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Mr. Brenden Kenny Central Valley Regional Water Quality Control Board March 7, 2022 Landfill. Additionally, downgradient monitoring wells were installed immediately downgradient of the POC (AMW- 40S through AMW-43S)to confirm and monitor the capture of impacted groundwater. Supplemental investigation activities in the area between the Landfill and the CDCR were completed from July 21 to August 11, 2020 and consisted of groundwater monitoring well installations and subsequent groundwater sampling (Arcadis 2020b). Based on the results of the supplemental investigation, the concentrations of the VOCs carbon tetrachloride and chloroform observed at shallow groundwater monitoring well location AMW-22S on the CDCR property were interpreted to be not associated with the historic Landfill release. Carbon tetrachloride and chloroform were also not detected in newly installed shallow groundwater monitoring wells AMW-44 through AMW-46 (east to west transect of shallow groundwater monitoring wells along the southern boundary of the CDCR), and AMW-40S through AMW-43S (west to east transect of shallow groundwater monitoring wells along the northern boundary of the Landfill), all of which are located hydraulically upgradient of well AMW-22S on the CDCR property. Carbon tetrachloride concentrations in shallow groundwater in the vicinity of the CDCR is presented on Figure 2. CDCR Property - File Review and Utility Survey: Attempts have been made since 2019 to finalize a right-of-entry (ROE) agreement with the CDCR to complete a review of historical files and obtain access to the CDCR property to complete the site investigation activities proposed in the Work Plan (Arcadis 2019). The ROE agreement was executed on February 19, 2021, and in accordance with the first phase of investigation detailed in the Work Plan, which was included as an attachment to the February 2021 ROE agreement, historical as-built drawings of former CDCR facilities were obtained from the CDCR. Review of the historical as-built drawings identified the following CDCR facility operations that may have impacted groundwater beneath the property (Figure 1): • A January 1991 Demolition Plan includes as-built drawings of a historical dry-cleaning facility located in the CDCR administrative core that operated from the 1960s to the 1980s. Construction and decommissioning of the former dry-cleaning facility were also documented in the California state budget reports. The historical dry-cleaning facility identified in Figure 1 is located northeast(downgradient) of AMW-22/22S. Additionally, a geophysical utility survey was conducted from July 19 through July 22, 2021. A subsurface utility map is provided on Figure 1. The geophysical utility survey was completed to help identify larger subsurface utilities in lieu of utility maps not being provided by the CDCR due to the security-sensitive nature of the information. The results of this geophysical investigation identified a confluence of stormwater and sewer drainage lines located adjacent to the historical dry-cleaning facility presented above. These subsurface features may act as potential pathways for chemical releases on the CDCR property and assessment of these areas are critical to understanding the lateral and vertical extent of potential source areas on the CDCR property. Proposed Scope of Work The purpose of this revised Work Plan Addendum is to present an updated approach to the remedial actions and SI presented in the Work Plan (Arcadis 2019) based on new data and information obtained since the October 2019 submittal, and direction from the RWQCB following discussions in a virtual meeting conducted on February 7, 2022, following the submittal of Work Plan Addendum in October 2021. At the direction of the RWQCB, the proposed field activities will be conducted as a multi-phased effort, with the first phase including the installation of www.arcadis.com 3/9