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WASTE DISCHARGE REQUIREMENTS ORDER NO. R5-2010-0016 -6- <br />SAN JOAQUIN COUNTY DEPARTMENT OF PUBLIC WORKS <br />NORTH COUNTY LANDFILL <br />SAN JOAQUIN COUNTY <br /> <br /> <br /> <br />Landfill Gas <br />28. In 2003, the Discharger installed a methane migration monitoring system, as <br />required by the Local Enforcement Agency under Title 27 solid waste regulations <br />(Section 20919 et seq.). Previous WDRs required that the system also be used to <br />monitor soil gas for volatile organic compounds (VOCs). The system has since <br />been expanded and presently includes 17 triple completion monitoring wells (i.e., <br />SGs-1, 2, 3, and 8 through 22) installed along the site perimeter with nested <br />probes screened in the upper, intermediate, and lower portions of the unsaturated <br />zone. In addition to methane migration monitoring system wells, the site includes <br />four singly completed soil gas monitoring wells (SGs-4, 5, 6 and 7) historically <br />installed at the site along the interior perimeter of landfill modules M1 and M3 and <br />screened in the upper portion of the unsaturated zone. Section E.1 of Monitoring <br />and Reporting Program (MRP) No. R5-2010-0016 specifies the monitoring points <br />and parameters for soil gas monitoring under this Order. <br /> <br />29. Subsequent LFG monitoring showed generally less than 1.0% methane by volume <br />in all probes, except those within the site interior immediately adjacent to the <br />landfill (i.e., not perimeter migration monitoring wells) where maximum methane <br />concentrations ranged from 9.5% by volume in SG-2S to 59.7% by volume in SG- <br />6, and carbon dioxide was detected up to 44.6% in SG-6. Fewer VOCs and lower <br />VOC concentrations (generally less than 100 ppbv or non-detect) were similarly <br />detected in the site perimeter wells compared to SG-6, where, for example, <br />detected VOCs (e.g., Freon 12) were generally greater than 100 ppbv, but less <br />than 1,000 ppbv. Since initiation of LFG extraction in June 2006 (see Finding 38), <br />the concentration of methane in well SG-6 has been reduced to less than 2% by <br />volume. <br /> <br />30. The unsaturated zone monitoring system consists of suction lysimeters installed <br />beneath the landfill modules during their construction. There are currently eight <br />lysimeters (VZs-1 through 8) installed at the locations shown in Attachment C. <br />Lysimeter monitoring prior to 2000 showed elevated concentrations of general <br />minerals and low concentrations of several VOCS in pore water at Module 1, such <br />as follows: <br />Module 1 <br />Lysimeter Monitoring Results, January 1995 <br />Constituent Concentration <br /> Background Detection <br />General Minerals, mg/L VZ-11 VZ-3 1 <br />Chloride 25 500 <br />Bicarbonate Alkalinity 63 380 <br />Total Dissolved Solids (TDS) 750 2,100