ARCHIVED REPORTS XR0013118 - Laserfiche WebLink

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M. Scott Mansholt -2- 8 March 2005 ChevronTexaco Pombo Property SOIL According to the reports, the majority of residual petroleum hydrocarbons present in soil were found between 8 and 12 feet with decreasing concentrations to approximately 15 feet. Impacted soil with total petroleum hydrocarbon as crude oil(TPH-c)concentrations up to 18,000 mg/kg were identified at two separate onsite locations. During site development in August 2001,petroleum-impacted soil in the western portion of the site was removed during construction of a 50,000 square foot stormwater retention basin. Soil was excavated to approximately 15 feet and used on the adjacent parcel.(now aretail shopping center)as fill material beneath an asphalt parking lot. Post excavation confirmation soil sampling reported that with the exception of two detections of 120 and 130 mg/kg,the majority of the impacted soil was removed. TPH as gasoline(TPH-g),TPH as diesel(TPH-d),and low concentrations at or slightly over the respective method detection limits of toluene, ethylbenzene, and total xylenes, and select polymuclear aromatic hydrocarbons(PAHs)were detected in on-site and off-site soil samples. Benzene has not been detected in soil samples submitted for analysis. The source of the TPH-g(maximum reported was 1,000 mg/kg at 12.5 feet),toluene, ethylbenzene and xylenes has not been identified;however, aretail gasoline station was previously located near the central-southern property boundary. During the November 2003 and February 2004 investigations,the focus was off-site groundwater delineation,within a residential area north of the Pombo site. Soil samples submitted for analysis were non detect(ND) for TPH-c except for one sample.reporting 5.6 mg/kg at five feet; and,with the exception �'' of toluene and xylenes reporting values at their respective detection limits of 0.005 mg/kg and 0.02 m&Ig, respectively,BTEX constituents were ND. GROUNDWATER Depth to groundwater ranges in the vicinity of the site ranges between 7 and 12 feet and generally flows to the north with a relatively low gradient. Shallow groundwater in the Tracy area is of poor quality and prohibited for use as domestic or municipal supply in accordance with the City of Tracy Municipal Code (Section 11.1.1.6), for administrative reasons unrelated to the presence of degraded crude oibbunker C fuel oil. The City of Tracy Municipal Code Section 11.1..16 prohibits installation of water supply wells in the upper 50 feet. Recent sampling and analysis of shallow groundwater on another nearby OVP project by Regional Board staff indicates the following: total dissolved solids—2,500 mg/L,boron—3.3 mg/L, nitrate—45 mg/L, chloride—620 mg/L, sodium—370 mg/L,sulfate—700 mg/L,and.hardness(as CaCO3) of 1,100 mg/L. On-site boring GMX-29 groundwater sample reported 140 gg/L TPH-g, 660 µg2:TPH-d,benzene at 0.6 gg/L,2.6 gg/L toluene, 1.6 µg/L ethylbenzene, and 4 µQ/L total xylenes; low concentrations of two PAHs at the detection limits were also reported. A groundwater sample from a second on-site boring (GMX-6), approximately 50 feet from GMX-29)was non detect for TPH compounds. These results are from the borings that also reported the elevated TPH-c concentrations in soil. The November 2003 and February 2004 site investigations focused on off-site groundwater delineation, �., within a residential area,north of the Pombo site. Borings were advanced, soil and groundwater samples