AQUIFER DENITRIFICATION AS INTERPRETED FROM IN-SITU MICROCOSM EXPERIMENTS

Denitrification of 40 mg L-1 NO3-N groundwater in the vicinity of Cent ral City, NE was stimulated with ethanol in in situ microcosms that we re vibrated into the aquifer sediment and, thus, filled with a relativ ely undisturbed, saturated sand and gravel matrix, In microcosms recei ving the 1.25 C/N ratio, nitrate disappeared within 40 h; however, NO2 -N accumulated and persisted after NO3-N was depleted, Nitrite has bee n documented in groundwater redoxclines and is expected to be reported more frequently with increased use of ion chromatography and dense sa mpling networks. Dissolved oxygen was consumed to <2.0 mg L-1 within 1 5 h, and stoichiometric production of HCO3- occurred, Average denitrif ication 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 res ulted in the delta(15)N enrichment of residual nitrate as denitrificat ion progressed, thus, causing the initial delta(15)N value of similar to+6 parts per thousand to increase to >+20 parts per thousand. The da ta support measuring additional parameters to prevent misinterpretatio ns when using delta(15)N values in NO3- source identification investig ations, Apparent isotope-enrichment factors, epsilon value, were deriv ed from delta(15)N increases vs. NO3- and NO3- plus NO2- reduction, Th ey ranged from -11 to -16 parts per thousand for delta(15)N vs. residu al 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 de lta(15)N analysis, a delta(15)N depleted NO2- phase was not identified , Nevertheless, isotopically light enrichment factors are consistent w ith the presence of NO2-.