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Rationale for Considering Conditional - 3 - 25 March 2026 No Further Action Olin Chlor Alkali Facility Tracy, San Joaquin County, California Ponds 1 and 2 were considered the on-Site source of COCs in soil. In 1999, Pioneer excavated 3,200 cubic yards of soil from Ponds 1 and 2 and disposed of the soil off- Site. The excavations were backfilled with clean soil and the ponds were closed. In 2000, the CVWB concluded there had been sufficient cleanup of Site soil and issued a staff acceptance letter for the soil closure report. In 2001, a Feasibility Study/Remedial Options Evaluation Report (FS/ROE) reported low concentrations of CCI4 and CF in on-Site and off-Site groundwater and recommended MNA as the remedial alternative. From 2001 to the present, groundwater monitoring continued under numerous Monitoring and Reporting Programs (MRPs). As the COCs migrated off-Site, the groundwater monitoring area was expanded to the northeast via installation of additional monitoring wells (Figure 5). In accordance with the current Revised MRP No. R5-2012-0813, the current monitoring program consists of annual measurements of groundwater elevation and biennial sampling for analysis of volatile organic compounds (VOCs) at eight monitoring wells: • Upper zone (10 to 27 feet bgs): MW-5, MW-9, MW-11, and MW-13 • Middle zone (42 to 58 feet bgs): MW-10, MW-12, and MW-14 • Lower zone (68 to 89 feet bgs): MW-15 In June 2024, step-out grab samples were collected downgradient of the Site (Figure 5) to update the interpretations of the lateral and vertical extent of CCI4 and CF in each groundwater zone. The results from the investigation confirmed the lateral and vertical extent of COCs. Current Conditions and Protectiveness: Extent of COCs Defined in Soil: Between 1994 and 1997, the Former Stormwater Pond, Former Small Wastewater Pond, and Former Fire Ponds 1 and 2 were sampled and closed with Central Valley Water Board staff concurrence. In 2000, staff accepted the closure report for the excavation at Ponds 1 and 2. Since the former source areas were effectively removed and there does not appear to be continuing or additional sources contributing to detections of CCI4 and CF in soil, contamination in soil is no longer a concern at the Site. Extent of COCs Defined in Groundwater: The Site's groundwater quality data have been compared to the State of California maximum contaminant levels (MCLs) for CCI4 (0.50 micrograms per liter [pg/L]) and trihalomethanes (80 pg/L) since the CF compound does not have an individual MCL but is included in the trihalomethane mixture. During the March 2025 groundwater monitoring event, CCI4 concentrations above the MCL in groundwater are limited to one monitoring well (MW-12). In 2024, CCI4 was detected above the MCL in one grab groundwater sample. Alternatively, CF concentrations generally remain below the MCL for trihalomethanes.