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1 _7 Ms. Wendy Arano Project No.: 791291 June 7, 2001 Page 6 • The most significant cost burden of new deeper wells is not the installation cost, but the monitoring cost through the postclosure period of the landfill. The monitoring of these new wells will be a drain on the postclosure funding of the landfill. Public funds are better allocated on the operation and maintenance of LFG extraction system and the final cover. The operation and maintenance of these corrective action measures will greatly assure that the landfill no longer is astable source of VOC contamination to both the deep and shallow zones. • Even in the event that a deep monitoring well were to detect migration of the trace levels of VOCs that have been found only in limited instances in the shallow zone, the preferred remedial action would be to install a LFG control system. This step has already been taken. • Further, the potential for this event is extremely unlikely given that the VOCs have never been detected in shallow well MW-4, which are located within 50 feet of waste. This indicates that pressure driving the gases is not high compared to many landfills. The LFG control system, which was just made operational in May 2001,will further reduce these pressures. RECOMMENDATIONS Based on our review of the groundwater and LFG monitoring data, and the corrective action measures taken by the County it is our opinion that a deeper series of monitoring wells is not warranted at the CHSL. We believe that compliance with the requirements of Title 27 Section 20415(b) can be achieved by the existing monitoring points at the landfill. These include the following: • The primary series of shallow wells utilizing MW-4 as an alternative background well. • The LFG probes for monitoring of vadose zone gases in the southwestern portion of the landfill, as well as the other portions of the landfill. • The secondary series of deep wells to continually check for the potential rise of the deep groundwater zone. • The LFG extraction well system to monitor the presence of LFG and fluids within the refuse mass. \\SACRFPi\COMMON\sacDP\2001\PRJ\PJ7\'791291.1 aa.doc (00006000)