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Pre-Design Investigation Workplan Former Manufactured Gas Plant—Lodi, California associated with the former MGP; and, several areas of suspected metal debris. Additional details including a map of the identified anomalies are provided in the RI Completion report (Stantec, 2010a). 2.4 SITE HYDROGEOLOGY The City of Lodi is part of the Eastern San Joaquin Groundwater Basin. Lodi is underlain by alluvial soils deposited by runoff from surrounding mountain ranges. The alluvial layers are part of the major aquifer system that runs the length of the valley. The aquifer is recharged by the Mokelumne River which borders the City of Lodi to the north (Stantec,2008). Groundwater in the City of Lodi ranges in depth from 20 feet bgs in the northwest section of the City to 60 feet bgs in the south section of the City with a general groundwater flow direction to the south. Subsurface investigations conducted at the Site have identified first-encountered groundwater at approximately 65 feet bgs (Stantec,2010a). The Site is located within the Lodi groundwater investigation area. An investigation of tetrachloroethene (PCE)-impacted and trichloroethene (TCE)-impacted soil and groundwater has been ongoing throughout northeast and central area of the City of Lodi since 1989, when the contaminants were first detected in City's water supply wells. These contaminants were released primarily from dry cleaning facilities located approximately 3,000 feet north-northwest of the Site. The Site is located within the southern end of the PCE plume, approximately 600 feet southwest of City of Lodi well MW-24A and approximately 1,000 feet north-northwest of City of Lodi well MW- 21A (Stantec, 2010a). 2.5 SUMMARY OF PREVIOUS INVESTIGATIONS, REMEDIAL ACTIONS AND DEED RESTRICTIONS Subsurface investigations at the Site have been conducted since circa 1985 to evaluate subsurface impacts related to operation of the former MGP. These investigations included a site screening investigation in circa1985; surface soil sampling in 2006;cultural resources survey (2007);RI (2008— 2010); water supply well destruction activities (2009); and, soil vapor extraction pilot test (2010). The following sections present a summary of previous investigations that have been conducted at the Site as described in the Draft RAW (Stantec, 2011b), RI Completion report (Stantec, 2010a) and the RI Workplan (Stantec,2008). 2.5.1 Site Screening Investigation (1985) One soil sample was collected and analyzed circa 1985 by PG&E at the Site. The analytical results indicated the presence of PAHs at a concentration of 34 milligrams per kilogram (mg/kg), arsenic at 2.2 mg/kg, lead at 530 mg/kg, mercury at 0.21 mg/kg, and cyanide at 1.5 mg/kg. Additional investigations were not conducted based on the results of this investigation. However,a Preliminary Endangerment Assessment (PEA) was requested by DTSC in a letter dated May 21, 2003. It is not known if a PEA was completed for the Site based on the referenced reports and documents available for the Site. TERRA PACIFIC GROUP Page 4 April 4, 2012