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.