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• The pea gravel-filled borehole does not act as a conduit for deeper migration of soil gases. • Monitoring probes do not need to extend through the entire vadose zone. This is because VOCs detected in probes that are either installed to shallow depths or deep right above groundwater are not good indicators of impending VOC impact to groundwater. • Pairing up vadose zone and groundwater monitoring points provide useful geologic data on the potential for impending groundwater impact. All of the existing vadose zone ' monitoring points which detected VOCs in soil gas are either shallow or deep. However, the two locations that were paired with groundwater monitoring wells (SG-1/MW-3 and SG-9/MW-2R), indicated that groundwater impact only occurred very close to the waste mass at the SG-1/MW-3 location. Proposed Vadose Monitoring Network: Based on the above information, the County proposes that the Vadose Zone Monitoring Network for FSL consist of the following seven (7) probes in the existing monitoring wells. The deep probe is proposed for VOC monitoring where three probes are installed: • SG-117 (deep probe) a 3-probe well, located to the east of Area 2 and a replacement for SG-7. • SB-118 (deep probe) a 3-probe well, located to the east of Area 2 and in the general vicinity of the LFG flare station. • SG-119 (deep probe) a 3-probe well, located along the southern limit of Area 2, a replacement for SG-8, and paired with new monitoring well MW-6. i • SG-120 (deep probe) a 3-probe well, located at the southwestern corner of Area 2 and adjacent to monitoring well MW-5. • SG-9 a single probe well, located to the west of Area 2 and adjacent to monitoring well MW-2R. • SG-1 a single probe well, located to the west of Area"I" and adjacent to monitoring well MW-3. • SG-3 a single probe station, located to north of Area"I". ' This network has the following attributes: • They monitoring the Point of Compliance for Areas "I", 1, and 3. As additional Areas are constructed, the deep probes of the existing 3-probe wells will be added to the monitoring program. • Section 20415 (d) (2) of Title 27 requires the installation of a sufficient number of soil pore monitoring points. These seven (7) locations are sufficient to track the escape of LFG from the waste mass. Foothill Sanitary Landfill 7 Department of Public Works/Solid Waste Engineering Feasibility Study County of San Joaquin—August 2014