COMPLIANCE INFO_Q3 2025 MONITORING REPORT 10-30-25

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Quarterly Monitoring Report – Third Quarter 2025 www.arcadis.com Forward Landfill 3Q25 Quarterly Mon Report 7 3.2 Unsaturated Zone Corrective Action The MRP, Section C.2 and Table 18 (RWQCB 2024), requires additional analysis of dissolved inorganic constituents and metals for unsaturated zone lysimeters FU-04, FU-05, FU-06, and FU-10 semiannually (first and third quarters). Lysimeter FU-10 is listed in the MRP’s Unsaturated Zone Corrective Action Monitoring Table (MRP Table 18 [RWQCB 20224]); however, no historical data exist for this location, and it is not included in the previous WDRs Order No. R5-2014-0006 (RWQCB 2014). The inclusion of this sampling location was a typographical error; therefore, the proposed corrective action is for this location to be removed from the MRP in the next revision. All analytical results for the unsaturated zone lysimeters are presented in Tables 5a and 5c. 3.3 Groundwater Extraction Treatment System The groundwater treatment system (GWTS) at the Site has been operating since 1994 to mitigate VOC impacts in groundwater. The GWTS currently operates in accordance with WDR Order No. R5-2003-0080 (RWQCB 2003). The monitoring and reporting of the extraction wells is in accordance with the WDR and MRP Order No. R5-2024- 0002 (RWQCB 2024). The MRP Table 19 (RWQCB 2024) lists six groundwater extraction wells as part of the GWTS (EW-1, EW-2, EW-3R, EW-4, EW-5, and CDCR-EW-1). Five of the six groundwater extraction wells listed in Table 19 of the MRP (EW-1, EW-2, EW-3R, EW-4, and EW-5) are monitored monthly as they are the only extraction wells that are part of an operational remedial system. Extraction well CDCR-EW-1 is currently not part of an operational remedial system and, therefore, is not monitored monthly. Additional offsite extraction wells (CDCR-EW-2, CDCR-EW-3, CDCR-EW-4, and CDCR-EW-5) are not included in the MRP and are not connected to an operational remedial system and, therefore, historically have not been monitored monthly. However, these five offsite extraction wells were added to the routine quarterly monitoring program in second quarter 2025 and the analytical results for third quarter 2025 are included in Tables 3a and 3b. During third quarter 2025, the GWTS was inspected and monitored bi-weekly by Del-Tech during July, August, and September. Operational details are documented in the Quarterly Corrective Action Progress Report – Third Quarter 2025 (Arcadis 2025d) and the Groundwater Treatment System Monitoring Report for third quarter 2025 (Arcadis 2025e). 3.4 Landfill Gas Corrective Action The landfill gas corrective action system is outlined in the MRP, Section C.4 and Tables 20 through 24 (RWQCB 2024). The results of landfill gas monitoring of the landfill gas collection and control system, onsite structures, and perimeter gas probes conducted at the Site during third quarter 2025 are presented in the Quarterly Corrective Action Progress Report – Third Quarter 2025 (Arcadis 2025d). In accordance with the Cleanup and Abatement Order (R5-2017-0703; RWQCB 2017a) and based on exceedances measured during the year, samples were collected on September 16, 2025, for laboratory testing by USEPA Method TO-15.VOC detection results are presented in the Quarterly Corrective Action Progress Report – Third Quarter 2025 (Arcadis 2025d). According to Geosyntec Consultants, the landfill gas collection and control system operated continuously with minimal downtime during third quarter 2025 (Arcadis 2025d).