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. Monitoring Well Installation and VET Report,and IRAP—Former Unocal Station#0187,Stockton,CA
<br /> December 22 2000
<br /> vicinity of the subject site are discussed in GSI Report No 4827 04-1, Limited Phase I Environmental
<br /> Site Assessment,dated May 11, 1995
<br /> To further assess on-site and off-site subsurface conditions, GSI installed three on-site groundwater
<br /> monitoring wells(U-4 through U-6)and two off-site groundwater monitoring wells(U-7 and. U-8), drilled
<br /> two on-site exploratory soil borings (U-N and U-O), and installed one on-site vapor extraction well
<br /> (UV-1) in August 1995 TPHg or benzene were not detected in soil samples collected from borings U-5
<br /> through U-7 or U-O TPHg were detected in a soil sample collected from boring U-N at 25 feet bgs
<br /> (1 7 ppm) TPHg and benzene were detected in a soil sample collected from well boring U-4 at 10 feet
<br /> bgs (41 and 0 011 ppm, respectively) TPHg were detected in the three soil samples collected from well
<br /> boring UV-1 at 25, 35, and 40 feet bgs at concentrations ranging from 3,400 to 20,000 ppm Benzene
<br /> was detected in the soil samples collected at 35 feet bgs (99 ppm) and 40 feet bgs (15 ppm) in boring
<br /> UV-1 Results of this subsurface investigation are presented in GSI Report No 4827 04-3, Well
<br /> Installation and Soil Boring Report, dated October 9, 1995
<br /> Based on the subsurface data gathered to date, a Soil Remediation Feasibility Study Work Plan (GSI
<br /> Report No 4827 05-1, April 30, 1996) was prepared that proposed a vapor extraction test (VET) to
<br /> evaluate whether soil vapor extraction was a possible remedial alternative at this site This work plan was
<br /> not implemented due to rising groundwater that submerged the impacted soil targeted for inclusion in the
<br /> feasibility study
<br /> • In May 1997, GSI conducted a search of Department of Water Resources files in Sacramento, California,
<br /> for domestic and municipal supply wells located within a 0 5-mile radius of the subject site GSI
<br /> identified twelve sites with wells within the area searched Six sites were identified as domestic water
<br /> supply wells, four sites as municipal water supply wells, and two sites as industrial supply wells The
<br /> nearest downgradient well (classified as municipal) was reported to be located approximately 1,580 feet
<br /> from the subject site Well search data were included in GSI's Well Installation Report (No 482721-2,
<br /> October 21, 1997)
<br /> As reported in the same document (Well Installation Report No 482721-2, GSI, October 21, 1997), GSI
<br /> installed off-site groundwater monitoring well U-9 in an attempt to delineate the downgradient edge of
<br /> the dissolved plume Petroleum hydrocarbons were not detected in the three soil samples from the well
<br /> boring that were submitted for analysis The initial groundwater sample from the well contained TPHg
<br /> (990 parts per billion, or ppb), benzene(130 ppb), and methyl tert-butyl ether, or MtBE(12 ppb)
<br /> Two nested piezometer wells NP-1 and NP-2 were installed by ARCADIS Geraghty & Miller in
<br /> November 1998 (NP-1) and January 1999 (NP-2) Well NP-I was installed near the southeast corner of
<br /> the subject site and well NP-2 was installed off-site, downgradient of on-site well U-3, across California
<br /> Street(Figure 3) TPHg(up to 420 ppm) and benzene(up to 3 0 ppm) were detected in soil samples from
<br /> well NP-1, mainly in the interval from 32 to 80 feet bgs Discrete groundwater samples collected from
<br /> NP-1 had concentrations of TPHg and benzene of up to 73,000 and 4,200 ppb, respectively MtBE by
<br /> EPA Method 8260 was not detected in either soil or grab groundwater samples from this boring Soil
<br /> samples from well NP-2 contained concentrations of TPHg and benzene of up to 15 and 0 18 ppm,
<br /> respectively, and discrete groundwater samples contained TPHg and benzene at concentrations up to
<br /> 4,000 and 1,200 ppb, respectively MtBE by EPA Method 8020 was not detected in soil or grab
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