ESTIMATION OF RATES OF AEROBIC HYDROCARBON BIODEGRADATION BY SIMULATION OF GAS-TRANSPORT IN THE UNSATURATED ZONE

The distribution of oxygen and carbon dioxide gases in the unsaturated zone provides a geochemical signature of aerobic hydrocarbon degradat ion at petroleum product spill sites. The fluxes of these gases are pr oportional to the rate of aerobic biodegradation and are quantified by calibrating a mathematical transport model to the oxygen and carbon d ioxide gas concentration data. Reaction stoichiometry is assumed to co nvert the gas fluxes to a corresponding rate of hydrocarbon degradatio n. The method is applied at a gasoline spill site in Galloway Township , New Jersey, to determine the rate of aerobic degradation of hydrocar bons associated with passive and bioventing remediation field experime nts. At the site, microbial degradation of hydrocarbons near the water table limits the migration of hydrocarbon solutes in groundwater and prevents hydrocarbon volatilization into the unsaturated zone. In the passive remediation experiment a site-wide degradation rate estimate o f 34,400 g yr(-1) (11.7 gal. yr(-1)) of hydrocarbon was obtained by mo del calibration to carbon dioxide gas concentration data collected in December 1989. In the bioventing experiment, degradation rate estimate s of 46.0 and 47.9 g m(-2) yr(-1) (1.45 x 10(-3) and 1.51 x 10(-3) gal . ft.(-2) yr(-1)) of hydrocarbon were obtained by model calibration to oxygen and carbon dioxide gas concentration data, respectively. Metho d application was successful in quantifying the significance of a natu rally occurring process that can effectively contribute to plume stabi lization.