SITE INFORMATION AND CORRESPONDENCE

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excavation indicated TPH-g and benzene concentrations ranged from 3 to 18 ppm and less than 0.05 to 1.7 ppm; respectively. In August 1989, two additional groundwater monitor wells were constructed off- site approximately 100 feet east and northeasterly of the former pump island where high i of Analyses groundwater.constituent concentrations had previously been detected in g Y I groundwater samples from these wells revealed low concentrations of TPH-g (150 and 240 ppb), xylene (1.4 and 1.9 ppb), and ethylbenzene (less than 0.5 and 8.1 ppb); benzene and n toluene were not detected. In February 1992, three vapor extraction wells and two more groundwater monitor well were installed. Two of the vapor extraction wells were located where the easterly pump island had been while the third well was positioned at the former location of the I site's second, westerly pump island. One of the new groundwater monitor wells was constructed about 300 feet down-gradient of the site while the other was positioned on- site and up-gradient, about 60 feet southwest of the pair of vapor extraction wells. Analyses of soil samples collected during the drilling of these wells indicated an absence of gasoline constituents in the vicinity of the westerly pump island and at the locations of the two newly installed groundwater monitor wells. However, in the vicinity of the easterly pump island, TPH-g and benzene were detected at concentrations as high as 7,300 ppm and 3.4 ppm, respectively, at a depth of 26 feet; the data indicated that concentrations diminished to 120 ppm and 0.083 ppm, respectively, at a depth of 51 feet. In July 1994, two more groundwater monitor wells were installed southeast and southwest of the site to complete definition of the plume. Water quality data from the array of nine groundwater monitor wells showed that the dissolved phase benzene plume, centered at the location of the easterly pump island, extended down-gradient about 120 feet, no more than 50 to 60 feet off-site. In January 1996, recovery of residual .petroleum hydrocarbons in the vadose zone was initiated using Soil Vapor Extraction (SVE) technology. The SVE system was operational through November 1996 and recovered an estimated 450 gallons of TPH-g, including about two gallons of benzene. Beginning in May 1996, MTBE was analyzed for via EPA Method 8020 in quarterly groundwater samples from site monitor wells. These analyses have not detected MTBE in groundwater from any site monitor wells. By letter to UNOCAL dated January 9, 1997, County staff concluded that"...the assessment of the lateral extent of groundwater contamination is complete, the assessment of soil contamination is complete, and the plume appears to be stable." The County recommended that a feasibility study and corrective action plan (CAP) be submitted, quarterly groundwater monitoring continue, and data in support of the presence of natural attenuation be collected.. By letter to the County dated February 7, 1997, UNOCAL summarized the extent of remedial activities undertaken at the site and the findings of its investigative efforts. UNOCAL then concluded that the site was suitable for closure as a low-risk groundwater case and that further site assessment or corrective action was not necessary. cubic yards of contaminated soil..."were excavated. Based on the reported dimensions of the excavation, we have estimated 850 cubic yards were excavated. I 2