COMPLIANCE INFO_JTD 9/3/2025 - Laserfiche WebLink

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Foothill Sanitary Landfill 17 Department of Public Works/Solid Waste First Semiannual 2025 Groundwater Monitoring ©2025 County of San Joaquin. All rights reserved. 6.4 Leachate Semiannual leachate monitoring results are provided in Table 4. Concentrations were consistent with previous sampling results. The WDRs require monitoring leachate flow rate, pH, and electrical conductivity on a monthly basis (Table 5 and Appendix G). Leachate volumetric data for the period reflects average flow rates. Leachate, along with LFG condensate, is returned to the refuse in accordance with the WDRs for this landfill. The landfill operator has constructed injection points (“dry wells”) in the refuse to receive this leachate. The injection wells are used during the winter months. Beginning in 2023 and continuing into 2025 excess leachate resulting from multiple wet years has been pumped out and applied by water truck over module 2A when weather conditions are conducive to rapid drying. 6.5 Vadose Zone Soil Gas Vadose zone soil gasses are generally measured at soil gas wells approximately 400 feet from the edge of refuse, but not from perimeter migration monitoring probes at the property boundary. Soil gasses were sampled on January 22, 2025 and May 6, 2025 (Appendix I). Samples were collected by first measuring the methane content using a CES-LANDTEC GEM™ 5000 portable gas analyzer. The gas extraction pump of the GEM™ 5000 was used until gas composition readings stabilized, which indicated that the well was purged. After purging, gas samples were collected in stainless steel Summa canisters and sent to Eurofins Air Toxics, Inc. for analysis by EPA method TO-15 (Appendix I). Methane was detected at four monitoring points (perimeter soil gas well SG-01 at 0.2%-vol, and SG-02AS, SG-11AD and SG-11AS at 0.1%-vol) during the January 2025 sampling event and in six monitoring points (SG-1, SG-2AD, SG-6AD, SG-6AI, SG-6AS, and SG-9) during the May 2025 sampling event at concentrations ranging from 0.1 to 0.3%-vol. During January 2025, VOCs were detected in all 10 soil gas samples collected (Appendix I). The highest concentration of any VOC (toluene) was found in probe SG-1 (25,000 ppbv). The greatest number of VOCs (7) were reported in probe SG-2AD. During May 2025, VOCs were detected in 9 of the 10 soil gas samples collected (Appendix I). The highest concentration of any VOC (toluene) was found in probe SG-1 (2,400 ppbv). The greatest number of VOCs (7) were reported in probe SG-6AD. The source of the VOCs in the vadose zone is likely the old unlined portion of the landfill (LF-1). These soil gas wells are installed near the edge of waste and are not considered CCR Title 27 LFG migration monitoring wells. The CCR Title 27-compliant LFG migration monitoring wells (perimeter migration monitoring wells) are located closer to the facility permitted boundary. 6.6 Lysimeters All four lysimeter locations were dry (Table 7) during the first semiannual 2025 monitoring event.