Nitrate-consuming processes in a petroleum-contaminated aquifer quantified using push–pull tests combined with 15N isotope and acetylene-inhibition methods

Nitrate consumption in aquifers may result from several biogenic and abiotic processes such as denitrification, assimilatory NO3− reduction, dissimilatory NO3− reduction to ammonium (DNRA), or abiotic NO3− (or NO2−) reduction. The objectives of this study were to investigate the fate of NO3− in a petroleum-contaminated aquifer, and to assess the feasibility of using single-well push–pull tests (PPTs) in combination with 15N isotope and C2H2 inhibition methods for the quantification of processes contributing to NO3− consumption. Three consecutive PPTs were performed in a monitoring well of a heating oil-contaminated aquifer in Erlen, Switzerland. For each test, we injected 500 l of test solution containing 0.5 mM Br− as conservative tracer and either 0.5 mM unlabeled NO3− or ∼0.3 mM 15N-labeled NO3− as reactant. Test solutions were sparged during preparation and injection with either N2, Ar or 10% C2H2 in Ar. After an initial incubation period of 1.5–3.2 h, we extracted the test solution/groundwater mixtures from the same location and measured concentrations of relevant species including Br−, NO3−, NO2−, N2O, N2, and NH4+. In addition, we determined the 15N contents of N2, N2O, NH4+, and suspended biomass from 15N/14N isotope-ratio measurements. Average total test duration was 50.5 h. First-order rate coefficients (k) were computed from measured NO3− consumption, N2–15N production and N2O–15N production. From measured NO3− consumption we obtained nearly identical estimates of k for all PPTs with small 95% confidence intervals, indicating good reproducibility and accuracy for the tests. Estimates of k from N2–15N production and N2O–15N production indicated that denitrification accounted for only 46–49% of observed NO3− consumption. Production of N2–15N in the presence of C2H2 was observed during one of the tests, which may be an indicator for abiotic NO3− reduction. Moreover, 15N isotope analyses confirmed occurrence of assimilatory NO3− reduction (0.58 at.% 15N in suspended biomass) and to a smaller extent DNRA (up to 4 at.% 15N in NH4+). Our results indicated that the combination of PPTs, 15N-isotope and C2H2 inhibition methods provided improved information on denitrification as well as alternative fates of NO3− in this aquifer. [Copyright &y& Elsevier]