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Soil Vapor Assessment Work Plan Page 2 CPSA,Lodi,California January 11,2023 Following a closure evaluation of the CPSA GWET and SVE systems, the City recommended decommissioning and removal of the existing CPSA GWET and SVE systems, abandoning/removal of all groundwater extraction, groundwater monitoring, and SVE wells, remediation piping, and excavating soil containing elevated VOCs along the sewer line in the alleyway near the intersection with S. Pleasant Avenue. Additionally, the City also proposed, as part of the post-closure activities, to perform indoor and ambient air monitoring in the residential and commercial properties within the CPSA,and address any increased PCE concentrations during indoor air monitoring activities using targeted remediation. The Water Board approved the City's recommendations for closure in their May 18, 2020 Review of Central Plume Source Area Groundwater Extraction and Soil Vapor Extraction Systems Closure Report concurrence letter to Andrew Richle at the City. CHT, on behalf of the City, then submitted the Central Plume Source Area Groundwater Extraction and Treatment and Soil Vapor Extraction Systems Decommissioning Activities Work Plan, on March 4, 2021 to the Water Board outlining the activities associated with decommissioning of the CPSA GWET and SVE system. The 2021 Work Plan was approved by the Water Board in the March 10, 2021 letter to Andrew Richle with the City. Between April and July 2021, the City decommissioned the CPSA GWET and SVE systems and excavated a portion of the alleyway near Pleasant Avenue, where passive soil gas(PSG) sampling had indicated high concentrations of PCE in soil vapor(Figure 4). As part of the post-closure and decommissioning activities, CHT, on behalf of the City, performed semi-annual indoor and ambient air monitoring at several commercial and residential properties within the CPSA in February and October 2022. The results of the indoor and ambient air monitoring were presented to the Water Board in the August 13, 2022 Semi-Annual Indoor and Ambient Air Investigation Letter Report and the January 9, 2023 Results of the October 22 Semi-Annual Indoor and Ambient Air Investigation Letter Report. The results, which are depicted in Figure 3, indicated that PCE concentrations in the February, June, and October 2022 indoor air samples at the 212 W. Pine Street property were observed to be above the PCE commercial indoor air ESL level of 2.0 µg/m3. Additionally, the February 2022 indoor air sample at the 18 S. Pleasant Ave. residence also indicated the presence of PCE in indoor air that was above the PCE residential indoor air ESL level of 0.46 µg/m3. Moreover, one of the two October 2022 indoor air samples collected from the 11 S. Church St. property also had PCE concentrations in indoor air (2.28 µg/m3) which were just above the PCE commercial indoor air ESL level of 2.0 µg/m3. 649 DISTEL DRIVE,LOS ALTOS,CA 94022 TEL:415-424-3009