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 1 5 N isotope and C 2 H 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 1 5 N-labeled NO 3 - as reactant. Test solutions were sparged during preparation and injection with either N 2 , Ar or 10% C 2 H 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 2 O, N 2 , and NH 4 + . In addition, we determined the 1 5 N contents of N 2 , N 2 O, NH 4 + , and suspended biomass from 1 5 N/ 1 4 N 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 - 1 5 N production and N 2 O- 1 5 N 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 - 1 5 N production and N 2 O- 1 5 N production indicated that denitrification accounted for only 46-49% of observed NO 3 - consumption. Production of N 2 - 1 5 N in the presence of C 2 H 2 was observed during one of the tests, which may be an indicator for abiotic NO 3 - reduction. Moreover, 1 5 N isotope analyses confirmed occurrence of assimilatory NO 3 - reduction (0.58 at.% 1 5 N in suspended biomass) and to a smaller extent DNRA (up to 4 at.% 1 5 N in NH 4 + ). Our results indicated that the combination of PPTs, 1 5 N-isotope and C 2 H 2 inhibition methods provided improved information on denitrification as well as alternative fates of NO 3 - in this aquifer.