Estimation of hydrocarbon biodegradation rates in gasoline-contaminated sediment from measured respiration rates

摘要

An open microcosm method for quantifying microbial respiration and estimating biodegrada- tion rates of hydrocarbons in gasoline--contaminated sediment samples has been developed and validated. Stainless--steel bioreactors are filled with soil or sediment samples, and the vapor-phase composition (concentrations of oxygen (O_2), nitrogen (N_2), carbon dioxide (CO_2), and selected hydrocarbons) is monitored over time. Replacement gas is added as the vapor sample is taken, and selection of the replacement gas composition facilitates real-time decision--making regarding environmental conditions within the bioreactor. This capability allows for maintenance of field conditions over time, which is not possible in closed microcosms. Reaction rates of CO_2 and O_2 are calculated from the vapor--phase composition time series. Rates of hydrocarbon biodegradation are either measured directly from the hydrocarbon mass balance, or estimated from CO_2 and O_2 reaction rates and assumed reaction stoichiometries. Open microcosm experiments using sedi- ments spiked with toluene and p-xylene were conducted to validate the stoichiometric assump- tions. Respiration rates calculated from O_2 consumption and from CO_2 production provide estimates of toluene and p-xylene degradation rates within about ± 50/100 of measured values when complete mineralization stoichiometry is assumed. Measured values ranged from 851.1 to 965.1 g m~(-3) year~(-1) for toluene, and 407.2--942.3 g m~(-3) year~(-1) for p-xylene. Contaminated sediment samples from a gasoline--spill site were used in a second set of microcosm experiments. Here, reaction rates of O_2 and CO_2 were measured and used to estimate hydrocarbon respiration rates. Total hydrocarbon reaction rates ranged from 49.0 g m~(-3) year~(-1) in uncontaminated (background) to 1040.4 g m~(-3) year~(-1) for highly contaminated sediment, based on CO_2 production data. These rate estimates were similar to those obtained independently from in situ CO_2 vertical gradient and flux determinations at the field site. In these experiments, aerobic conditions were maintained in the microcosms by using air as the replacement gas, thus preserving the ambient aerobic environment of the subsurface near the capillary zone. This would not be possible with closed microcosms.