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ORDER NO. R5-2015-0058 -2- SAN JOAQUIN COUNTY DEPARTMENT OF PUBLIC WORKS FOOTHILL SANITARY LANDFILL, INC. FOOTHILL LANDFILL SAN JOAQUIN COUNTY boundary. Each contiguous monitoring well (i.e., MW-5, MW-6) is a corrective action monitoring well for LF-1 and a detection monitoring well for LF-2. Low to trace concentrations of volatile organic compounds (VOCs), primarily of Trichloroethene (TCE) and 1,1-Dichloroethene (DCE), have been intermittently detected in monitoring well MW-3 adjacent to unlined LF-1 since 1995, indicating a historical release from LF-1. TCE, for example, has been historically detected in MW-3 up to 4.0 µg/L (January 1999) and was recently detected in MW-3 at 4.0 µg/L (January 2014). No VOCs have been confirmed in any of the other groundwater monitoring wells at the site. No significant exceedances of inorganic parameters have been historically confirmed at the site. Landfill Design and Construction Landfill 1 Previous WDRs classified unlined unit LF-1 as a Class III landfill unit based on a finding that the natural geologic materials underlying the unit were sufficiently protective of underlying groundwater beneficial uses to meet Class III unit containment standards. See Title 27, sections 20240 and 20260. Monitoring data has since indicated that waste constituents from LF-1 have migrated into the unsaturated zone and groundwater indicating that the natural geologic materials underlying the site do not meet Class III containment standards. Instead of declassifying or reclassifying LF-1, these WDRs require that LF-1 be completely closed. Landfill 2 Previous WDRs Order R5-2003-0020 approved an engineered alternative design (EAD) to the Title 27/Subtitle D prescriptive liner design for an MSW landfill proposed by the Discharger for the containment systems of LF-2, Module 1 and future expansion modules at the site. LF-2, Module 1 was constructed consistent with this approved EAD and CAO R5- 2004-0706, which required installation of an HDPE barrier layer between overlapping portions of Module 1 and Module I as a corrective action measure (i.e., to prevent landfill gas migration into Module I from Module 1). Module 2, Phase A (to be constructed immediately south of Module 1) and future modules/phases will also be constructed in accordance with the approved EAD. See WDR Finding 74. In 2006, the Discharger implemented various corrective action measures (e.g., partial landfill closure, landfill gas extraction) to address the VOC release under Cleanup and Abatement Order (CAO) R5-2004-0706 issued by the Executive Officer (see WDR Finding 95). ET Cover Demonstration Project The 2006 Site Improvement Program (see WDR Finding 96) included partial closure of Module I with an evapotranspirative (ET) final cover on its top deck as an EAD to Title 27 prescriptive closure requirements. The ET cover demonstration project included a 2,500 square foot test pad equipped with soil moisture monitoring equipment (e.g., time domain reflectometers coupled with heat dissipation units) placed at specified depths (i.e., 1, 2 and 3 feet below surface) within the 4.5-foot thick ET cover soil; an underlying percolation layer and a geomembrane to allow for recovery and measurement of percolation/seepage through the ET cover; plumbing for recovery and measurement of surface runoff; and a small weather