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 c an provide insight into the complexities of the reaction. A method was developed that chromatographically separated nitrite (NO2-) from nitr ate (NO3-) in quantities that permitted isotopic analysis of both N sp ecies. Only a 3% peak-to-peak crossover contamination was measured in the column eluates during chromatographic separation using N-15-enrich ed NO2-. During in situ microcosm-amended denitrification, low delta(1 5)N values of NO2- ranging from -2 to 4 parts per thousand verified th at NO2- produced early persisted throughout much of the reaction. Sign ificant isotopic depletion of the residual NO3- plus NO2- fraction res ulted. This depletion effect was reflected in lower apparent isotopic- enrichment factors (flatter slopes) in plots of delta(15)N vs. In [NO3 - + NO2-] than those vs. In [NO3-] alone. Dinitrogen concentrations pr oduced during the reaction were determined by measuring delta Ar/N-2 v alues and showed that significant N-2 losses occurred in the latter pa rt of the reaction due to supersaturated conditions. The delta(15)N va lues for the N-2 produced during denitrification ranged from -5.2 part s per thousand to higher values of +17 parts per thousand when the rea ction was similar to 70% complete. A slope reversal in delta(15)N of p roduced N-2 followed when NO2- reduction became more dominant, as isot ope dilution of product N-2 occurred. At the culmination of the reacti on, the delta(15)N value of N-2 approached but did not return to the o riginal delta(15)N value of the NO3-. The results caution the blind us e of Ar/N-2 ratios in groundwaters that initially contain high NO3- le vels and emphasize the importance of NO2- in environmental sampling.