Spatial Variability in In Situ Aerobic Respiration and Denitrification Rates in a Petroleum‐Contaminated Aquifer

An extensive series of single‐well, push‐pull tests was performed to quantify horizontal and vertical spatial variability in aerobic respiration and denitrification rates in a petroleum‐contaminated aquifer. The results indicated rapid consumption of injected O 2 or NO 3 − in shallow and deep test intervals across a large portion of the site. Computed first‐order rate coefficients for aerobic respiration ranged from 0.15 to 1.69 h −1 in the shallow test interval, and from 0.08 to 0.83 h −1 in the deep test interval. The largest aerobic respiration rates occurred on the upgradient edge of the contaminant plume where concentrations of petroleum hydrocarbons and dissolved O 2 were relatively high. Computed first‐order rate coefficients for denitrification ranged from 0.09 to 0.42 h −1 in the shallow test interval, and from 0.11 to 0.28 h −1 in the deep test interval. The largest denitrification rates occurred on the downgradient edge of the plume where hydrocarbon concentrations were relatively high but dissolved oxygen concentrations were small. The rates reported here represent maximal rates of aerobic respiration and denitrification, as supported by high concentrations of electron acceptors in the injected test solutions. Production of dissolved CO 2 during aerobic respiration and denitrification tests provided evidence that O 2 and NO 3 − consumption was largely due to microbial activity. Additional evidence for microbial NO 3 − consumption was provided by reduced rates of NO 3 − consumption when dissolved O 2 was injected with NO 3 − , and by increased N 2 O production when C 2 H 2 was injected with NO 3 − .