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Monitoring well installation Report--Former Unocal Service Station 0187,Stockton,CA August 2,2000 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-I 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 Nater Resources files in Sacramento, California, foi domestic and municipal supply wells located within a 0 5-mile radius of the subject site GSl 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 dovvngradient 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), GSl installed off-site ground rater monitoring well U-9 in an attempt to delineate the dovvngradient edge of the dissolved plume Petroleum hydrocarbons were not detected in the three soil samples from the well boring that were submitted for analysts The groundvvater 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 piesometer wells NP-1 and NP-2 were installed by ARCADIS Geraghty & Miller in November 1998 (NP-1), and January 1999 (NP-2) Well NP-1 was installed near the southeast corner of the subject site and well NP-2 was installed off-site, dovvngradient of on-site well U-3, across California Street (Figure 3) TPHg (up to 420 ppm) and benzene (up to 3 0 ppm) were detected in soil samples from well NP-1, mainly in the interval from 32 to 80 feet bgs Discrete groundwater samples collected from NP-I had concentrations of TPHg and benzene of up to 73,000 and 4,200 ppb, respectively MtBE by EPA Method 8260 was not detected in either soil or grab groundwater samples from this boring Soil samples from well NP-2 contained concentrations of TPHg and benzene of up to 1 5 and 0 IS ppm, respectively, and discrete groundwater samples contained TPHg and benzene at concentrations up to 4,000 and 1,200 ppb, respectively MtBE by EPA Method 8020 was not detected in soil or grab groundwater samples from this boring Results of this subsurface investigation are presented in ARCADIS Geraghty & Miller's Supplemental Site Characterization Report dated June 3, 1999 24004205-1 3