COMPLIANCE INFO_JTD 9/3/2025 - Laserfiche WebLink

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Foothill Sanitary Landfill 16 Department of Public Works/Solid Waste First Semiannual 2025 Groundwater Monitoring ©2025 County of San Joaquin. All rights reserved. additional extraction wells in the vicinity of well MW-3 (see Section 2.1 for a summary of corrective actions completed). The County suspects that the periodic detection of VOCs at MW-3 are related to the placement of a gas-tight HDPE membrane over Module “I” refuse (see Sections 2.1 and 2.2). Placing a plastic cover over unlined refuse can reduce natural venting of LFG and increase transport of VOC-containing LFG through the vadose zone towards the water table. The VOC detections, which started approximately five (5) years after installing the HDPE membrane over refuse, is consistent with the lag time experienced for other actions at this landfill as noted above. Therefore, the additional LFG extraction efforts of the 2016 installation of four (4) additional LFG extraction wells in LF-1 were expected to take several years to show positive effects in the reduction of groundwater or vadose zone impacts. The number and the concentration of VOCs detected at MW-3 have declined since second quarter 2017. VOCs have not been detected at MW-3 since the Second Quarter 2021, indicating the effectiveness of the additional LFG extraction wells. As recommended in the Second Semiannual and Annual 2021 Groundwater Monitoring report, VOCs have not been detected since Second Quarter 2021, therefore, a reduction of sample frequency of MW-3 to semiannually should be considered. 6.2.3 5 Year COC Sampling Groundwater samples were analyzed for the 5-Year COCs in the second semiannual period of 2024. The 5-Year COCs did not suggest that the water from the monitoring wells had been impacted by landfill activities. No VOCs, SVOCs, organophosphorous pesticides, or chlorinated herbicides were detected for the samples collected. 6.3 Surface Water Surface water sample locations were reported as dry and therefore not sampled during the first or second semiannual periods of 2025 (Table 3). Surface water samples were last collected on February 4, 2019. Surface water samples are collected upstream and downstream from the creek flowing along the eastern property boundary when water is flowing at these two points. In general, historical results indicate lower concentrations are generally found in the SW-2 sample (downgradient) than in the SW-1 sample (upgradient). No VOCs, SVOCs, chlorinated herbicides, or organophosphorus compounds were detected in the last 5-year COC sampling event completed in February 2019. Note that surface water entering the creek on landfill property includes water flowing across land on the other side of the creek. Livestock grazing on that property can affect the constituents of the downstream sample.