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Mr.Todd Del Frate 0 -2- 0 GAS -PERMEABLE 2 - GAS -PERMEABLE BARRIER OVER REFUSE AT FOOTHILL LANDFILL Base liners must retain integrity against up to a foot of hydraulic head, and protect the groundwater below the landfill from contamination. Closure layers are designed to shed water away from the refuse beneath to minimize water intrusion. Some have required that the HDPE fabric now over refuse of Module "I" meet the same criteria as a base liner over native soil per Section 20330, the same requirements met by the liner under Module 1. Breaching the base liner under Module 1 would be serious and significant, and would require repair welding and associated Construction Quality Assurance and documentation. We would avoid such deliberate penetration at all costs because of the potential for future contamination of the water table beneath the landfill, but I cannot imagine a situation where it would be needed. However, we will need to breach the separation layer between old and new refuse at Foothill occasionally, perhaps regularly, to maintain buried components of the landfill gas system as the landfill settles. The well head of one other well (GX-4) is also beneath the HDPE fabric, and I expect that we will also need eventually to break through the HDPE at that well head to repair it. In addition, the developer of the LFG-to-energy facility at the Foothill Landfill has requested permission to install additional wells through this fabric. I have reluctantly denied that request because of the possible reaction by RWQCB staff to this idea, but I would much rather allow additional wells to increase LFG collection from the older refuse below the HDPE. Even more important than dealing with the existing HDPE fabric at Module "I" is possible significant negative consequences of requiring a gas-tight barrier between old and new refuse in the future. The development plan for Foothill landfill includes the overlay of future refuse on all sides of Module "I" such that all surfaces except for the top deck portion would be overlain. Experience at the Beale Air Force Base Landfill has shown that preventing air from entering refuse can produce anaerobic environment when an aerobic environment would normally exist. Experience at landfills with a HDPE closure cap has shown that the application of an air -tight barrier over refuse can cause the pressurization of the refuse beneath the fabric by landfill gas (Photo attached). Such pressurization would certainly increase the potential for groundwater impact. The photo shows a situation where small pressures lift the HDPE fabric. Imagine how great the pressure could be if the fabric itself was under tens of feet of additional refuse, and how much landfill gas would be forced into the soil beneath the landfill. I believe we both want to avoid this situation. I understand the importance of protecting water quality beneath the unlined refuse by minimizing leachate; however, other landfills have used other separation methods for this purpose. The Neal Road landfill in Butte County separates old refuse from new with a two -foot thick layer of miscellaneous soil. I understand that the Forward Landfill separates old refuse in an unlined area from overlaid refuse with a barrier equivalent to a closure layer. So it appears that precedents exist within the San Joaquin Valley Region for gas -permeable separations between old and new refuse. In light of the potentially serious negative consequences of an air -tight barrier over refuse, we propose that a separation such as the solid layer at Neal Road Landfill would be sufficient to reduce infiltration of water into the refuse beneath the separation, as long as that layer can be sloped to drain into an active leachate collection and recovery system (LCRS).