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Mr. Robert McClellon March 7, 2011 Page 5 success of this design is the GCCS. This design has been demonstrated elsewhere including the unlined or clay -only lined areas of the original portions of the Forward landfill (i.e. WMUs B, C, and D-87 through D-88) which contain a LFG collection system. It is our understanding that probes within 50 feet of the unlined or clay -only lined areas containing a LFG collection system have not shown evidence of LFG migration. These observations prove that a properly installed and operating LFG collection system can control LFG migration regardless of the set- back from LFG monitoring probes even in an unlined cell. Historical Setback Evaluation Based on Relocation/Offset of LFG Probes As discussed with Scott Walker of CalRecycle during a January 26, 2011 meeting at the Landfill with you, Lewis Engineering, and Republic personnel, there are no existing mathematical models that can accurately predict an appropriate LFG probe setback. It was agreed at this meeting by all parties that a LFG probe setback could best be estimated by a review of the past performance of LFG probes at the site relative to the proximity to the edge of waste. Many of the past LFG issues at the Landfill have stemmed from exceedences in Probes GP - 11 and GP -12 (see Figure 2). Cornerstone believes that these issues are primarily due to LFG migrating under the lining system from the former unlined Austin Road Landfall. Because of the nature of this LFG migration, it is difficult to evaluate the appropriate setback distance based on the performance of Probes GP -11 and GP -12, and therefore no correlation in this regard has been made herein. However, Probes GP -8, GP -9, and GP -10 are also located near lined waste management units. To address prior EHD comments, they have been relocated further from the lined cell's edge of waste to avoid detecting LFG that may be expected given the proximity to the edge of waste, and therefore within the GCCS radius of influence. The purpose of this is to avoid making risk management decisions based on LFG data that is not indicative of what could be actually migrating from the lined landfill cell. Forward originally installed Probes GP -8, GP -9, and GP -10 sometime between 2000 and 2002. As shown on Figure 2, Probe GP -8 is located southwest of Waste Management Unit (WMU) D-01, GP -9 is located directly west of WMU D-01, and GP -10 is located northwest of WMU D-02 and south of WMU FU -06. All of these probes were relocated in 2006 approximately 30 feet west of their original location due to their proximity of the edge of waste. The probes were relocated to avoid detecting LFG that may be expected given their proximity to the edge of waste and therefore within the radius of influence of the GCCS. A summary of these probes' original and current distance from the edge of waste is shown below in Table 1.