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

Abstract

Nitrate consumption in aquifers may result from several biogenic and abiotic processes such as denitrification, assimilatory NO 3 − reduction, dissimilatory NO 3 − reduction to ammonium (DNRA), or abiotic NO 3 − (or NO 2 −) reduction. The objectives of this study were to investigate the fate of NO 3 − in a petroleum-contaminated aquifer, and to assess the feasibility of using single-well push–pull tests (PPTs) in combination with 15N isotope and C 2H 2 inhibition methods for the quantification of processes contributing to NO 3 − 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 NO 3 − or ∼0.3 mM 15N-labeled NO 3 − as reactant. Test solutions were sparged during preparation and injection with either N 2, Ar or 10% C 2H 2 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 −, NO 3 −, NO 2 −, N 2O, N 2, and NH 4 +. In addition, we determined the 15N contents of N 2, N 2O, NH 4 +, 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 NO 3 − consumption, N 2– 15N production and N 2O– 15N production. From measured NO 3 − 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 N 2– 15N production and N 2O– 15N production indicated that denitrification accounted for only 46–49% of observed NO 3 − consumption. Production of N 2– 15N in the presence of C 2H 2 was observed during one of the tests, which may be an indicator for abiotic NO 3 − reduction. Moreover, 15N isotope analyses confirmed occurrence of assimilatory NO 3 − reduction (0.58 at.% 15N in suspended biomass) and to a smaller extent DNRA (up to 4 at.% 15N in NH 4 +). Our results indicated that the combination of PPTs, 15N-isotope and C 2H 2 inhibition methods provided improved information on denitrification as well as alternative fates of NO 3 − in this aquifer.