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February 14, 1997 Page 1 hydrocarbons in groundwater Therefore downgi adient sampling 1 esults are deemed representative of site conditions The analysis of groundwater for methyl tert-butyl ether (MtBE) was initiated in the third quarter 1996 groundwater monitoring program Groundwater collected from Well MW-9 was the only well sampled to report detectable concentrations of MtBE (2 10 ppb) During the fourth quarter monitoring program, MtBE was detected and confirmed in Well MW-9 MtBE was also detected at 20 ppb in Well MW-13, however I no detectable concentrations of MtBE were reported (above the 2 ppb detection limit) upon confirmation by EPA Method 8260 Although the MtBE concentration has been confirmed in Well MW-9, no other wells report concentrations of MtBE (including proximal Well MW-4 screened from 28 to 43 feet bgs) The occurrence is therefore isolated and the extent of migration is limited The absence of MtBE in any other site-related wells supports this Residual petroleum hydrocarbons in groundwater are attenuating In fact, the dissolved petroleum hydrocarbon plume is not only stable, but is decreasing in size The Chevron Research and Technology Company (CRTC) Memorandum (April 19, 1996) evaluated intrinsic bioremediation parameters and found that present subsurface conditions are conductive to natural biodegradation of residual petroleum hydrocarbons in ground- water This evaluation is based on the measurement of groundwater for the presence of various electron acceptor compounds (dissolved sulfate, dissolved ferrous iron, dissolved nitrate, and dissolved oxygen) which if absent, limit the degree of biodegradation DISCUSSION OF RESIDUAL PETROLEUM HYDROCARBONS Petroleum Hydrocarbons Above Background Concentrations Residual petroleum hydrocarbons remain in soil and groundwater above background concentrations as discussed in the previous sections Site remediation consisted of removing the product storage and distribution equipment, remedial excavation of areas of hydrocarbon-impacted soil, and the operation of an SVE system After nearly 2 years of operation, mass removal of petroleum hydrocarbons by the SVE system had reached an asymptotic condition when it was deactivated in November 1992 It is recognized that while SVE was an effective remediation technology which contributed considerably to the remedial effort at the site, there were environmental and system design factors that contributed to the decline of system effectiveness prior to achieving "background" soil concentrations These factors are presented below • Vapor flow to the SVE wells is affected by frictional resistance along I the well sceeen, the difference between horizontal and vertical soil permeability to air flow, and general variations in soil permeability to 3201337NCLOSURE3