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WASTE DISCHARGE REQUIREMENTS ORDER NO. R5-2010-0016 SAN JOAQUIN COUNTY DEPARTMENT OF PUBLIC WORKS NORTH COUNTY LANDFILL SAN JOAQUIN COUNTY 101 Landfill Gas 28. In 2003, the Discharger installed a methane migration monitoring system, as required by the Local Enforcement Agency under Title 27 solid waste regulations (Section 20919 et seq.). Previous WDRs required that the system also be used to monitor soil gas for volatile organic compounds (VOCs). The system has since been expanded and presently includes 17 triple completion monitoring wells (i.e., SGs-1, 2, 3, and 8 through 22) installed along the site perimeter with nested probes screened in the upper, intermediate, and lower portions of the unsaturated zone. In addition to methane migration monitoring system wells, the site includes four singly completed soil gas monitoring wells (SGs-4, 5, 6 and 7) historically installed at the site along the interior perimeter of landfill modules M1 and M3 and screened in the upper portion of the unsaturated zone. Section E.1 of Monitoring and Reporting Program (MRP) No. R5-2010-0016 specifies the monitoring points and parameters for soil gas monitoring under this Order. 29. Subsequent LFG monitoring showed generally less than 1.0% methane by volume in all probes, except those within the site interior immediately adjacent to the landfill (i.e., not perimeter migration monitoring wells) where maximum methane concentrations ranged from 9.5% by volume in SG -2S to 59.7% by volume in SG - 6, and carbon dioxide was detected up to 44.6% in SG -6. Fewer VOCs and lower VOC concentrations (generally less than 100 ppbv or non -detect) were similarly detected in the site perimeter wells compared to SG -6, where, for example, detected VOCs (e.g., Freon 12) were generally greater than 100 ppbv, but less than 1,000 ppbv. Since initiation of LFG extraction in June 2006 (see Finding 38), the concentration of methane in well SG -6 has been reduced to less than 2% by volume. 30. The unsaturated zone monitoring system consists of suction lysimeters installed beneath the landfill modules during their construction. There are currently eight lysimeters (VZs-1 through 8) installed at the locations shown in Attachment C. Lysimeter monitoring prior to 2000 showed elevated concentrations of general minerals and low concentrations of several VOCS in pore water at Module 1, such as follows: Module 1 Lysimeter Monitoring Results, January 1995 Constituent General Minerals, mg/L Chloride Bicarbonate Alkalinity Total Dissolved Solids (TDS) Concentration Background_ Detection VZ -1 Vim_ 25 500 63 380 750 2,100 L_J