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Foothill Sanitary Landfill 7 Department of Public Works/Solid Waste <br />First Semiannual 2025 Groundwater Monitoring ©2025 County of San Joaquin. All rights reserved. <br /> <br /> <br /> The four to five-year time lag between a surface action at LF-1 and groundwater response <br />is due to the approximate 270-foot thickness of the vadose zone at the MW-3 location. <br /> Winter of 1995/96 and continuing through three winters: water ponded over refuse near <br />MW-3 (Ponding of surface water over refuse increases moisture content of the refuse, <br />resulting in increased LFG production and the eventual impact to groundwater years <br />later). <br /> 1998: The ponding of storm water was corrected. <br /> March 2000 (five years after the start of ponding): VOCs were detected at MW-3 – <br />Conclusion: it took about five years for the VOC impact from the increased LFG <br />production within the refuse to reach groundwater. <br /> February 2002 (four years after ponding was corrected): VOC impact is no longer seen <br />at MW-3 – Conclusion: it took approximately four years after the removal of the source of <br />VOC impact at the surface to be observed in groundwater. <br /> 2002 to 2006: Groundwater at MW-3 was found absent of VOCs without an LFG <br />extraction system in operation. <br /> 2006: The Site Improvement Project was executed including the installation of an LFG <br />extraction system and a gas-tight HDPE barrier over refuse as required by CVRWQCB <br />staff. County staff expressed concern that adding a gas-impermeable barrier over unlined <br />refuse could increase chances for groundwater impacts because it may force LFG <br />migration vertically downward toward groundwater. <br /> September 2010 (almost five years after installing impermeable barrier over refuse): <br />VOCs were again detected in MW-3. <br />Based on the above observations, there appears to be a 4 to 5-year lag from the time that an <br />event at the surface of the landfill occurs and the time when VOCs are actually <br />detected/remediated. This correlation exists from the time that the barrier was installed, until <br />VOCs were again detected in MW-3. The County believes that installing HDPE additional barrier <br />over placed refuse causes a downward migration of LFG.