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V Soil Boring Report—Former Unocal Station No 0123,Tracy,California March 10 2000 DISCUSSION The SJCEHD requested Unocal to install an additional groundwater monitoring well downgradient (northwest) of the subject site at the northwest corner of 11`h Avenue and Holly Drive in an effort to further delineate groundwater impact Due to limited sidewalk access at the northwest corner, the proposed well location was placed in the street City of Tracy policy does not allow the installation of monitoring wells in any street within the city Because of this restriction, GR proposed a temporary soil boring in the street at the northwest corner to collect a grab groundwater sample in order to comply with the SJCEHD request The SJCEHD approved this scope of work and the City of Tracy agreed to the temporary boring The location for boring B-5 was chosen to meet both City of Tracy and SJCEHD requirements, and was done without any pnor knowledge of historical land use in the vicinity of the boring Petroleum hydrocarbons detected in soil and groundwater samples from boring B-5 were either weathered gasoline or unidentified hydrocarbons in the gasoline and diesel ranges Benzene, toluene, ethylbenzene, and xylenes (BTEX) were detected in groundwater, but except for benzene at 36 2 ppb, the other constituents were below their respective California Primary Maximum Contaminant Levels (MCLs) None of the oxygenates, except for ethanol in the soil, were detected When the elevated levels of weathered hydrocarbons and BTEX detected in groundwater from boring B-5 are compared to hydrocarbon levels detected recently at the subject site (GR report #280047, Third Quarter 1999 Groundwater Monitoring and Sampling Report dated October 6, 1999), the levels in boring B-5 defy the expected trend of hydrocarbon concentrations diminishing away from the source area Based on these results, GR, at the request of Unocal, performed a limited offsite source survey From the Enviros Phase I Environmental Site Assessment report and Sanborn Fire Insurance (Sanborn) Maps, two other possible sources for the residual hydrocarbons detected in boring B-5 were identified at the northwest and northeast corners of 11 h Street and Center Avenue (now Holly Drive) Locations of these former facilities are shown in pink on Figure 3, and are based on a 1947 Sanborn Map (Appendix E) At the northwest corner was a former Shell Service Station, and at the northeast corner was a garage and service station An aerial photograph from 1957 does not show the former Shell Service Station, but instead shows Holly Drive rerouted through the former Shell site This indicates that the Shell Service Station was closed between 1947 and 1957 According to the Enviros report, the station at the northeast corner closed in the mid to late 1960's, around the same time as the subject site Both of these former stations could be potential contributors to the hydrocarbons detected in boring B-5, but because of low hydrocarbon concentrations recently detected in well U-6 (located on the northeast corner of 11`h Avenue and Holly Drive),the former station at that corner is probably not a significant source Based on the configuration of the former Shell Station shown on the 1947 Sanborn Map, the inferred former dispenser islands location is just east of boring B-5, and boring B-5 could potentially be in the area of the former USTs at that site The exact location of the former Shell USTs is not known based on information supplied by Unocal or the Sanborn maps, and because of their closures prior to the mid to late 1960's, neither of the facilities on the north side of I Vh Street have had environmental assessments It is GR's recommendation that no further environmental assessment be done downgradient of the subject site 4 240073 03-1 4 x