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�`M§. Mary Meays October 8, 1996 Page 2 quarterly report reflects product removed from monitoring well E-10 and vapor extraction well V-3 after a release occurred in the product piping (subsequently repaired) near the northern pump island in 1995. The 12 million gallons of groundwater extracted as stated in the quarterly report only reflects extraction after 1991 because the system was not in continuous operation between 1987 and 1991. BC is not currently including groundwater extraction tables and charts in the quarterly reports because the groundwater extraction system is not in operation as per ARCOs request and PHS/EHD approval. ARCO will include a "Summary of Groundwater Remediation" in the Operations and Maintenance (O&M) section of future quarterly reports for this site which will include the overall total of free product and groundwater removed. Soil Vapor Extraction and Air Sparging Performance Evaluations The August 14 PHS/EHD letter states that "In order to evaluate system performance and to accurately estimate and report the cumulative destruction, benzene destruction concentrations must be determined at least quarterly". The destruction efficiency of the soil vapor extraction and treatment system (SVETS) is monitored on a monthly basis (when operating) using a flame-ionization or photo-ionization detector(FID or PID) as a condition of Permit to Operate No. N-3601-1-0 issued by the San Joaquin Valley Unified Air Pollution Control District. Laboratory analysis of influent and effluent air samples for total petroleum hydrocarbons as gasoline and benzene, toluene, ethylbenzene, and xylenes are not required by the air permit, therefore, these analysis are no longer performed. The laboratory analysis of the three samples collected between August and November 1995 did not contain detectable benzene concentrations. Based on this, we can reasonably assume that the benzene concentrations in the vapor stream will be insignificant in the future and, therefore, it is not necessary to monitor for this compound. In addition, since benzene is not distinguishable using the FID or PID, BC has not been reporting the influent concentrations, removal rate, or net removal for this compound since the use of the FID or PID instruments was initiated in December 1995. Co-mingled Plume The August 14 PHS/EHD letter indicates that interim remediation will be initiated at the Chevron site located at 2905 Benjamin Holt Drive in late 1997. The letter states "It may be helpful if you increase coordination/system evaluation efforts and develop some type of agreement whereby remediation of the co-mingled plume can proceed to regulatory closure". As you are aware, ARCO and Chevron are sharing data on these sites to the extent of exchanging quarterly groundwater monitoring reports as well as any investigation reports that are submitted to PHS/EHD. When interim remediation is initiated at the Chevron Facility, ARCO will evaluate the Chevron system operation data and make necessary adjustments to the ARCO remediation program. 3316\CORRE SP\LTR-3.DOC