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Rationale for Considering Conditional - 7 - 25 March 2026 No Further Action Olin Chlor Alkali Facility Tracy, San Joaquin County, California On-Site Supply Wells: There are currently four supply wells located on-site, Olin-1 and Banta-1 through Banta-3. The on-Site supply wells are owned by Olin and are no longer in use. Olin-1 was historically used for potable water by the Site operators and Banta-1 through Banta-3 were used for process water during Site operations. Based on the well completion reports, Olin-1 is installed below the Corcoran Clay (screened 490-530 feet bgs), and therefore, does not pose a risk to potential receptors. Supply wells Banta-1 through Banta-3 are screened above the Corcoran Clay. Data collected from wells Banta-1 and Banta-2 indicate that COCs are not present in the wells at elevated concentrations. Banta-3 has not been sampled. However, Banta-3 is located adjacent to Banta-1 and Banta-2 and the wells have comparable construction details. Olin plans to decommission wells Banta-1 through Banta-3, and therefore, the wells will not pose a future risk to potential receptors. Off-Site Private Wells: Historically, monitoring was conducted to assess potential impacts to neighboring properties at four off-Site, privately owned wells: PW002, PWO03, PW009, and PWO12. The wells are located north of the Site (Figure 5). Off-Site private wells PW002, PW009, and PWO12 are located on nonresidential properties, and groundwater extracted from these wells is used for nondomestic purposes, such as toilet flushing and dust control. These wells were sampled regularly from 2001 through 2011, under the requirements of the MRPs. In 2012, PW002, PW009, and PWO12 were removed from the MRP sampling requirements because VOCs had not been detected above their respective laboratory reporting limits in the 11 years of sampling. Off-Site private well PWO03 is associated with a private residence where water is used for domestic purposes. Samples were collected from PWO03 as part of the DLA-Tracy Monitoring Program between 1994 and 1999. In samples collected in November 2001, CCI4 and CF were detected above reporting limits but below MCLs. The well was regularly sampled until November 2007, when PWO03 was replaced with a deeper well, screened below the Corcoran Clay aquitard at 360 to 400 feet bgs. Sampling continued at the replacement well through 2011. In 2012, sampling was discontinued because CCI4 and CF had not been detected in the replacement well. On 26 February 2026, Central Valley Water Board staff collected a groundwater sample from the domestic drinking water well at 26391 South Bird Road, approximately 1,400 feet east of the Site. The laboratory analytical results did not detect CCI4 above laboratory reporting limits. Currently, a few residential properties are present in the downgradient agricultural area that could potentially use groundwater for potable use in the future. The San Joaquin County Environmental Health Department (SJCEHD) requires a 200-foot grout seal for any proposed water well at or in the vicinity of the Site and DLA-Tracy, meaning that any future water well will be screened within or below the Corcoran Clay aquitard (see