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North County Recycling Center & Sanitary Landfill Department of Public Works/Solid Waste First Semiannual 2025 Groundwater Monitoring Report ©2025 COUNTY of San Joaquin. All Rights Reserved 8 4 WATER QUALITY PROTECTION STANDARD The Water Quality Protection Standard (WQPS) for groundwater consists of the COCs, the Concentration Limits (CLs) for each COC, the Point of Compliance (POC, and related groundwater monitoring wells), and the Compliance Period. 4.1 Constituents of Concern The COCs include all the waste constituents, their reaction products, and hazardous constituents that are reasonably expected to be in or derived from waste contained in the landfill. The COCs for the landfill are listed in Tables G.1 and G.2 of the WDR Order No. R5-2010-0016 and are presented above in the Compliance Summary. 4.2 Concentration Limits CLs for COCs, including general minerals and dissolved metals generally detected in background, were based on an interwell data analysis method specified below in the Statistical Analysis and Appendix D (Statistical Methods to Determine Concentration Limits). A new replacement background well was installed in the first semiannual 2022 period and will be used to update interwell CLs after a minimum of 8 data points have been collected. CLs for non- statistical COCs, including VOCs, other organic COCs, and dissolved metals not detected in background are equal to the method detection limit (MDL). As indicated in the August 4, 2021 letter from the Regional Board, Staff is comfortable with the Discharger using the established concentration limits shown in Table G.1 of the Monitoring and Reporting Program R5-2010-0016 and that any deviation from these adopted CLs will be a violation of the WDRs. Concentration limits were historically updated annually using data from the background monitoring well, G-1A. For comparison purposes, the concentration limits from the MRP Table G.1 are provided in Table 3. Comparison between the 2016 concentration limits and the MRP concentration limits shows that the limits for pH, dissolved sodium, chloride, and sulfate are very similar. The 2016 concentration limit for dissolved calcium is lower that the MRP concentration limit. The 2016 concentration limits for electrical conductivity, turbidity, dissolved magnesium, dissolved potassium, bicarbonate alkalinity, total alkalinity, nitrate as nitrogen and TDS are higher than the MRP concentration limits and were calculated based on observed background concentrations. The MRP concentration limits will be used for evaluation until new background well G-12, installed approximately 1,000 feet north of currently dry well G-2A, has sufficient data points (a minimum of 8 are needed) to update concentration limits.