Aquifer Denitrification as Interpreted from In Situ Microcosm Experiments

Abstract

AbstractDenitrification of 40 mg L−1 NO3‐N groundwater in the vicinity of Central City, NE was stimulated with ethanol in in situ microcosms that were vibrated into the aquifer sediment and, thus, filled with a relatively undisturbed, saturated sand and gravel matrix. In microcosms receiving the 1.25 C/N ratio, nitrate disappeared within 40 h; however, NO2‐N accumulated and persisted after NO3‐N was depleted. Nitrite has been documented in groundwater redoxclines and is expected to be reported more frequently with increased use of ion chromatography and dense sampling networks. Dissolved oxygen was consumed to <2.0 mg L−1 within 15 h, and stoichiometric production of HCO−3 occurred. Average denitrification rate decreased from 28.5 to 19.3 and to 16 mg N L−1 d−1 for C/N ratios of 1.25, 2.5, and 5.0 mg L−1. Kinetic N‐isotope effects resulted in the δ15N enrichment of residual nitrate as denitrification progressed, thus, causing the initial δ15N value of ∼+6‰ to increase to > +20‰. The data support measuring additional parameters to prevent misinterpretations when using δ15N values in NO−3 source identification investigations. Apparent isotope‐enrichment factors, ε value, were derived from δ15N increases vs. NO−3 and NO3 plus NO−2 reduction. They ranged from −11 to −16‰ for δ15N vs. residual NO−3 plus NO−2 and from −2.5 to −9‰ for δ15N vs. NO−3 alone. Since NO−3 could not be resolved from NO−2 in the δ15N analysis, a δ15N depleted NO2 phase was not identified. Nevertheless, isotopically light enrichment factors are consistent with the presence of NO−2.