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Foothill Landfill - 2 - 1 August 2014 San Joaquin County Review of Soil Gas Probes Soil gas probes SG-1 through SG-11 are located approximately 400 feet from the edge of refuse of Module I. Review of the boring logs and construction details of the soil gas probes indicates they were installed as single probes with 10 feet of well casing. In all probes with the exception of SGA, the boreholes were drilled to depths between 29 and 71 feet bgs and backfilled with 3/8-inch pea gravel to approximately five feet bgs. In some cases, there is 60 feet of permeable borehole open to the vadose zone, which is not being monitored and likely acting as a conduit for gas migration. Gas probe SGA was installed to a depth of 255 feet bgs and is screened from 251 to 253 feet bgs. Depth to groundwater in the vicinity of SGA was measured in well MW-3 at 274.64 feet bgs. Gas probe SGA recorded the highest concentration of VOCs reported during the second half 2013 monitoring event. This indicates that VOCs, which are transported by LFG, are being detected at depths that are within reach of groundwater. During the second semester 2013 monitoring event, methane concentrations were reported in soil gas probes SG-7, SG-9, and SG-10. These probes are located south-southeast and southwest of the landfill. Annual 2013 Report Summary Groundwater flow direction appears to be south-southeast based on the well configuration associated with Module I and Module 1. As groundwater passes beyond the boundary of Module 1, it swings south-southwest. It is staff's understanding that the County is within one year of lining future Module 2. The current detection monitoring system lacks a point of compliance well south-southeast of Module 1 and future expansion Module 2 and therefore is not compliant with WDRs Detection Monitoring Specification E.2 which states: the Discharger shall comply with the detection monitoring program (DMP) provisions of Title 27 for groundwater, surface water, and the unsaturated zone, and in accordance with Monitoring and reporting Program No. R5-2003-0020. A detection monitoring program for a new Unit shall be installed, operational, and one year of monitoring data collected prior to the discharge of wastes. Due to the large vadose zone and historical detections of VOCs detected in the alternate perimeter soil gas probes, there continues to be a release of landfill gas (LFG) from Module I. As discussed above, in some cases there is 60 feet of borehole open to the unsaturated zone. Staff is concerned that higher LFG concentrations are migrating through the unsaturated zone and are not being monitored nor mitigated. Furthermore, according to the Discharger, soil gas probe SG-8 had been destroyed. SG-8 was located directly south of Module 1 and should be replaced to monitor LFG in the unsaturated zone south of future Module 2 in compliance with Specification E.2. Engineering Feasibility Study Based on the evidence of a LFG release from Module I, by 15 October 2014 the Discharger shall submit a revised Engineering Feasibility Study (EFS). The EFS shall include: 1) a LFG containment plan that provides a time schedule such that landfill gas is no longer detected migrating beyond the footprint of Module I,