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Rutan & Tucker, LLP 13 July 2023 Page 4 At 30707 Corral Hollow Road where the historical City of Tracy Landfill was situated, methane was detected in only one location above the 27 CCR explosive gas control compliance level, with the concentration localized to an area at this property's southwestern boundary adjacent to Interstate 580. Methane concentrations decreased moving away from this probe to levels that were two to four orders of magnitude below this compliance level in the other three probes which are located approximately 450 to 800 feet away, thereby demonstrating the lack of methane migration from this localized area. Given the age of the buried waste, disposed at least 76 years ago, continued generation of methane at higher concentrations would not be expected. Rather, methane concentrations being generated by the buried waste will continue to attenuate overtime, thereby, further reducing the potential for migration. At 30600 Corral Hollow Road, methane concentrations were not detected in either vapor probe. The lack of methane concentrations detected in these two probes provide further evidence that methane migration is not occurring from the buried waste present at the adjacent historical City of Tracy Landfill. At the Tracy Hills Phases 2, 3, and 4 property, which is located northwest of both the closed Corral Hollow Landfill and historical City of Tracy Landfill, methane concentrations were not detected in any of the 12 vapor probes. The closed Corral Hollow Landfill contains an active landfill gas collection system operated by the County as required by law, with the methane generated within this landfill being captured and controlled by this system. Quarterly monitoring performed demonstrates the continued effectiveness of this system in preventing methane migration as no methane concentrations are being detected outside the landfill in the nine perimeter gas monitoring wells. Three of these monitoring wells are located between this landfill and the adjacent Tracy Hills Phases 2, 3, and 4 property, and have consistently detected no methane concentrations since quarterly monitoring began in 2006. The landfill gas collection system is required to remain operational and routinely monitored until methane is no longer being generated at concentrations above the 27 CCR compliance level. Given the findings presented in this memorandum, methane mitigation and monitoring measures are not recommended for either the Tracy Hills Phases 2, 3, and 4 property or 30600 Corral Hollow Road/KT Property/Hillview. \\haleyaldrich.com\sharelCF\Projec[s\0208051\Deliverables\2023.06XX_Memo\2023-0713-HAI-Rutan Memo F.dou DL.DRICH