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Additional Soil Vapor Assessment Work Plan Page 3 CPSA,Lodi,California March 22,2024 Based on the results of the indoor air sampling,CHT,on behalf of the City, submitted a Soil Vapor Assessment(SVA) Work Plan(Work Plan) on January 11, 2023 to the Water Board which outlined additional assessment activities to evaluate the extent of residual PCE in soil vapor at three primary locations — 210 and 212 W Pine St. and 18 S. Pleasant Ave. residence. The Work Plan was approved by the Water Board in the January 30, 2023 letter to Andrew Richle with the City. Between March and May 2023, the City conducted the following assessment activities at / near the three properties— • 212 West Pine Street: o Installed two (2) temporary soil vapor (TSV) borings (SV1 and SV2) south of the building along the sewer lateral emanating from the building and collected depth discrete soil vapor(SV) samples; o Installed five (5) dedicated sub-slab (SS) Vapor Pins® beneath the building and collected sub-slab vapor samples. o Collected five (5)passive indoor air(IA) samples within the building. • 210 West Pine Street: Collected two passive IA samples from within the building; • 18 South Pleasant Avenue: Installed two passive soil gas (PSG) samplers to the northwest and northeast of the residence at approximately 12 to 14 inches below ground surface(bgs). The results of the assessment activities were presented by the City to the Water Board in their January 15, 2024 Soil Vapor Assessment Report. Results from the depth discrete SV sampling behind the 212 W. Pine St. building indicated PCE was detected in the soil vapor at concentrations ranging from 25,000 micrograms per meter cubed (µg/m3) to 2,600,000 µg/m3. These concentrations exceed the PCE commercial soil vapor ESL of 67 µg/m3. The highest PCE concentrations in each boring were at depths of 14 to 15 feet below ground surface (ft-bgs) along the sewer lateral. Sub-slab soil vapor samples beneath the 212 W. Pine St. building were observed to contain PCE concentrations ranging from 630 µg/m3 to 46,000 µg/m3, which again exceeded the PCE commercial soil vapor ESL values. The IA samples collected from the building were observed to have PCE concentrations ranging from 5.69 to 33.6 µg/m3, which exceeded the PCE commercial indoor air ESL of 2.0 µg/m3. The SS and discrete depth SV samples indicate the presence of significant residual PCE in soil vapor beneath the building and along the sewer lateral emanating from the building. Additionally, the IA results indicate the proliferation of the PCE beneath the building slab and along the sewer lateral into the building interior. For the 210 W. Pine St. building, PCE concentrations in indoor air were observed to be 1.7 µg/m3 and 5.39 µg/m3. Hence, only one of the IA samples exceeded the PCE commercial indoor air ESL of 2.0 µg/m3. The detection of PCE in the IA samples could be attributed to the remnants of PCE in soil vapor that has been observed both outside and beneath the adjacent 212 W Pine St property. 581 MONTEGO TERRACE,SUNNYVALE,CA 94089 TEL:415-424-3009