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Tony Gonzales Trucking 2 - 22 August 2007 <br /> Escalon, San Joaquin Count}. -.Poe <br /> The Report recommends verifying the results of the SESOIL modeling by advancing a Cone <br /> Penetrometer Testing (CPT) boring adjacent to soil boring SB-1, sampling soil at the same <br /> depths (15, 25, 40, 45, 50, 55, and 60 feet bgs), and taking a grab groundwater sample at <br /> approximately 60 feet bgs. The results of this investigation would provide the basis for Site <br /> closure or a recommendation for further work, including the asphalt cap recommended by <br /> GZA in the 2002 Final Remediation Plan. <br /> Comment: <br /> The Report appears to be complete and adequately addresses the Fate and Transport <br /> modeling of residual contaminants at the Site. Conservative inputs were used to address a <br /> worst-case scenario for threats to groundwater. SESOIL modeling results show that the <br /> groundwater resource is adequately protected by natural attenuation. As such, additional <br /> investigation need only confirm the model results, as adequate definition of the vertical and <br /> lateral extent of the contamination has been achieved. In order to not disturb the existing soil <br /> column at SB-1 and to confirm that groundwater has not been impacted 29 years after the <br /> USTs removal, I suggest taking a single grab groundwater sample near soil boring SB-4 <br /> (located ten feet from the USTs pit) in the regional groundwater flow direction (towards the <br /> northwest) of the USTs. Also, in the grab groundwater Workplan, include a shallow soil vapor <br /> survey near the residence and compare the soil vapor results to the San Francisco Bay <br /> Regional Board Environmental Screening Levels (ESLs) for all of the constituents of concern. <br /> If groundwater is not impacted and soil vapor results do not exceed the ESLs, then the model <br /> has been validated, soil vapor intrusion into the residence is not a concern, and a Closure <br /> Report of the results of the investigations should be generated under the guidance of the <br /> Appendix A — Report, Tri-Regional Board Staff Recommendation for Preliminary Investigations <br /> and Evaluation of Underground Tank Sites, April 2004, which may be downloaded from our <br /> website at <br /> http://www.waterboards.ca.gov/centraIvaIley/available documents/index.html#anchor617894. <br /> If you have any questions, please contact me at (916) 464-4615 or by email at <br /> jbarton@waterboards.ca.gov. <br /> 4 <br /> !1James L.L. Barton, P.G. <br /> Engineering Geologist <br /> cc: Mr. Mark Owens, State Water Resources Control Board, UST Cleanup Fund, Sacramento <br /> Ms. Lorie Duncan, San Joaquin County Environmental Health Services, Stockton <br /> Mr. Russell Juncal, Ground Zero Analysis, Inc., 1714 Main St., Escalon 95320 <br />