SITE INFORMATION AND CORRESPONDENCE

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CONESTOGA-ROVERS & ASSOCIATES January 6, 2011 5 Reference No. 632277 • In Tosco/BP wells MW-8 and AMW-3 located downgradient of the site, TPHg, BTEX, and MTBE generally have not been detected for the past several years. • Since an unexplained increase in TPHg and BTEX concentrations in the Tracy Express sump during second quarter 2009, concentrations are decreasing. • Graphs of TPHd, TPHg, benzene, and/ or MTBE concentrations in groundwater over time in the site wells are included as Attachment C. ADDITIONAL SOIL VAPOR SAMPLING During the initial sampling event in October 2009, elevated concentrations of TPHg (up to 120,000,000 micrograms per cubic meter [pg/mal) and benzene (up to 20,000 µg/m3) were detected in wells VP-1 and VP-4. This investigation was documented in the December 21, 2009 Soil Vapor Survey Report. Based on the site conditions and use, it was concluded that there was no significant vapor intrusion risk to site occupants; to address potential future redevelopment, a soil management plan would be prepared. However, the SJCEHD requested that the vapor wells be re-sampled to verify the previous concentrations and evaluate any trends. On July 6, 2010, CRA collected soil vapor samples from wells VP-1, VP-2, and VP-3 in 1-liter SUMMATM canisters. Please note that a sample was unable to be collected from VP-4 as a large storage container had been placed on top of the well. A field duplicate sample (Dupe) was also collected from VP-1 at the same time as the original sample. The samples were collected in general accordance with the Department of_Toxic S_u_bst_an_ces Con_trol_(DTSC) Advisoi - Active Soil Gas Investigations guidance document dated January 28, 2003. CRA's standard field procedures are included as Attachment D. In accordance with the DTSC guidance, leak testing was performed during sampling. Helium was used as a source gas for leak detection to evaluate if significant ambient air was entering the canisters during sampling. To perform the leak testing, helium was injected into an enclosed shroud covering the entire manifold sampling system and wellllead. In addition, prior 1 to sampling a field leak test was performed on the manifold sampling system. Testing was conducted by opening the purge canister, thus applying a negative pressure to the manifold sampling system and collecting vacuum readings. The vacuum reading did not drop after 10 minutes, thus it was considered a leak-free apparatus. The soil vapor samples were kept at ambient temperature and submitted under chain of custody (COC) to Air Toxics Ltd. (Air Toxics) of Folsom, California, for analysis. The three soil vapor samples and the field duplicate sample were analyzed for the following constituents: Worldwide Engineering, Environmental, Construction, and IT Services