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September 18, 1996 Page 7 RESIDUAL PETROLEUM HYDROCARBONS IN GROUNDWATER Quarterly groundwater monitoring was initiated in November 1988 Dissolved hydro- carbon concentrations in groundwater samples collected from Well MW-9, located in the source area, have generally decreased with the overall increase in groundwater elevation This trend indicates that SVE was effective in remediating hydrocarbon-affected soils in the source area Furthermore, during 1996, groundwater elevations in site monitonng wells have risen allowing\collection of groundwater samples from wells which were previously dry (Wells MW-1, and MW-3 through MW-8) Third quarter groundwater monitoring (July 30, 1996, Attachment B) indicates the maximum residual concentration of TPPH-g in groundwater to be 480 ppb (Well MW-4) The maximum residual concentration of benzene in groundwater is 2 4 ppb (Well MW-9) Wells MW-4 and ' MW-9 are located within and adjacent to the northeast of the former UST complex All remaining groundwater monitoring wells reported no detectable concentrations of TPPH-g or benzene, with the exception of 110 ppb TPPH-g in Well MW-6 Therefore, minor petroleum hydrocarbons'exist in groundwater locally in the vicinity of the north- eastern corner of the former UST complex The lateral extent of petroleum hydrocar- bons in groundwater have been delineated to non-detectable concentrations Residual petroleum hydrocarbons in groundwater are attenuating 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 rutrate, and dissolved oxygen) which if absent, limit-the degree of biodegradation SUMMARY AND CONCLUSIONS { PACIFIC recommends that the site be closed based on the following 1 All USTs and associated piping have been removed from the site 2 SVE operation has effectively remedlated petroleum hydrocarbons in - soil such that there is little potential for future impact to groundwater • Based on data collected from bonngs drilled before and after SVE system operation, hydrocarbon concentrations in soil and groundwater have significantly been reduced Soil chemistry data from borings indicate that TPPH-g mass in the vicinity of the former USTs was estimated to be 26,000 pounds mass prior to SVE system operation, after SVE system operation TPPH-g mass in the vicinity of the former USTs is estimated to be 4,000 pounds mass Based on these figures, a total reduction of TPPH-g mass of approximately 85 percent was achieved during the time period between SVE system activation and 3201337BICLOSLTRE I