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' ERI 227903 R03 Former Unocal Service Station 5098,Stockton,California September 30, 2999
<br /> Environmental work at the site was initiated in 1985 when Unocal concluded retail operations and
<br /> removed two 10,000-gallon fuel underground storage tanks (USTs), one used-oil UST, and associated
<br /> piping and dispensers (Kaprealian Engineering, Inc [KEI], March 6, 1996) Excluding ERI'S recent
<br /> Supplemental Environmental Investigation (July 30, 1999), environmental work completed at the site
<br /> has included the following
<br /> • Drilling of at least 19 soil borings (KEI, March 6, 1996),
<br /> ' • Excavation of approximately 6,000 cubic yards of hydrocarbon impacted soil to approximately
<br /> 53 feet below ground surface (bgs) from the site (KEI, December 15, 1988),
<br /> • Installation of seven vapor wells (unknown designation), 13 on-site groundwater monitoring wells
<br /> (RW1, W1 through W5, MWl through MW5, MW7, and MW8), and 10 off-site groundwater
<br /> monitoring wells (MW6 and MW9 through MW 17) (KEI March 6, 1996),
<br /> • Destruction of the vapor extraction wells during soil excavation and destruction of wells Wl, W2,
<br /> MW1, MW2, MW7, MW8, and RW1 (wells W3 through W5, MW3 through MW5, and MW9
<br /> could not be located) (KEI, March 6, 1996),
<br />' • Completion of a well search with the California Department of Water Resources (DWR) which
<br /> identified approximately 40 wells within one mile of the site (KEI, March 6, 1996),
<br /> • Completion of a hydrogeologlc investigation of groundwater conditions beneath and in the vicinity
<br /> of the site (ERI, January 5, 1999), and,
<br /> • Ongoing quarterly groundwater monitoring and sampling
<br /> Cumulativegroundwater monitoring and sampling data are provided in Attachment B Historical
<br /> laboratory analysis results of soil samples collected during the proceeding work and the borings and
<br /> well locations are provided in Attachment C
<br /> Based upon Unocal's remedial efforts to remove the petroleum hydrocarbon impacted soil from beneath
<br /> the site, the Central Valley Regional Water Quality Control Board (Regional Board) provided a letter,
<br /> dated August 25, 1989 The letter stated that the Regional Board " confirms that the soil
<br /> contamination at the subject site has been adequately cleaned up " (Attachment D)
<br /> PRESENT SUPPLEMENTAL ENVIRONMENTAL INVESTIGATION (ERI, JULY 30, 1999)
<br /> During January 25 and May 25, 1999, ERI performed a soil and groundwater investigation at and in
<br /> the vicinity of the site (ERI, July 30, 1999) The soil and groundwater investigation included the
<br /> following
<br /> • Drilling 12 soil borings (111, B2, MW10A, MW14A, MW18, MW19S, MW19D, MW21S,
<br /> MW21D, MW22S, MW22D, and HP1) to depths ranging from approximately 49 to 81 5 feet bgs,
<br /> and constructing and developing groundwater monitoring wells MW 10A, MW 14A, MW 18,
<br /> MW19S, MW19D, MW21S, MW21D, MW22S, and MW22D in the respective borings The
<br /> boring logs for these wells are provided in Attachment E
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