BIODEGRADATION OF HYDROCARBON VAPORS IN THE UNSATURATED ZONE

The time-averaged concentration of hydrocarbon and oxygen vapors were measured in the unsaturated zone above the residually contaminated capillary fringe at the U.S. Coast Guard Air Station in Traverse City, Michigan. Total hydrocarbon and oxygen vapor concentrations were observed over a 13-month period. Supplementary gain size, porosity, and moisture content data support the assumption of a uniform, homogeneous site geology which, in view of the planar hydrocarbon source term, abundant oxygen, and sparse data base, is suitable for simple analytical modeling. In the assumed absence of advection, leaching, and transience, the analysis is a straight-forward balance of gaseous diffusion and biological degradation coupled stoichiometrically in the two reacting constituents. Volatilization is shown to be a significant transport mechanism for hydrocarbons at Traverse City, and biodegradation prevents the escape of appreciable contamination to the atmosphere for most locations at this site. Little oxygen is expected to reach the water table because of the aerobic biodegradation process in the unsaturated zone.