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Site Closure Request Pombo Property June 14,2005 `*••e The rationale is described in the Updated HHSE included as Appendix C and the Conformance to Low Risk Vadose Zone Criteria section in this report(Section 4.3). 19. List of technical reports submitted for site assessment,corrective action, confirmation sampling, and closure. The list of technical reports is provided in Table 4 of this report,and all available documentation has been included as appendices. 20. Additional comments supporting site No Further Action Required. The analytical results from the off-site groundwater investigations(SAIC,2005),indicate low or non- detectable concentrations,and completed the analytical suite required to fully delineate the hydrocarbon impacts. The RWQCB agrees the plume is limited in extent and appears to stable. The RWQCB also concurs with the conclusion of the 2,000-foot well survey(SAIC,2005)that there are no known sensitive receptors for the Pombo Property. Results of the updated HHSE indicate that petroleum constituents currently measured in soil and shallow groundwater at the site should not pose an unacceptable noncarcinogenic or carcinogenic risk to future on-site residential receptors,or pose an unacceptable human health risk to future commercial workers at the site;the DTSC agrees that the current land use designation(GHC) for the Pombo Property is acceptable. 4.3 CONFORMANCE TO LOW RISK VADOSE ZONE CRITERIA This section provides the technical framework for classifying the site as a low-risk vadose zone case by conforming to each of the following three criteria. 1. Demonstrate that the release has been stopped and the source of contamination has been removed or remediated. The former OVP crude oil pipeline was abandoned in the late 1960s to 1970. In addition, petroleum- impacted soil was removed from the western portion of the property to 15 feet during construction of a 50,000 square foot storm water retention basin in August 2001. Excavated material was reused on the adjacent parcel as fill material beneath an asphalt parking lot. 2. Demonstrate that the site has been adequately characterized. Thirty-two soil samples and seventeen grab groundwater samples have been taken from 31 borings both on site and off site to delineate the vertical and lateral extent of the subsurface impact. There is a decreasing trend of TPHd in groundwater, as seen on the TPHd isoconcentration map(Figures 3 and 4) and on cross-sections A-A' and B-B' of the site(Figure 6). TPHc was not detected in soil down gradient of the WSID easement. 3. Demonstrate that no waters of the State or other sensitive receptors are likely to be impacted. No sensitive receptors or potential sensitive receptors were identified that would be impacted by the Pombo Property,according to the 2,000-foot well survey performed by SAIC in 2005 and the results of the updated HHSE(Geomatrix,2001). The presence of petroleum constituents measured in soil and shallow groundwater at the site should not pose an unacceptable non-carcinogenic or carcinogenic risk to future on-site residential receptors or unacceptable human health risk to future commercial workers at the site. 5. CONCLUSION AND RECOMMENDATIONS In accordance with State Water Resource Control Board(SWRCB)Resolutions,the shallow groundwater at the Pombo Property is considered a potential source of drinking water,unless proven otherwise and exempted by the RWQCB. Accordingly,restoration of the beneficial use is defined by achieving drinking water WQOs at a minimum. The following discussion provides rationale for the closing this site 7