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PARCADIS SUBJECT TO Revised CDCR Site Investigation Mr. Brendan Kenny and Remedial Design Work Plan Addendum Engineering Geologist Central Valley Regional Water Quality Control Board DATE OUR REFERENCE March 7, 2022 30105785 DEPARTMENT Environment On behalf of Forward Inc., Arcadis U.S., Inc. (Arcadis) has prepared this revision to the California Department of Corrections and Rehabilitation (CDCR) Site Investigation and Remedial Design Work Plan Addendum (Work Plan Addendum) submitted to the Regional Water Quality Control Board (RWQCB) in October 2021 (Arcadis 2021). The October 2021 Work Plan Addendum proposed a multi-phased site investigation (SI)on the CDCR property Newcastle Road, Stockton, California (site), to delineate the nature and extent of potential release(s) and volatile organic compound (VOC) sources masses beneath the CDCR property, leading to remedial design and implementation. This site is north of the Forward Landfill located at 9999 S. Austin Road in Manteca, California (the Landfill). This revised Work Plan Addendum provides an update to the Background and Proposed Scope of Work sections presented in the CDCR Site Investigation and Remedial Design Work Plan (Work Plan)submitted to the RWQCB in October 2019 (Arcadis 2019). The updates presented in this revised Work Plan Addendum are based on new data and information obtained since the submittal of the Work Plan in 2019, 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. The field activities proposed in this revised Work Plan Addendum will be conducted as a multi-phased effort, with the first phase including the installation of an interim remedial action on the CDCR property, as directed by the RWQCB, to be followed by the completion of the proposed SI activities on the CDCR property to determine the source and extents of VOC mass(es) beneath the site. The interim remedial action will consist of the installation of a groundwater extraction well (CDCR-EW-1) located in the vicinity of shallow well AMW-22S on the CDCR property, positioned to capture and remediate total VOC concentrations in groundwater greater than 25 ug/L, as directed by the RWQCB under the 2017 Cleanup and Abatement Order(CAO) R5-2017-0703, along with the installation of two piezometers (CDCR-PZ-1 and CDCR-PZ-2)for hydraulic assessment. The proposed locations are presented on Figure 1. Hydraulic testing will be conducted after installation of the groundwater extraction well and piezometers, to assess the extraction well capacity, aquifer response and area of capture. The results of the hydraulic testing will assist with finalizing the design of the interim remedial action. The initial phase of the proposed SI will focus on updated areas of interest(AOIs)and potential VOC masses beneath the CDCR property, which include a residential property at 7833 Newcastle Road located west of the CDCR, and one CDCR facility identified in historical as-built drawings as a former laundry facility. Based on the results of previous investigations at 7833 Newcastle and the potential for the historical use of chemicals containing VOCs at the former CDCR laundry facility, the updated AOIs have been identified as properties and former facilities that may have impacted groundwater beneath the CDCR. The results of this proposed SI will investigate the lateral and vertical extent of VOCs in groundwater beneath the CDCR property, and the potential extent of VOCs in soil vapor beneath current CDCR buildings/facilities in the vicinity of identified sources of www.arcadis.com 1/9