallel, first-order kinetics, and reaction rate constants that depend on the nitrogen or oxygen isotope present in nitrate. The derived mathematical relationship between delta N-15 and delta O-18 indicates that nitrogen and oxygen isotopes should fractionate at a constant ratio. Using the laboratory reaction rate constants obtained by Olleros (1983), our equations give a theoretical fractionation ratio value of b = 0.51. A review of published field studies in which denitrification in groundwater has been confirmed, and delta N-15 and delta O-18 data has been collected, shows that the mean fractionation ratio value is 0.55 (standard deviation = 0.08, n = 6). The field values therefore agree well with the predicted fractionation ratio value of b = 0.51. It is concluded that a linear relationship between delta N-15 and delta O-18 values, with a slope (b) close to 0.51, provides additional, unambiguous evidence that denitrification is responsible for nitrate concentration decline and enrichment in nitrogen isotope values in shallow groundwater.