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FORWARD Man S.Austin Road ]J Manteca,CA 95336 Landfill P:(209)982-4298 F:(209)982-1009 Mr. Brendan Kenny Engineering Geologist Central Valley Regional Water Quality Control Board 11020 Sun Center Drive,Suite 200 Rancho Cordova,CA 95670 March 7,2022 RE: Revised CDCR Site Investigation and Remedial Design Work Plan Addendum Dear Mr. Kenny, Forward, Inc. (Forward)is pleased to submit this revision to the CDCR Site Investigation and Remedial Design Work Plan Addendum(Work Plan Addendum),submitted to the Central Valley Regional Water Quality Control Board (RWQCB) in October 2021.This revision to the Work Plan Addendum is based on direction from the RWQCB following discussions in a virtual meeting conducted on February 7,2022. During this virtual meeting,the RWQCB directed Forward to install interim remedial action groundwater treatment on the CDCR property before completing the proposed site investigation (SI)activities to determine the source and extent of volatile organic compound (VOC) mass(es)beneath the CDCR property,specifically carbon tetrachloride and its degradation product chloroform in the shallow groundwater zone. As previously presented to the RWQCB in the October 2021 Work Plan Addendum,the Newcastle Road Supplemental Site Investigation Report submitted in November 2020,and numerous virtual meetings since January 2021,the concentrations of carbon tetrachloride and chloroform in groundwater monitoring well AMW-22S,are only present in the shallow groundwater monitoring well on the CDCR property.These constituent concentrations are generally increasing,and currently comprise the majority of VOC mass in groundwater at this location. However,shallow and intermediate wells located hydraulically upgradient and downgradient of AMW-22S have not indicated detections of these constituents;therefore,a potential source of carbon tetrachloride and chloroform may be present on the CDCR property in the vicinity of AMW-22S. As directed by the RWQCB under the 2017 Cleanup and Abatement Order(CAO) R5-2017-0703,the proposed interim treatment remedy includes the installation of groundwater extraction well(s) north of Forward Landfill to capture and remediate total VOC concentrations exceeding 25 ug/L,which based on all available data is localized in the vicinity of AMW-22S.A typical remedial plan would include a determination of the source and extent of VOC mass(es) prior to the installation of a treatment remedy; however,the RWQCB has requested the installation of the interim treatment remedy prior to any additional source mass investigations on the CDCR Property. By installing a treatment remedy prior to the proposed SI activities,the VOC source mass(es) may not be sufficiently captured and could result in plume migration. Forward will continue coordinating with the CDCR to implement the required interim remedial action for groundwater in the vicinity of AMW-22S on the CDCR property and will keep the RWQCB apprised on progress. Following completion of the proposed field activities,Arcadis will evaluate the data and prepare a summary of the results.A report will document the extraction well installation and subsequent hydraulic testing,and the results of the proposed SI. Sincerely, Joshua Mills Area Environmental Manager West Region (510)691-4337/(209) 982-1009 fax Jmills3@republicservices.com