Aquifer denitrification as interpreted from in situ microcosm experiments

Denitrification 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 HCO3- 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 delta(15)N enrichment of residual nitrate as denitrification progressed, thus, causing the initial delta(15)N value of similar to+6 parts per thousand to increase to >+20 parts per thousand. The data support measuring additional parameters to prevent misinterpretations when using delta(15)N values in NO3- source identification investigations, Apparent isotope-enrichment factors, epsilon value, were derived from delta(15)N increases vs. NO3- and NO3- plus NO2- reduction, They ranged from -11 to -16 parts per thousand for delta(15)N vs. residual NO3- plus NO2- and from -2.5 to -9 parts per thousand for delta(15)N vs. NO3- alone. Since NO3- could not be resolved from NO2- in the delta(15)N analysis, a delta(15)N depleted NO2- phase was not identified, Nevertheless, isotopically light enrichment factors are consistent with the presence of NO2-.