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WASTE DISCHARGE REQUIREMENTS ORDER R5-2015-0058-01 -31- SAN JOAQUIN COUNTY DEPARTMENT OF PUBLIC WORKS FOOTHILL SANITARY LANDFILL, INC. FOOTHILL LANDFILL SAN JOAQUIN COUNTY Additional LFG collection facilities will be installed at the site, as necessary, to meet landfill development and corrective action needs. Existing LFG facilities within the planned landfill footprint, including the flare station and gas-to-energy plant, will be moved prior to initiating module construction in those areas. 132. Previous WDRs predated installation of the LFG system at the site and therefore did not require LFG extraction monitoring. Given that an LFG extraction system has since been installed at the site, the monitoring program in these WDRs requires that the Discharger conduct semiannual LFG extraction monitoring for parameters and constituents, including VOCs. 133. On 19 December 2014, in response to a 17 October 2014 email request by Central Valley Water Board staff, the Discharger submitted an amended EFS proposing the installation of additional LFG controls and monitoring wells at the landfill as part of the corrective action program to address VOC impacts to groundwater at the site. The amended EFS included a two-phase plan to address areas of the unit where LFG may be trapped and migrating toward groundwater, including the lower portion of the landfill waste column near MW-3 and under the HDPE barrier layer in the overlap area between LF-1 and LF-2. Phase 1 of the plan would include the installation of 4 additional LFG extraction wells at LF-1, including two wells (GX-10 and 11) screened in the lower portion of the waste column in the western corner of the unit near MW-3 and two wells (GX-12 and 13) screened beneath the HDPE barrier layer in the LF-1/LF-2 overlap area. Existing LFG extraction wells would be vacuum-tested to estimate the radius of investigation for placement of the new wells. Five of the single probe soil gas monitoring wells along the northern and western perimeter of the unit (SG-2, -3, -4, -5 and -11) would be replaced with triple completion wells (SG-2A, -3A, -4A, -5A and -11A) to monitor the effectiveness of the extraction system in reducing VOC concentrations in soil gas. Phase 1 would also include unsaturated zone modelling using EPA’s VLEACH software to evaluate LFG migration in the unsaturated zone and threshold concentrations of specific VOCs in soil gas that represent a threat to groundwater. Based on the results of unsaturated zone modelling and gas probe monitoring data (at least one year), up to 8 additional gas extraction wells would be installed within the LF-1 area, as needed, as Phase 2 of the plan. Central Valley Water Board staff is currently reviewing the amended EFS, but the plan has not yet been approved. CLOSURE AND POSTCLOSURE MAINTENANCE 134. The Title 27 prescriptive standard for landfill final cover includes the following components, from top to bottom: a. Erosion Resistant Layer -- at least one foot of vegetative cover soil; b. Barrier Layer (compositely-lined landfills only) – Geomembrane equivalent to geomembrane in composite base liner. c. Low Hydraulic Conductivity (LHC) Layer -- Minimum one foot of compacted clay