COMPLIANCE INFO_REV.4TH QTR-ANNUAL REPORT 6/26/2025

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Revised Quarterly Monitoring Report – Fourth Quarter/Annual 2024 www.arcadis.com Revised - Forward Landfill 4Q24 Quarterly Mon Report 06262025 3 laboratory practical quantitation limit (PQL); estimated (J-flagged) VOC data between the PQL and the laboratory method detection limit (MDL) are also included to evaluate exceedances. Cumulative data for all groundwater monitoring points and constituents for 2024 are included in Appendix D. The isoconcentration contours for tetrachloroethene (PCE), trichloroethene (TCE), cis-1,2-dichloroethene (cis-1,2- DCE), carbon tetrachloride, and chloroform in shallow wells are shown on Figures 9, 10a, 10b, 11, 12, and 13, respectively. The isoconcentration contours for PCE, TCE, cis-1,2-DCE in intermediate wells are shown on Figures 14, 15, and 16, respectively. The isoconcentration contours for PCE and TCE in deep wells are shown on Figures 17 and 18, respectively. A total VOC isoconcentration map for fourth quarter 2024 is included as Figure 19. The following is a summary of the fourth quarter 2024 monitoring results:  Non-statistical data analysis: The California non-statistical data analysis method (CNSDAM) was performed for VOCs detected in background samples less than 10 percent (%) of the time. Under the CNSDAM, a monitoring well is to be resampled during the following quarter when concentrations of two or more VOCs exceed their respective MDLs, or the concentration of one or more VOCs equals or exceeds its respective PQL. Tables 3a through 3c present well locations with VOC concentrations exceeding the applicable MDLs or PQLs during fourth quarter 2024.  Concentration limit (CL) thresholds. CLs were recalculated in fourth quarter 2024. In accordance with the MRP (RWQCB 2024a), the routine inorganic constituent monitoring data for the fourth quarter 2024 monitoring period were compared to CLs established for the wells, as presented in Table 4. As shown in Table 4, there were no exceedances of the CLs for inorganic constituents during fourth quarter 2024. Details on the statistical analysis for the established CLs are presented in Appendix E-1.  PCE and TCE. The monitoring results suggest that the PCE and TCE plumes in the shallow zone (Figures 9 and 10a) and intermediate zone (Figures 14 and 15) both continue to remain separated between Forward Landfill and the CDCR property. Additionally, the monitoring wells installed at the CDCR property in August 2023 (AMW-58S, AMW-58M, AMW-59S, and AMW-59M) and in second and third quarters 2024 (AMW-60S, AMW-60M, AMW-61S, and AMW-61M) continue to define and monitor the distribution of carbon tetrachloride (Figure 12) and chloroform (Figure 13) near monitoring well AMW-22S. Based on the fourth quarter 2024 monitoring results, the elevated carbon tetrachloride and chloroform concentrations historically observed in monitoring well AMW-22S appear to be limited in extent, but have now been defined to extend locally within the shallow and into the intermediate zone with migration hydraulically downgradient. The wells with VOC detections (Tables 3a through 3c) were identified for resampling in first quarter 2025 as required by MRP Section B.4.h (RWQCB 2024a). The RWQCB was notified of these VOC detections in a letter dated January 30, 2025 (Forward 2025c). 2.1.4 Quality Assurance/Quality Control Results The quality assurance (QA)/quality control (QC) samples collected at the Site during fourth quarter 2024 and the associated analytical results are described in this section. QA/QC samples at the Site included 10 trip blanks, two field blanks, two equipment blanks, laboratory method blanks, and two duplicate samples. The method blanks were analyzed for all the analytes included in the MRP