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Groundwater denitrification in ethanol amended in situ microcosms
Abstract
Shallow ground water denitrification was investigated in stainless steel in situ microcosms that were sited in nitrate (NO$\sb3\sp-)$ contaminated sand and gravel aquifer in Merrick County, Nebraska. Total denitrification, estimated by the combined reduction of NO$\sb3$-N plus NO$\sb2$-N, decreased from 26 to 14 mg N L$\rm\sp{-1}\ d\sp{-1}$ with increasing C in the 50, 100, and 200 mg C L$\sp{-1}$amendments, respectively. The kinetics of $\sp{15}$N enrichment of the unreacted NO$\sb3\sp-$ during denitrification were described by the slope, $\varepsilon$, from the plot of $\delta\sp{15}$N versus the natural logarithm of the molar concentrations of NO$\sb3\sp-$ or NO$\sb3\sp-$ plus NO$\sb2$-N. When only NO$\sb3\sp-$ concentration was plotted against the $\delta\sp{15}$N of NO$\sb3\sp-$ plus NO$\sb2\sp-$, the isotopic enrichment ($\varepsilon$) values averaged $-6.5\perthous$ and $-2.6\perthous$ for the 50 and 100 mg C L$\sp{-1}$ amendments, respectively; whereas, combining the NO$\sb3\sp-$ plus NO$\sb2\sp-$ concentrations resulted in respective increased enrichments of $-14\perthous$ and $-12\perthous$. A liquid chromatographic technique was used to separate NO$\sb3\sp-$ and NO$\sb2\sp-$ to analyze the respective $\delta\sp{15}$N values. The enrichment factors for the plot of $\delta\sp{15}$N of NO$\sb3\sp-$ against the ln (mM NO$\sb3\sp-)$ were more negative than the plot of $\delta\sp{15}$N of NO$\sb3\sp-$ against the ln (mM NO$\rm\sb3\sp- + NO\sb2\sp-).$ Denitrification was also followed by changes in the $\rm\delta Ar/N\sb2$ ratio and in the $\delta\sp{15}$N of the N$\sb2$ gas. Adjusted $\rm\delta Ar/N\sb2$ values of the background samples were initially $\sim{1200}\perthous$ and rapidly declined to near zero as denitrification proceeded. Initially the $\delta\sp{15}$N of the total dissolved nitrogen, $\rm\delta\sp{15}N\sb2,$ reflected air-saturated-water (ASW) levels of about 0.0$\perthous$. With the advent of N$\sb2$ production the $\rm\delta\sp{15}N\sb2$ declined to negative values in association with greater proportions of isotopically lighter nitrate being reduced. Values of the $\rm\delta\sp{15}N\sb{Nex}$ were estimated from knowing the ASW concentrations and the $\delta\sp{15}$N of ASW N$\sb2$. As the reaction progressed, the $\rm\delta\sp{15}N\sb{Nex},\ N\sb2$ produced during denitrification, became more enriched. These microcosm results suggested that denitrification followed a multistep reaction instead of the single step reaction described by Rayleigh distillation theory.
Subject Area
Soil sciences
Recommended Citation
Bates, Herbert Kim, "Groundwater denitrification in ethanol amended in situ microcosms" (1996). ETD collection for University of Nebraska-Lincoln. AAI9637060.
https://digitalcommons.unl.edu/dissertations/AAI9637060