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Aquifer Denitrification as Interpreted from In Situ Microcosm Experiments

Water Sciences Laboratory, University of Nebraska, Lincoln, NE 68583-0844.

^* Corresponding author (rspaldin{at}unlinfo.unl.edu).

ABSTRACT

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 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 ¹⁵N enrichment of residual nitrate as denitrification progressed, thus, causing the initial ¹⁵N value of +6 to increase to > +20. The data support measuring additional parameters to prevent misinterpretations when using ¹⁵N values in NO^–₃ source identification investigations. Apparent isotope-enrichment factors, value, were derived from ¹⁵N increases vs. NO^–₃ and NO₃ plus NO^–₂ reduction. They ranged from –11 to –16 for ¹⁵N vs. residual NO^–₃ plus NO^–₂ and from –2.5 to –9 for ¹⁵N vs. NO^–₃ alone. Since NO^–₃ could not be resolved from NO^–₂ in the ¹⁵N analysis, a ¹⁵N depleted NO₂ phase was not identified. Nevertheless, isotopically light enrichment factors are consistent with the presence of NO^–₂.

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