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Taj Bahadori Page 2 of 6 July 6, 2018 release from the unit. The RWQCB also stated in the NOV that volatile organic compounds (VOCs) continue to be detected in groundwater well MW -3 and continue to be detected in vadose zone monitoring devices. Based on our discussion with you, it appears that the County acknowledges that unlined Module I is the source of VOCs in the groundwater and the vadose zone at FSL. It also appears that the County acknowledges that landfill gas (LFG) migrating from the base of Module I is the source of VOCs in the groundwater and the vadose zone. VOCs have been consistently detected at the following locations as shown of Figure 1: Groundwater monitoring well MW -3 Single completion probe soil gas devices SG -1, -4, -5, -7, & -9 Triple completion probe soil gas devices SG -2A, -3A, -6A, -10A, & -I IA Figure I shows that the soil gas probe devices form a relative tight monitoring network around Module I. In addition, the triple probe devices show that VOCs are present all the way down to groundwater at all triple probe locations. Because VOCs were detected all the way to groundwater at every location that a single shallow probe was replaced with a triple probe, it is highly likely that at the remaining single probe locations, VOCs have also migrated all the way down to groundwater. Deep probe location SG -I is paired with monitoring well MW -3, which contains VOCs. In addition, it is highly likely that at every probe location, whether or not it is a single or triple probe, VOCs have migrated into groundwater. Therefore, the County may want to consider acknowledging that it is likely at each probe location, groundwater may be impacted by VOCs that have migrated from the base of unlined Module I. Based on this conservative, but highly likely interpretation of VOC impact, APTIM does not believe that a new monitoring well near SG -6 would provide any new information on the extent of impact from Module I. APTIM believes that the County's resources are better -allocated on the installation of more LFG extraction wells. As such, APTIM recommends the County install three (3) to five (5) new LFG extraction wells in lieu of installing a groundwater monitoring well near SG -6. The only mechanism to remove the LFGNOC impact from the vadose zone and groundwater is to extract more LFG from Module I. Re -install Background Well MW -4 The RWQCB stated in the NOV that the DMP requires a background well that can provide consistent groundwater samples to determine if up -gradient water quality is changing. Background well MW -4 has been dry for two consecutive years. The last water level measurement taken in MW -4 was during the fourth quarter 2015 monitoring event. APTIM understands that the County will install a new background well MW -4A, adjacent to MW -4. MW -4A will be used to determine if up -gradient water quality is changing for 2018-07-16 FH Workplan