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Water Board Question: The EFS must provide a plan for defining the extent of contamination in all zones affected by the release (groundwater and landfill gas). County Response: The five (5) deep monitoring wells installed in Phase 1 (Appendix B, Figure 3) will be sampled to determine the extent of VOCs in the vadose zone. The deep probes of these wells will be monitored following the installation of four (4) new wells in Phase 1 (Appendix B, Figure 1), and following installation of additional wells if necessary in Phase 2 (Appendix B, Figure 2). Groundwater monitoring wells will continue to be monitored semi-annually in accordance with the current monitoring program, with the exception of MW -3 which will be monitored quarterly. Water Board Question: The EFS must propose and update the MRP, such that staff is capable of evaluating compliance with the WDRs. County Response: The County will begin monitoring MW -3 quarterly, no additional updates to the MRP are deemed necessary at this time. The County will re-evaluate the MRP after the conclusion of Phase 1 construction and monitoring results. Water Board Question: Please also review CCR Title 27 Section 20425(e)(2) which states: the discharger shall propose for approval by the RWQCB a list of Monitoring Parameters for each medium (ground water, surface water, and the unsaturated zone) to be monitored pursuant to §20415. The list for each medium shall include all hazardous constituents that have been detected in that medium and those physical parameters, waste constituents, and reaction products that provide a reliable indication of changes in water quality resulting from any release from the Module to that medium. For MSW landfills, the list of Monitoring Parameters must also meet the federal requirements incorporated by reference into SWRCB Resolution No. 93-62 [see §258.54(x) and §258.55(a) of 40CFR258]. The RWQCB shall specify each list of Monitoring Parameters in the WDRs after considering the following factors: What changes is the County proposing? County Response: The County will be able to better assess any necessary changes following the completion of the field testing proposed in this revised EFS. To help in the assessment of magnitude of potential impact of vadose zone gas on groundwater quality, the County proposes to conduct the following vadose zone modeling. Foothill Sanitary Landfill 13 Department of Public Works, Solid Waste Division Revised Engineering Feasibility Study County of San Joaquin — December 19, 2014