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

Abstract

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 δ15N and δ18O 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 δ15N and δ18O 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 δ15N and δ18O 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 δ15N and δ18O 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. Key words: nitrate, groundwater, denitrification, isotopes, enrichment factor, fractionation ratio.