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

摘要

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 ~(15)N 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 1 of test solution containing 0.5 mM Br~- as conservative tracer and either 0.5 mM unlabeled NO, or ～ 0.3 mM ~(15)N-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 ~(15)N contents of N_2, N_2O, NH_4~+, and suspended biomass from ~(15)N/~(14)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― ~(15)N production and N_2O―~(15)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―~(15)N production and N_2O―~(15)N production indicated that denitrification accounted for only 46-49% of observed NO_3~- consumption. Production of N_2―~(15)N 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, ~(15)N isotope analyses confirmed occurrence of assimilatory NO_3~- reduction (0.58 at.% ~(15)N in suspended biomass) and to a smaller extent DNRA (up to 4 at.% ~(15)N in NH_4~+). Our results indicated that the combination of PPTs, ~(15)N-isotope and C_2H_2 inhibition methods provided improved information on denitrification as well as alternative fates of NO_3 in this aquifer. Keywods: nitrate reduction; denitrification; assimilation; acetylene inhibition; N-15 isotope; push―pull test