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Discrete Depth Soil&Soil Vapor Assessment Work Plan Page 2 CPSA,Lodi,California March 12,2025 Following a closure evaluation of the CPSA remedial systems, the City, with the Water Board's approval, decommissioned the CPSA remedial systems between April and July 2021. Additionally, the City excavated a portion of the alleyway near the 18 S. Pleasant Ave. residence, where passive soil gas (PSG) sampling had indicated high concentrations of PCE in soil vapor. Post-Closure Indoor&Ambient Air Sampling(April—July 2021) As part of the post-closure decommissioning activities, CHT, on behalf of the City, performed indoor and ambient air monitoring at several commercial and residential properties located within the CPSA in February and October 2022. The results of the indoor and ambient air monitoring indicated that PCE concentrations were observed to be above the PCE commercial indoor air Environmental Screening Level (ESL) of 2.0 µg/m3 in one or more samples at the 212 W. Pine St. and 11 S. Church St. properties, and the PCE residential indoor air ESL of 0.46 µg/m3 in the 18 S. Pleasant Ave. residence. Passive Soil Gas(PSG)Sampling(May 2023) Based on the results of the indoor air sampling at the 18 S.Pleasant Ave.residence,CHT,on behalf of the City, installed two passive soil gas (PSG) samplers to the northwest and northeast of the residence at approximately 12 inches below ground surface(bgs). The results of the PSG sampling indicated the presence of residual PCE in soil vapor at concentrations of 14,500 µg/m3 (northwest sample) and 67,900 µg/m3 (northeast sample),which exceeded the PCE residential soil vapor ESL of 15 µg/m3. Trichloroethene (TCE)was also detected in the soil vapor samples at concentrations of 147 and 155 µg/m3, which exceeded the TCE residential soil vapor ESL of 16 µg/m3. Passive Soil Gas(PSG)Sampling(May 2024) In May 2024, CHT, on behalf of the City, installed step-out PSG samplers to further define the extent of residual PCE in soil vapor in the vicinity of the 18 S. Pleasant Ave. residence. The results of the May 2024 PSG sampling, as shown on Figure 3, indicated the presence of residual PCE in soil vapor at concentrations of 6,050 µg/m3, 6,130 µg/m3, and 1,050 µg/m3 in PSG-3, PSG-4, and PSG-5, respectively. TCE in soil vapor was also detected at concentrations of 595 µg/m3, 348 µg/m3, and 218 µg/m3 in PSG-3, PSG-4, and PSG-5, respectively. Both the PCE and TCE concentrations in the PSG samples were observed to be greater than the PCE and TCE residential soil vapor ESL of 15 µg/m3 and 16 µg/m3,respectively. Figure 3 shows the results of the However, the PCE concentrations in soil vapor in the vicinity of the residence were observed to attenuate quickly by an order of magnitude (PG-3 compared to PG-2 and PG-5 compared to PG-1) with distance from the sewer trunk line located within the alleyway. 581 MONTEGO TERRACE,SUNNYVALE,CA 94089 TEL:415-424-3009