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North County Recycling Center & Sanitary Landfill Department of Public Works/Solid Waste <br />First Semiannual 2025 Groundwater Monitoring Report ©2025 COUNTY of San Joaquin. All Rights Reserved <br />9 <br />4.3 Point of Compliance <br />The POC is the vertical surface located at the hydraulically downgradient limit of the landfill that <br />extends through the uppermost aquifer underlying the landfill. Due to groundwater flow direction <br />fluctuations, a traditional upgradient to downgradient data comparison is not recommended. The <br />POC groundwater monitoring wells consist of the following eight downgradient (and cross- <br />gradient) wells: <br />G-5B <br />G-7 <br />G-8 <br />G-9 <br />G-10 <br />G-11 <br />G-13 <br />G-14 <br />4.4 Compliance Period <br />The compliance period is equal to the active life of the landfill plus the closure period. <br />4.5 Statistical Analysis of Groundwater Data <br />The NCRC&SL has been approved to perform interwell tolerance limits (well G-1A initially served <br />as the background well). Concentration limits were updated annually using interwell tolerance <br />limit methods, and were most recently updated during the second semiannual period of 2016. <br />Analytical results from this monitoring event were compared to the 2016 concentration limits that <br />were developed from G-1A and the WDR concentration limits. Due to dropping water levels at <br />the site, which caused G-1A to go dry in 2016, well G-10 had been proposed as a background <br />replacement well for G-1A. However, based on groundwater flow direction evaluation provided <br />in the November 2021 DMP Evaluation, the predominant flow direction appears to be towards <br />the east or southeast and a new proposed background well (G-12) was installed in the northwest <br />portion of the facility during the first semiannual 2022 period. This new background well will be <br />used for interwell concentration limit updates after a minimum of 8 data points have been <br />collected. In the interim period, until sufficient data points are available for a concentration limit <br />update, the MRP concentration limits will be used. See Appendix D—Statistical Methods to <br />Determine Concentration Limits and Appendix E—Tabular Summary of Historical Monitoring <br />Data. <br />Detection monitoring wells on northern side of the landfill consist of G-7 and G-8. Detection <br />monitoring wells on western side of the landfill consist of G-5B and G-9. Detection monitoring <br />well G-11 is on southern side of the landfill. Detection monitoring well G-10 is on eastern side of <br />the landfill.