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<br />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 /> 12 <br />since 2016. <br />5.1.3 Statistical Analysis <br />The concentration limits from the WDR/MRP were used in this report and will be used until new <br />interwell concentration limits can be calculated from the new background monitoring well <br />installed during the first semiannual 2022 period. <br />As recommended in the DMP Evaluation submitted in November 2021, new background well <br />G-12 located in the northwest portion of the facility was installed during the first semiannual 2022 <br />period and will be used to update concentration limits after a sufficient number of data points <br />have been collected. Therefore, the current concentration limits calculated from G-1A and <br />provided in the WDR will be used in the interim. <br />5.2 Storm Water <br />During the first semiannual period 2025 period, storm water samples were not collected as no <br />significant storm events were reported. Storm water is monitored according to the site Industrial <br />General Permit. <br />5.3 Surface Water <br />Surface water sampling locations were dry during the first semiannual monitoring event and <br />could not be sampled. <br />5.4 Leachate <br />The WDRs require monthly monitoring of leachate volume, pH, and electrical conductivity. In <br />addition, the County is monitoring any liquid level in the secondary sumps of Areas 3, 4, and 5. <br />Area 1 is not equipped with a secondary sump. Some liquid was reported in secondary sumps at <br />all areas during January 2025. The presence of liquid in the secondary sumps in January 2025 <br />appears to be a reporting error. No liquid was measured in the secondary sumps during February <br />through June. <br />A comparison of the leachate monitoring data to groundwater monitoring data shows that <br />leachate generally contains significantly higher TDS, electrical conductivity, and chloride than <br />groundwater. Leachate is also slightly more acidic than groundwater. <br />Leachate was sampled at collection points L-1, L-3, L-4, and L-5 during January 2025 <br />(Appendix E) and (Table 6). VOCs (some at trace concentrations) were reported in the January <br />2025 leachate samples. The highest VOC concentrations reported at each sample point were <br />acetone in L-1 and L-5, methyl ethyl ketone in L-3, and naphthalene in L-4. <br />VOC results are consistent with past leachate monitoring events. Presence of VOCs in the <br />leachate does not indicate groundwater contamination, but rather the success of the liners at