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® GAVIN NEWSOM C'CJVFHNOH Water Boards ENV E GARCIA ENVIRONMENTAL PROTECTION Central Valley Regional Water Quality Control Board 8 January 2023 Mr. Mark Houghton Senior Civil Engineer San Joaquin County Department of Public Works 1810 E. Hazelton Avenue Stockton, CA 95201 REVIEW OF EVALUATION MONITORING PROGRAM AND WELL INSTALLATION WORK PLAN, CORRAL HOLLOW LANDFILL, SAN JOAQUIN COUNTY The Corral Hollow Landfill is owned and maintained by San Joaquin County Department of Public Works (Discharger) and is regulated by Waste Discharge Requirements (WDRs) Order R5-2013-0013 for Post-Closure Maintenance and Corrective Action. The Discharger is also required to comply with Cease and Desist Order (CDO) R5-2013-0024 for progressive corrective actions. The current corrective action measures employed at the Corral Hollow Landfill consists of landfill gas (LFG) extraction. The LFG extraction system is intended to control the source at the compliance boundary and keep volatile organic compounds (VOCs) from entering groundwater and further degrading groundwater. Evaluation Monitoring Program During routine third quarter 2023 groundwater sampling and analysis of deep aquifer well MW-12, one volatile organic compound (VOC) trichlorofluoromethane (TCFM) was detected above the practical quantitation limit (PQL). To verify the exceedance, the Discharger initiated a retest of well MW-12 in accordance with the WDRs. The retest verified the exceedance above the PQL, which triggered the requirement to establish an Evaluation Monitoring Program in accordance with the WDRs and Title 27, Section 20420 (k)(5). In accordance with the WDRs and Title 27, the Discharger submitted an Amended Report of Waste Discharge and Evaluation Monitoring Program (EMP) Report submitted 6 November 2023. The EMP proposed focuses on the deep aquifer VOC detections that include installing one deep groundwater well downgradient of well MW-12. The proposed well will be installed to evaluate TCFM concentrations in the deep aquifer. The shallow aquifer monitoring system of concern consists of wells MW-5, -8, -9A, -10A, and -11A. Wells MW-5, -8, and -9A have reported consistent detections of VOCs, including TCFM. Shallow wells MW-9A through MW-11A were installed offsite to delineate the VOC plume. Over time, total VOC concentrations in these wells have remained consistent or decreased in concentration. Well MW-9A reported TCFM concentrations in July and September sampling at 0.66 pg/L and 0.62 pg/L, respectively. Since the shallow aquifer well field has been MARK BRADFORD, CHAIR I PATRICK PULUPA, ESQ., EXECUTIVE OFFICER 11020 Sun Center Drive#200, Rancho Cordova, CA 95670 1 www.waterboards.ca.gov/centralvalley