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Interim Corrective Action Plan—Tosco(76)Service Station No. 11195,Lathrop,California August 28,2002 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 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(ETBE), 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 MW-17)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. 2.3 Geology_and Hydrog_eology The site is situated on generally flat topography at an elevation of approximately 10 feet above mean sea level (MSL). The San Joaquin River is located approximately 1.1 miles southwest of the site and Weatherbee Lake is located approximately 2.7 miles south of the site. The static depth to groundwater in the eleven existing monitoring wells at the site was measured at 7.69 to 11.94 feet below top of casing(btoc)on June 18,2002. The groundwater flow direction on June 18, 2002 was toward the south at a gradient of 0.03. The historical groundwater flow direction is generally southward at an average gradient of 0.001 to 0.01 (GR 2nd Quarter, 2002). Soil encountered during drilling consisted primarily of silty sand, poor to well graded sand, and silt. Silty sand has been encountered from ground surface to approximately 8 to 16 feet bgs. Underlying the silty sand is poorly-graded to well-graded sand to a depth of approximately 25 feet bgs,the maximum depth explored. GR prepared a geological cross-section following a transect along the long axis of the dissolved MtBE plume. The surface projection of the cross-section is presented on Figure 2. Cross-section A-A' (Figure 3)reveals a predominantly sand rich lithology with a discontinuous fine-grained silt and clay layer at approximately 15 feet bgs. The fine-grained layer is also underlain by sand rich sediments. 140193.13 4 j