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Mr. Kevin Basso -2- 14 May 2003 4. Te French Camp Landfill Responses to Regional Waster Quality Control Board Letter addresses Comments 6 (evaluate the groundwater monitoring system) and 7 (VOCs in landfill gas)in Staff's comments to the Final Closure Plan, French Camp Landfill. The evaluation of tie monitoring system continues work that was started when the City of Stockton installed new `bnitor wells to replace and/or supplement existing wells. The document recommends that old monitor wells MW-3 and—4 should be abandoned, MW-2 should be used to monitor water levels on y, and five staff gages should be installed in French Camp and Walker Sloughs to monitor surface water levels. Staff concurs with the abandonment of MW-3 and—4 and with the installation of the staff gages. MW-2 should continue to be monitored. These changes will be included in revised Waste Discharge Requirements when a Joint Technical Document for closure of the landfill has been approved. 5. The landfill gas investigation involved sampling and analysis by TO-15 of twelve vadose zone gas probes. The investigation detected ni1rVOCs in what are apparently background probes, which are believed to represent releases from sources other than the landfill. Fifteen VOCs were detected in downgradient probes. Predicted VOC concentrations in water calculated from observed concentrations in probes within the landfill footprint and Henry's law constants are generally greater than method detection limits. The intermittent VOC detections in MW-2 (four times since 1989) are likely the result of LFG. Conversely,predicted VOC concentrations in water calculated from observed concentrations in probes downgradient of the landfill are generally below method detection limits. VOC concentrations in groundwater detected to date are low level, intermittent and may have multiple sources. 6. Staff concludes from the data presented that VOCs in landfill gas are currently causing low-level impacts to groundwater beneath the landfill footprint. VOC concentrations in gas and water are sufficiently low that impacts rapidly diminish through dilution at or near the landfill boundary. Staff is concerned that the threat to water quality will increase when a final landfill cover is installed. Forward should develop a preliminary plan to capture and control landfill gas to be implemented if landfill gas production increases after closure. The plan should include a landfill gas sampling plan for the post-closure period and propose triggers to implement gas control measures. The proposed follow up sediment sampling should be completed; a description of exactly what is intended by selective removal of waste outside the landfill footprint should be submitted for Board (and other interested agencies)review; Staff will make changes to the groundwater monitoring requirements as noted above; and a preliminary plan to control landfill gas with triggers to implement should be submitted. Forward should also submit a Joint Technical Document per Title 27 Section 21585 for closure of the French Camp Landfill.