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Confirmation Sampling Investigation Work Plan STAND Affordable Housing Page 2 micrograms per liter(µg/L)to 450µg/L(well MW-6).Concentrations of contaminants in MW-6 appeared to be increasing slightly starting in 2008 until 2012 when monitoring ceased and slight odor/sheen was noted in two of the last three monitoring events,where no odor or sheen had been noted in earlier events. In November 2017, Condor conducted a Phase II Environmental Site Assessment (ESA), which included drilling and sampling of four temporary shallow soil gas probes and four soil borings/groundwater monitoring wells. Soil gas, soil and groundwater samples were collected at the east property boundary(SVP- 1 and MW-1),at the west property boundary(SVP-2 and MW-2),at the north property boundary(SVP-3 and MW-3),and at the location of the former dry cleaner(SVP-4 and MW-4). Low levels of petroleum hydrocarbon constituents were detected in soil gas at all four locations. Elevated concentrations of the dry-cleaning solvent tetrachloroethene (PCE)were detected at SVP-2, which is located southwest of the former dry cleaner location, and SVP-4, which is located at the former dry cleaner location. The soil gas analytical results were compared to the California Human Health Screening Levels (CHHSLs) established by the Office of Environmental Health Hazard Assessment(OEHHA).The PCE concentrations in the soil gas samples collected from SVP-2 and SVP-4 exceeded the CHHSL residential screening levels.The PCE concentration in SVP-4 (2,200 micrograms per cubic meter [[tg/m3]) also exceeded the CHHSL threshold for commercial settings. PCE was detected in only one of the nine soil samples collected from the borings.The sample collected near the former dry cleaner at a depth of approximately 2 feet bgs contained a very low level of PCE that was well below the EPA's most restrictive Regional Screening Level. Shallow groundwater beneath the Site was initially encountered at a depth of approximately 30 to 32 feet bgs during drilling but stabilized at a depth of approximately 28 to 29 feet bgs in the completed monitoring wells. Well MW-1, located at the eastern property boundary, contained PCE at a concentration of 14 µg/L, which exceeds the Maximum Contaminant Level(MCL) of 5 µg/L for PCE.Low levels of PCE were also detected in wells MW-2 (western property boundary) and MW-4 (near the former dry cleaner), but at concentrations below the MCL. No PCE was detected in the groundwater sample collected from well MW-3. None of the groundwater samples collected from Site monitoring wells contained detectable concentrations of the primary constituents of concern related to gasoline, which include benzene, toluene, ethylbenzene, xylenes (BTEX), naphthalene, and MTBE, although very low concentrations of total petroleum hydrocarbons (TPH) as gasoline were detected in wells MW-1,MW-2,and MW-3. A more detailed summary of the Phase H ESA can be found in January 28, 2018 report prepared by Condor, "Phase II Environmental Site Assessment Report, Airport and Eighth Street Property, 2222 and 2244 S. Airport Way,Stockton,San Joaquin County,California"and a March 19,2018 report addendum. In August of 2018, Condor conducted a passive soil gas survey by placing shallow samplers across the Site. A hand drill was used at all sample locations toplace the passive samplers at depths of approximately 32 inches bgs. The results of the passive soil gas investigation confirmed the general pattern of PCE beneath the property. The most frequently detected chemicals included PCE (24 detections), xylenes (13 detections), and TPH (10 detections). TPH, toluene, and undecane were detected in the field blank that was deployed during sampler installation and retrieval,although at relative low levels. The distribution of PCE in soil gas across the Site indicates that the greatest area of PCE impact is in the southwest corner of the property, not in the area of the former dry cleaner. This is also the area where TCE was detected in four samples; TCE is a daughter product of PCE as it breaks down in the environment. In contrast, the distribution of petroleum hydrocarbon constituents (BTEX) is somewhat random, with the greatest mass detected in samplers deployed near the former dry cleaner (northwest portion of the Site), beneath the former discount store (central portion of the Site), and at the southeasternmost corner of the Site. ;� CONDOR