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S E C O R 76 Service Station#11195 April 5, 2004 Page 3 Prior to backfilling and compacting of the former gasoline UST complex excavation, one conductor casing was placed near the northwest corner of the former UST excavation. The casing was installed to facilitate the removal of groundwater from within the former UST cavity. The conductor casing consisted of 11.5 feet of 6-inch diameter polyvinyl chloride(PVC) and 10 horizontal feet of 6-inch PVC in an "L" configuration extending eastward from the vertical section. The bottom 10 feet of the vertical casing and the horizontal casing consisted of 0.02-inch machine slotted well screen. Between February 26 and March 26, 1999, approximately 41,760 gallons of groundwater were removed from the former UST backfill utilizing the conductor casing. Groundwater removal and transportation was performed by WMiS. The water was transported to the Tosco Refinery for treatment and disposal. A total of approximately 102,760 gallons of groundwater were removed from the former and present UST pits and transported to the Tosco Refinery between January 1999 and March 26, 1999. Starting in January 2000, Tosco contracted Onyx Industrial Services, Inc. (Onyx)of Benicia, California to purge approximately 5,000 gallons of groundwater from_conductor casing CC-WS on a weekly basis. The purged groundwater is transported to the Tosco Refinery in Rodeo, California for disposal. As of July 29, 2002, Onyx has removed a total of 570,320 gallons of groundwater from the site. In February 2002, GR advanced five offsite geoprobe soil borings (B-1 through B-5) downgradient of the subject site. Soil and groundwater samples were collected at each of the geoprobe locations and analyzed for TPHg (by EPA Method 8015 modified), and benzene, toluene, ethylbenzene, and r xylenes (BTEX) and MtBE by EPA Method 8021. In addition, all soil and groundwater samples were analyzed for Ethanol, TBA, MtBE, di-isopropyl ether (DIPE), ethyl tert-butyl ether (ETRE), 1,2- dichloroethane (1,2 DCA), TAME, and ethylene dibromide (EDB) by EPA Method 8260B. None of the hydrocarbon constituents were detected in soil. Four groundwater samples (B-1, B-2, B-4, and B- 5)contained MtBE at concentrations ranging from 3.0 to 10,000 ppb. On May 28 and 29, 2002, GR installed five additional offsite groundwater monitoring wells (MW-13 through MWA 7) at the locations shown on Figure 2. The well borings were drilled to depths ranging from 20 to 21 feet bgs. Groundwater was encountered in the well borings during drilling at depths ranging from approximately 8 to 10 feet bgs. All soil samples analyzed were non-detect for all hydrocarbon constituents, except for soil sample MW-16-S9.5, which contained MtBE at a concentration of 0.040 ppm. MtBE was detected in all groundwater samples collected from the newly installed monitoring wells at concentrations ranging from 28 to 3,900 ppb. All other hydrocarbon constituents and fuel oxygenates were non-detect. An ozone sparge unit was placed into operation on December 20, 2002 and injects ozone in to sparge wells SP-1 through SPA 5. SCOPE OF WORK The scope of work for this site assessment includes the over-drilling of existing well MW-13 and reinstallation of the well in the borehole. The specific scope of work is discussed below: • Health and Safety Plan. As required by the Occupational Health and Safety Administration (OSHA) Standard "Hazardous Waste Operations and Emergency Response" guidelines (29 CFR 1910.120), and by California Occupational Health and Safety Administration (Cai-OSHA) "Hazardous Waste Operations and Emergency Response" guidelines (CCR Title 8, Section 5192), SECOR will prepare lAConocoPhillips1111951Work Plan For Reinstallation Of MW-13.Doc