EVIDENCE OF AEROBIC AND ANAEROBIC BIODEGRADATION OF COMINGLED GASOLINE PLUMES

摘要

The influence of intrinsic biodegradation was evaluated for two comingled gasoline plumes resulting from leaking underground storage tanks (USTs) at a former fueling station located at Oakland Army Base in Oakland, California. One plume contains benzene, toluene, ethylbenzene, and xylene (BTEX) and the second, younger plume contains BTEX and methyl-tert butyl ether (MtBE). Releases of fuel were discovered at both sets of tanks following their removal. Monitoring wells were installed in and around the area of both plumes to monitor the floating and dissolved gasoline constituents. The goal of this project was to demonstrate that reduction of petroleum hydrocarbons had occurred through intrinsic biodegradation and that cleanup goals could be met within an acceptable period of time. In order to assess whether intrinsic biodegradation was occurring at the site, electron acceptors (dissolved oxygen (DO), sulfate, nitrate) and other indicator parameters (oxidation-reduction potential (ORP), alkalinity, ferrous iron, and methane) from a semi-annual sampling event were contoured on site maps. Intrinsic biodegradation indicators provided strong evidence that both aerobic and anaerobic biodegradation has occurred in the petroleum-impacted groundwater. Biodegradation processes appeared to be limiting the extent of the BTEX contamination at both the 1990 and 1999 plumes, but have little demonstrable effect on preventing the migration of MtBE from the 1999 gasoline plume. In addition, a model was developed to simulate the fate of the petroleum hydrocarbon contaminants in the groundwater at the 1990 plume. Bioscreen was selected because it can simulate instantaneous biodegradation reactions for BTEX that are present at the site. Bioscreen was used to estimate the amount of contaminant adsorption, dispersion and biodegradation that would occur as the plume migrates. The Bioscreen model, using conservative input parameters, predicted that the dissolved concentration of total BTEX in the groundwater would be reduced by natural attenuation, including intrinsic biodegradation, to below standard analytical detection limits in 18 years and that the cleanup level at the source area will be reached between 14 and 16 years.