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Quarterly Monitoring Report— First Quarter 2026 QA/QC samples at the Site included 25 trip blanks, two field blanks, 10 equipment blanks, laboratory method blanks, and two duplicate samples. The method blanks were analyzed for all the analytes included in the MRP (RWQCB 2024); equipment, field, and trip blanks were analyzed for VOCs by United States Environmental Protection Agency(USEPA) Method 8260. Styrene was measured in four of the trip blanks at concentrations of 0.16 J pg/L, 0.12 J pg/L, 0.24 J pg/L, and 0.15 J pg/L. Ethanol was measured in two of the trip blanks at concentrations of 120 J pg/L and 130 J pg/L. No VOCs were measured in any field blanks. TCE was measured in one equipment blank, at 0.13 J. No VOCs were measured in the laboratory method blanks analyzed with the groundwater samples. Inorganic constituents (chloride, nitrate as N, sulfate, and dissolved copper)were measured at estimated trace concentrations in one or more of the laboratory method blanks analyzed with the groundwater samples. Several samples contained similar reported concentrations of these constituents, which are considered suspected laboratory contaminants. Results from the duplicate groundwater samples collected from monitoring wells AMW-34S and AMW-36S are presented in Table 3a along with the primary results for the wells. Primary and duplicate results for both wells indicate a relative percent difference of 20% or less for the routine monitoring parameters (i.e., turbidity, temperature, specific conductance, and pH). All analytical results had a relative percent difference of 20% or less. Based on evaluation of the laboratory QA/QC data, the laboratory results generated for first quarter 2026 were deemed acceptable and the water quality samples collected are representative of water quality at the Site. 2.2 Unsaturated Zone Monitoring This section describes monitoring conducted within the vadose zone of the Site during first quarter 2026 and the associated monitoring results. The unsaturated zone monitoring is conducted in accordance with MRP Section B.2 (RWQCB 2024). 2.2.1 Monitoring Program The unsaturated zone monitoring network in the MRP includes 23 suction lysimeters and eight pan lysimeters (MRP Table 6 [RWQCB 2024]). The current unsaturated zone monitoring network includes 18 suction lysimeters and 14 pan lysimeters, which includes four additional pan lysimeters than the MRP (TS-1 [FU-23], TS-2 [FU-23], TS-3 [FU-24], and TS-4 [FU-24]). The lysimeter locations are shown on Figures 2 and 3 and listed in Table 1. Lysimeter analytical results are provided in Tables 5a through 5c. Five suction lysimeters (LY-ElA, LY-El B, LY-E2A, LY-E2B, and D93B) are included in MRP Table 6 (RWQCB 2024) and shown on Figure 2; however, these suction lysimeters were assumed destroyed as shown in the Revised Quarterly Monitoring Report— Fourth Quarter/Annual 2024 (Arcadis 2025b) and were proposed to be removed from the MRP in the next revision. Pan lysimeters FU-19E and FU-19W are not listed in the MRP and were not monitored until second quarter 2025. Sumps FU-19E and FU-19W are listed (MRP Table 25 [RWQCB 2024]), as shown on Figure 4. When pan lysimeters FU-19E and FU-19W were visited for sampling in first quarter 2026, the locations were dry. The locations should be included in Table 6 of the MRP in the next revision. The lysimeter samples were analyzed in accordance with MRP Tables 8 and 9 (RWQCB 2024). Tables 5a and 5c detail results of soil pore liquids if they are present during the monitoring event. Supplemental analyses are www.arcadis.com 1 Forward Landfill 1Q26 QMR 4