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Site Closure Request
<br /> Pombo Property June 14,2005
<br /> `., oil. Municipal Code Section 11.1.16 also prohibits the installation of water supply wells in the upper
<br /> 50 feet(RWQCB,2005—Appendix A).
<br /> 2. PREVIOUS ENVIRONMENTAL ACTIVITIES
<br /> Previous investigations at the Pombo Property included soil sampling by Earth Systems Consultants
<br /> (ESC, 1999) in support of ROW relocation; a soil and groundwater investigation with a human health
<br /> screening risk assessment by Geomatrix Consultants, Inc. (Geomatrix, January 2000); additional soil and
<br /> groundwater sampling with an updated human health screening risk assessment by Geomatrix(December
<br /> 2000), and a soil and groundwater investigation report by SAIC (January 2005). Historical sampling
<br /> locations associated with these investigations are included on Figure 2. Historical soil and groundwater
<br /> analytical results are presented in Tables 1 and 2, and on Figures 3 and 4. A brief summary of each
<br /> investigation is presented below.
<br /> June 1999. Two of the four soil borings advanced by ESC exhibited petroleum hydrocarbon-affected
<br /> soils. Total petroleum hydrocarbons quantified as crude(TPHc),toluene,xylenes, and polynuclear
<br /> aromatic hydrocarbons(PAHs)were detected in soils from borings EB-7 and E13-9,at depths of 9.5 and
<br /> 15 feet bgs,respectively. There were no observable impacts in the other two borings,which were not
<br /> sampled. The ESC report is included as Appendix B.
<br /> January 2000. Based on the ESC findings, Geomatrix advanced 15 soil borings in late September 1999.
<br /> Evidence of petroleum impacts was observed in eight borings between depths of 7 and 19 feet bgs. Soil
<br /> samples collected from four of the eight impacted borings(GMX-2, GMX-6,GMX-7,and GMX-13)
<br /> were analyzed for TPH quantified as diesel(TPHd),TPH quantified as gasoline(TPHg),TPHc,methyl
<br /> tertiary butyl ether(MTBE),benzene,toluene,ethylbenzene,and xylenes(BTEX),and PAHs. TPHd,
<br /> TPHg, and TPHc were detected in all four samples. The maximum detection was 18,000 milligrams per
<br /> kilogram(mg/kg)TPHc at 8.5 feet bgs from boring GMX-2. Ethylbenzene,xylenes, and PAHs were
<br /> detected in low concentrations in two of the four samples. Confirmation samples taken from above the
<br /> water table(from 11 to 12.5 feet bgs)in seven borings outside the area of visibly affected soils(GMX-1,
<br /> GMX-3,GMX-4,GMX-8, GMX-15,GMX-17,and GMX-19)had no detections of BTEX,MTBE,or
<br /> PAHs, and concentrations of TPHg,TPHc,and TPHd were either non-detect or below 100 mg/kg in al
<br /> samples. A grab groundwater sample from beneath affected soil was collected in one boring(GMX-6)
<br /> and analyzed for BTEX,MTBE,and PAHs. Two PAHs were detected,both at less than 1 microgram per
<br /> liter(µg/L). The Department of Toxic Substances Control(DTSC)approved the findings of the 1999
<br /> screening risk evaluation,which concluded that residual petroleum constituents in soil do not pose
<br /> unacceptable heath risks. The January 2000 Geomatrix report is included as Appendix B, and the DTSC
<br /> response is included in Appendix A.
<br /> December 2000. Geomatrix conducted two off-site investigations in May and October 2000 to assess the
<br /> lateral extent of petroleum-affected soils and to further assess groundwater conditions. Seven soil borings
<br /> were advanced around the site. Soil and grab groundwater samples were collected based on visible
<br /> evidence of petroleum impacts; at a minimum,a grab groundwater sample and a soil sample from near the
<br /> top of the water table were collected. Soil samples were analyzed for TPHc,PAHs,and BTEX at depths
<br /> ranging from 8 to 12.5 feet bgs. TPHc was detected in borings GMX-26 and GMX-32 at concentrations
<br /> of 120 and 1,700 mg/kg at 8 and 9 feet bgs,respectively. In addition,pyrene and chrysene were reported
<br /> in GMX-32 at 0.057 and 0.079 mg/kg,respectively. Grab groundwater samples from five borings located
<br /> between 10 and 100 feet down gradient of affected soils(GMX-25,GMX-29 through GMX-31 and
<br /> GMX-33)were analyzed for TPHg,TPHd, PAHs, and BTEX. Xylenes and toluene were reported in trace
<br /> concentrations in GMX-29 through GMX-31, and benzene was detected at 0.6 gg/L in GMX-29, which is
<br /> below the RWQCB's water quality objective(WQO)of 1.0 µg/L for benzene. TPHd was detected in all
<br /> samples at concentrations ranging from 86 to 560µg/L,but all TPHd samples run with silica-gel
<br /> preparation reported non-detectable results. The Geomatrix investigation completed in December 2000
<br /> (Appendix B)concluded that the lateral extent of affected soil on the eastern,western,and southern
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