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

2003; Elsevier BV; Volume: 66; Issue: 1-2 Linguagem: Inglês

10.1016/s0169-7722(03)00007-x

1873-6009

A. Schürmann, Martin H. Schroth, Matthias Saurer, Stefano M. Bernasconi, Josef Zeyer,

Atmospheric and Environmental Gas Dynamics

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.