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WASTE DISCHARGE REQUIREMENTS ORDER NO. R5-2010-0016 -9- SAN JOAQUIN COUNTY DEPARTMENT OF PUBLIC WORKS NORTH COUNTY LANDFILL SAN JOAQUIN COUNTY years compared to historical averages, a possible impact from carbon dioxide in LFG. 37. A release to groundwater consisting of low to trace concentrations of VOCs, primarily BTEX (benzene, ethylbenzene, toluene, and xylenes) compounds, was confirmed in down gradient well G-4 in July 2002. Subsequent evaluation monitoring found that the VOC impacts to groundwater did not extend down gradient to well G-6. By January 2003, maximum VOC impacts at G-4 had attenuated to trace concentrations. The EMP concluded that the VOC impacts were sporadic and likely associated with LFG. No VOCs have been detected in any of the wells since 2006 Corrective Action 38. In 2006, the Discharger installed an LFG extraction system in accordance with a May 2005 Corrective Action Plan (Corrective Action Plan For the Prevention of Future Groundwater Impact by Landfill Gas at the North County Recycling Center and Sanitary Landfill). The system (which became operable in June 2007) was intended to control off-site migration of landfill gas and to address concerns regarding LFG as a suspected source of VOCs sporadically detected in groundwater. The system included installation of 7 vertical extraction wells in existing modules M1 and M3; 2 LCRS risers between M1 and M3; horizontal wells installed in collection trenches in expansion module M4; and associated collection system piping, condensate handling facilities, blowers and a flare station. The plan anticipated that future modules would be constructed with LFG extraction facilities similar to Module 4 and tied into the system. 39. Based on the two semiannual monitoring events since startup of the LFG collection system in 2007, lower LFG constituent concentrations have been detected in interior well SG -6 compared to prior to system startup, including methane (<30% by volume), and carbon dioxide (<30% by volume) and VOCs (generally <100 ppbv). See Finding 29. LANDFILL OPERATIONS 40. Refuse is spread and compacted in approximately two -foot lifts until 14 feet above surrounding refuse, as determined by laser. The top slope of the working face is usually graded (also by laser) to a 4% minimum slope for drainage. 41. The discharger uses onsite borrow for daily and intermediate cover soil. Tarps are also employed as alternative daily cover (ADC). Cover soil is obtained from excavation of the next module and/or from existing onsite stockpiles. A refuse to soil ratio of approximately 5:1 is maintained for daily cover, which is applied at the working face in 6 -inch minimum lifts. Intermediate cover of 12 -inches minimum thickness is placed in areas that will be inactive for at least 180 days per Title 27, is CCR Section 20705.