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Troy Estacio - 4 - 2 June 2020  Leaching Potential: The leaching potential of arsenic in the stockpiled materials removed from WU1 was further evaluated by comparing the estimated mass of DI soluble arsenic contained in the stockpile materials to a potential volume of receptor groundwater beneath the site. The soluble mass calculation includes the following assumptions: the entirety of the arsenic mass leached to groundwater, saturated zone void ratio of 0.5, and twenty-foot depth of dispersion and diffusion within the saturated zone. The soluble arsenic mass in the stockpiled materials is estimated at 0.007 mg/L which is below the MCL for arsenic of 0.01 mg/L. WU1 Materials Reuse To mitigate metals leaching potential from WU1 waste material, placement and remedial treatment of WU1 material was implemented in accordance with 23 May 2018 Waste Materials Reuse Work Plan. The stockpiled waste material from WU1 was placed beneath the concrete floor slab of the new warehouse in a cement treatment process that results in an impermeable monolith that is resistant to infiltration and thereby reduce the leaching potential of metals. In addition, a Land Use Covenant will also be established for the site to restrict use, prohibit the use of groundwater beneath the property, and establish future management and maintenance. Regional Water Board Comment 1. Staff of the Regional Water Board acknowledge and appreciate the effort to implement this work under several work plans as referenced throughout this letter. Verification soil sampling test results indicate that WU1 removal activities were successful and residual soil conditions in the footprint do not appear to pose a threat to future commercial/industrial site usage. While chromium and lead concentrations in previous groundwater sampling from beneath WU1 as part of site assessment work did not exceed MCLs, the DI WETS show a leaching potential for these constituents that was mitigated by the selected cement fixation remedy. Elevated concentrations of arsenic have been detected above the MCL in area groundwater and are likely to be naturally occurring. Although site activities may have contributed arsenic to site soils, the selected cement fixation remedy also mitigates potential arsenic leaching. Therefore, WU1 removal and reuse activities appear complete and implementation of the LUC should assure ongoing protection of human health and the groundwater. 2. As discussed in the above-stated Site Background section, portions of the NWC and SWC that extend onto the neighboring Former San Joaquin Cogeneration Facility property are currently being addressed by that property ownership. Therefore, the NWC and SWC areas will remain open until characterization and remediation of the remaining waste in both waste cells is complete. It is anticipated that these areas will be addressed in the near future as the property owner is actively addressing the issue by undertaking investigation activities and proposed remediation of the remaining waste cells.