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Work Plan for Additional Subsurface Investigation @ Former Unocal Station 90187 November 12, 1999 site. The ESA identified a former station located at 448 East Miner Avenue, south of the subject site. Historical groundwater data indicate that this former station is upgradient of the subject site. Environmental information related to the soil and groundwater conditions beneath this former service station were not obtained during the ESA. Environmental data related to surrounding properties in the vicinity of the subject site are discussed in GSI Report No.4827.04-1,Limited Phase IEnvironmental Site Assessment,dated May 11, 1995. To further assess on-site and off-site subsurface conditions, GSI installed three on-site groundwater monitoring wells(U-4 through U-6)and two off-site groundwater monitoring wells(U-7 and U-8), drilled two on-site exploratory soil borings(U-N and U-O),and installed one on-site vapor extraction well (UV-1) in August 1995. TPHg or benzene were not detected in soil samples collected from borings U-5 through U-7 or U-O. TPHg were detected in a soil sample collected from boring U-N at 25 feet bgs (1.7 ppm). TPHg and benzene were detected in a soil sample collected from well boring U-4 at 10 feet bgs (41 and 0.011 ppm, respectively). TPHg were detected in the three soil samples collected from well boring UV-1 at 25,35,and 40 feet bgs at concentrations ranging from 3,400 to 20,000 ppm. Benzene was detected in the soil samples collected at 35 feet (99 ppm) and 40 feet bgs (15 ppm) in boring UV-1. Results of this subsurface investigation are presented in GSI Report No. 4827.04-3, Well Installation and Soil Boring Report, dated October 9, 1995. Based on the subsurface data gathered to date,a Soil Remediation Feasibility Study Work Plan(GSI Report No.4827.05-1,April 30, 1996)was prepared that proposed a vapor extraction test(VET)to evaluate whether soil vapor extraction was a possible remedial alternative at this site. This work plan has not been implemented due to rising groundwater that submerged the impacted soil targeted for inclusion in the feasibility study. In May 1997, GSI conducted a search of Department of Water Resources files in Sacramento, California, for domestic and municipal supply wells located within a 0.5-mile radius of the subject site. GSI identified twelve sites with wells within the area searched. Six sites were identified as domestic water supply wells, four sites as municipal water supply wells, and two sites as industrial supply wells. The nearest downgradient well (classified as municipal)was reported to be located approximately 1,580 feet from the subject site. Well search data were included in GSI's Well Installation Report(No.482721-2,October 21, 1997). As reported in the same document (Well Installation Report No. 482721-2, GSI, October 21, 1997), GSI installed off-site groundwater monitoring well U-9 in an attempt to delineate the downgradient edge of the dissolved plume. Petroleum hydrocarbons were not detected in the three soil samples from the well boring that were submitted for analysis. The groundwater sample from the well initially contained TPHg(990 parts per billion, or ppb), benzene (130 ppb), and methyl tert-butyl ether (MtBE) (12 ppb). Drilling and well installation data are presented in GSI's Well Installation Report(No. 482721-2), dated October 21, 1997. Two nested piezometer wells NP-I and NP-2 were installed by ARCADIS Geraghty&Miller in November 1998(NP-1),and January 1999(NP-2). Well NP-I was installed near the southeast corner of the subject site and well NP-2 was installed off-site,downgradient of on-site well U-3,across California Street(Figure 3). 240042.05 3