Biodegradation of petroleum hydrocarbon vapors: laboratory studies on rates and kinetics in unsaturated alluvial sand.

Predictions of natural attenuation of volatile organic compounds (VOCs) in the unsaturated zone rely critically on information about microbial biodegradation kinetics. This study aims at determining kinetic rate laws for the aerobic biodegradation of a mixture of 12 volatile petroleum hydrocarbons and methyl tert-butyl ether (MTBE) in unsaturated alluvial sand. Laboratory column and batch experiments were performed at room temperature under aerobic conditions, and a reactive transport model for VOC vapors in soil gas coupled to Monod-type degradation kinetics was used for data interpretation. In the column experiment, an acclimatization of 23 days took place before steady-state diffusive vapor transport through the horizontal column was achieved. Monod kinetic parameters Ks and vmax could be derived from the concentration profiles of toluene, m-xylene, n-octane, and n-hexane, because substrate saturation was approached with these compounds under the experimental conditions. The removal of cyclic alkanes, isooctane, and 1,2,4-trimethylbenzene followed first-order kinetics over the whole concentration range applied. MTBE, n-pentane, and chlorofluorocarbons (CFCs) were not visibly degraded. Batch experiments suggested first-order disappearance rate laws for all VOCs except n-octane, which decreased following zero-order kinetics in live batch experiments. For many compounds including MTBE, disappearance rates in abiotic batch experiments were as high as in live batches indicating sorption. It was concluded that the column approach is preferable for determining biodegradation rate parameters to be used in risk assessment models.

Small-volume releases of gasoline in the vadose zone: impact of the additives MTBE and ethanol on groundwater quality.

A controlled gasoline spill experiment was performed under outdoor conditions typical for winter in temperate regions to study the fate of methyl tert-butyl ether (MTBE), ethanol, benzene, and selected other petroleum hydrocarbons. Artificial gasoline containing MTBE and ethanol (5% w/w of each) was placed at a defined depth into a 2.3 m thick unsaturated zone of alluvial sand overlying a gravel aquifer in a lysimeter. During an initial period of 41 days without recharge, MTBE and hydrocarbon vapors migrated by vapor-phase diffusion to groundwater, while ethanol vapors were naturally attenuated. In a subsequent period of 30 days with 5-mm daily recharge, all soluble compounds including ethanol were transported to the groundwater. Ethanol disappeared concomitantly with benzene and all other petroleum hydrocarbons except isooctane from the aerobic groundwater due to biodegradation. MTBE persisted for longer than 6 months at concentrations larger than 125000 microg L(-1). No evidence for MTBE biodegradation was found, whereas > 99.6% of ethanol removal from the lysimeter was due to biodegradation. It is concluded that MTBE-free gasoline would be less harmful for groundwater resources and that ethanol is an acceptable substitute.

Vapor phase transport and biodegradation of volatile fuel compounds in the unsaturated zone: a large scale lysimeter experiment.

The vapor phase transport and biodegradation of typical fuel compounds including volatile petroleum hydrocarbons and methyl tert-butyl ether (MTBE) was studied in a large scale field lysimeter representing a 2.3 m thick sandy unsaturated zone over a gravel aquifer. A mixture of 13 fuel compounds with MTBE (5%) was placed at a defined depth in the unsaturated zone to obtain a homogeneous source zone with a residual NAPL saturation. The upward and downward transport of fuel vapors and the biodegradation by indigenous microorganisms were monitored during 70 days. Using tracers in water and NAPL, it was shown that fuel compounds were transported by vapor phase diffusion only. All fuel compounds except MTBE disappeared from the lysimeter below the analytical detection limits within 70 days. MTBE accumulated in groundwater but volatilized from the unsaturated zone. First-order biodegradation rates were estimated in the unsaturated zone to range from <0.05 d(-1) for MTBE up to 8.7 d(-1) for octane. Aerobic biodegradation of degradable fuel compounds to CO2 started without any lag phase and removed about 3 times more fuel mass than volatilization. The study illustrates the recalcitrance of MTBE vapors compared to other fuel vapors, leading to a significant groundwater pollution with MTBE.