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Quarterly Monitoring Report— First Quarter 2026 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, carbon tetrachloride, and chloroform in intermediate wells are shown on Figures 14, 15, 16, 17, and 18 respectively. The isoconcentration contours for PCE and TCE in deep wells are shown on Figures 19 and 20, respectively. The following is a summary of the first quarter 2026 monitoring results: • Non-statistical data analysis:The California non-statistical data analysis method (CNSDAM)was used 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 concentrations of one or more VOCs equals or exceeds their respective PQLs. Table 4 presents detection and upgradient program wells (MRP Section B.1(a)Table 1 [RWQCB 2024])with VOC concentrations exceeding the applicable MDLs during first quarter 2026. There were two MDL exceedances in the detection and upgradient program wells during the first quarter 2026 monitoring period, at MW-13A for 1,1-dichloroethane (0.36 J micrograms per liter[pg/L]) and cis-1,2-DCE (0.15 J pg/L). • Concentration limit(CL) thresholds. CLs were recalculated in fourth quarter 2024. In accordance with the MRP (RWQCB 2024), the routine inorganic constituent monitoring data for the first quarter 2026 monitoring period were compared to CLs established for the wells. Table 4 presents the CLs established for detection and upgradient program wells (MRP Section B.1(a)Table 1 [RWQCB 2024]) and inorganic constituent concentrations. There was one exceedance of the CLs for inorganic constituents during first quarter 2026, at monitoring well MW-26 for pH (8.10 standard units [s.u.]). • 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) continue to remain separated between the Site and the California Department of Corrections and Rehabilitation (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-61 S, and AMW-61 M) continue to define and monitor the distribution of carbon tetrachloride (Figure 12) and chloroform (Figure 13) near monitoring well AMW-22S. Based on the first quarter 2026 monitoring results, the elevated carbon tetrachloride and chloroform concentrations historically observed in monitoring well AMW-22S appear to be limited in extent. However, the concentrations extend locally within the shallow zone and have migrated vertically and laterally into the intermediate zone from monitoring well AMW-22S (Figures 17 and 18), following the northeast groundwater flow direction. The specified detection and upgradient program wells (MRP Section B.1(a)Table 1 [RWQCB 2024])with VOC detections and/or CL exceedances (Table 4)were identified for resampling in second quarter 2026 as required by MRP Section B.4.h (RWQCB 2024). The RWQCB was notified of this exceedance in a letter dated April 30, 2026 (Forward 2026). 2.1.4 Quality Assurance/Quality Control Results The quality assurance (QA)/quality control (QC) samples collected at the Site during first quarter 2026 and the associated analytical results are described in this section. www.arcadis.com Forward Landfill IQ26 QMR 3