Kinetic Isotope Effects in Production of Nitrite‐Nitrogen and Dinitrogen Gas during In Situ Denitrification

In multistep redox reactions, stable isotopes of the product species can provide insight into the complexities of the reaction. A method was developed that chromatographically separated nitrite (NO−2) from nitrate (NO−3) in quantities that permitted isotopic analysis both N species. Only a 3% peak‐to‐peak crossover contamination was measured in the column eluates during chromatographic separation using 15N‐enriched NO−2. During in situ microcosm‐amended denitrification, low δ15N values of NO−2 ranging from −2 to 4‰ verified that NO−2 produced early persisted throughout much of the reaction. Significant isotopic depletion of the residual NO−3 plus NO−2 fraction resulted. This depletion effect was reflected in lower apparent isotopic‐enrichment factors (flatter slopes) in plots of δ15N vs. In [NO−3 + NO−2] than those vs. In [NO−3] alone. Dinitrogen concentrations produced during the reaction were determined by measuring δAr/N2 values and showed that significant N2 losses occurred in the latter part of the reaction due to supersaturated conditions. The δ15N values for the N2 produced during denitrification ranged from −5.2‰ to higher values of +17‰ when the reaction was ∼70% complete. A slope reversal in δ15N of produced N2 followed when NO−2 reduction became more dominant, as isotope dilution of product N2 occurred. At the culmination of the reaction, the δ15N value of N2 approached but did not return to the original δ15N value of the NO−3. The results caution the blind use of Ar/N2 ratios in groundwaters that initially contain high NO−3 levels and emphasize the importance of NO−2 in environmental sampling.