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Mr Knoll February 19, 1999 Page 2 ' Regional groundwater flow has been toward large pumping cones located in the northwest region of Stockton Historically, groundwater was encountered at approximately 40 feet below ground surface (bgs)at the site(San Joaquin County, 1996) Groundwater was not found within 40 feet of the surface during previous work at the ' site in 1995 (RMI, 1995) Groundwater levels may have changed since 1995 work and the 1996 mapping due to changes in regional withdrawal of groundwater and precipitation Seasonally, shallow groundwater flow may be altered by the influence of the San Joaqwn River ' Two public supply wells exist within a 1 mile radius of the site, however these wells have been inactive for the last 20 years due to poor water quality (Mar, 1999) The poor water quality was caused by the overdraft of the groundwater basin which induced the movement of poor quality water resulting from the Delta sediments ' eastward near the City of Stockton(DV,/R, 1980) SITE HISTORY ' According to information provided by BFS, the site was used as a gasoline station during the 1920s and 1930s After this period the site was used as a retail bre and automotive service business In 1967 four USTs(Tanks No 1 through 4) were closed in place by filling the tanks with sand At that time, closure for the in-place ' tanks was granted by the Stockton Fire Department In 1991 twelve USTs, including one of the previously closed in-place tanks, were removed from the site (Figure 2) Ryan Murphy, Inc (RW remediated the soil surrounding Tanks No 4 and 5 using soil vapor extraction during 1994 and 1995 Environmental site closure was received from the SJCPHS-EHD in November 30, 1995 In May 1997, during a property transfer site assessment, four slant soil borings were drilled at the site by Foothill Engineering in the area of the previously closed in place Tanks No 1 through 3 (Figure 2) Soil samples from two of the four soil borings near Tank No 3 contained petroleum hydrocarbons at depths of 20 and 25 feet Total Petroleum hydrocarbons (TPH) concentrations of 16,200 and 1,160 milligrams per kilogram(mg/kg) were detected at 20 feet in soil samples from bormgs PBI and P132, respectively Toluene, ethylbenzene and total xylenes were also detected in soil from boring PBI at concentrations of 49 2, 308, and 1,920 mg/kg, respectively Analytical results from the property transfer site assessment are summarized in Table 1 During this assessment, no groundwater samples were collected and the report stated that groundwater was not encountered however soil sample PB2 at 25 feet was described as wet Foothill Engineering concluded that samples collected near the south tank indicated the presence of aged gasoline (Foothill Engineering, 1997) Due to the detected concentrations of petroleum hydrocarbons from the May 1 1997 assessment, the County requested additional assessment surrounding Tank No 3 Refer to Figure 2 for soil boring locations ' SITE ASSESSMENT WORK PERFORMED On October 9, 1998, QST submitted a workplan to SJCPHS-EHD for additional assessment which proposed the advancement of two soil borings, one to the west of Tank No 3 and the other south of Tank No 3 The proposed soil bonngs were to be drilled to a depth of 35 feet bgs and soil and groundwater samples were to ' be collected for laboratory analysis The samples were to be analyzed for petroleum hydrocarbons and their constituents and oxygenate additives An encroachment permit for the proposed boring to the west of the Tank No 3 was denied by the City of Stockton The relocation of one of the proposed bormgs to the east of Tank ' No 3 was approved by Harlan Knoll of the SJCPHS-END