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sites. If uncorrectable exceedances within the 10-day limitation are detected or emissions are discovered during an inspection by Regulatory Agencies,the landfill must perform monitoring on a 25-foot pathway on a quarterly basis for active disposal sites. Upon completion of four consecutive SEM events without an uncorrectable exceedance of the 25 ppmv or 500 ppmv standards, other than non-repeatable momentary readings, the landfill may perform the monitoring on a 100-foot spacing on an annual basis for closed landfills or quarterly for active disposal sites. In accordance with the provisions of the LMR, the monitoring of the landfill will be done quarterly on a 25-foot pathway based on a regulatory inspection during April 2022, in which exceedances were observed. Note that the subsequent monitoring since the inspection has shown no uncorrectable exceedances. Finally,to help prevent potential future exceedances, SCS recommends that the landfill surface be routinely inspected and any observed surface erosion be routinely repaired. BACKGROUND The North County Landfill is an active organic refuse disposal site. By way of background, organic materials buried in a landfill decompose anaerobically(in the absence of oxygen) producing a combustible gas that contains approximately 50 to 60 percent methane gas, 40 to 50 percent carbon dioxide, and a trace amount of various other gases, some of which are odorous. The North County property contains a system to control the combustible gases generated in the landfill. The gases produced in a landfill will either vent vertically to the atmosphere or migrate horizontally through subsurface soil to locations on adjacent properties. At higher concentrations, methane is flammable. However, the presence of methane gas in site soil does not mean there is an immediate threat of explosion because flames cannot typically propagate through the soil. SURFACE EMISSIONS MONITORING On November 11, 2025,the instantaneous and integrated SEM was performed over the surface of the subject site. The intent of the monitoring was to identify any specific locations or areas of the landfill surface with organic compound concentrations exceeding the LMR threshold limit values of 500 ppmv measured as methane for instantaneous monitoring, or an average methane concentration of 25 ppmv for the integrated monitoring in the 50,000 square foot grids as required under the LMR. During this event, SCS performed the monitoring on a 25-foot pathway in accordance with the rules as required. EMISSIONS TESTING INSTRUMENTATION/CALIBRATION The instruments used to perform the landfill surface emission testing consisted of the following: • Thermo Scientific TVA 2020 portable Flame Ionization Detector(FID). This instrument measures methane in the air over a range of 1 to 50,000 ppmv. The TVA 2020 meets the State of California Air Resources Board (CARB) requirements for combined instantaneous and integrated monitoring and was calibrated in accordance with the United States Environmental Protection Agency(US EPA) Method 21. • Weather Anemometer with continuous recorder for meteorological conditions in accordance with the LMR. Instrument calibration logs and weather information are shown in Attachments 5 and 6. North County Landfill—Fourth Quarter 2025 www.scsenaineers.com 2