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Vapor Intrusion Assessment Work Plan Project number:60653972 4.2 Utility Clearance Prior to advancement of any tooling, an Underground Service Alert(USA)will be called into the 811 USA North system at least 48 hours in advance of any subsurface work. Once utility members have responded and/or marked their utilities at the Site,AECOM will have a private utility locator conduct utility clearance at each intrusive location. During this utility clearance,a 10-foot by 10-foot area surrounding each intrusive location will be cleared.In addition, each location will be hand-augered to a depth of 5 feet bgs. 4.3 Sub-Slab Soil Gas Sampling Sub-slab soil gas sampling will be used to assess whether vapors are present directly under selected buildings onsite, and to help assess the impact that sub-slab vapors may have on indoor air quality. Sub- slab samples will be collected in the air space immediately below the slab, and at depths of 5 and 15 feet below the slab,to characterize conditions beneath each building. Sub-slab samples will be co-located with IA samples. Sub-slab samples will be collected between 8 and 24 hours after the M samples are collected. Sample locations will be determined by accessibility to building interiors prior to sampling and the following rationale per DTSC VI Guidance(DTSC,2011 &2023). • Sub-slab soil gas samples will be located,when possible, at the center of the space away from doors and the foundation edge and within proximity of preferential pathways.Possible preferential pathways include bathrooms, areas of suspected chemical use, areas with underground utilities, floor drains, and/or plumbing features. Samples can also be placed in areas likely to have potential receptors. 4.3.1 Field Activities Permanent sub-slab soil gas probes with be installed at 12 locations, as shown on Figure 2 and described below.At each location soil gas probes will be installed as follows and as shown in Figure 3 • Access permitting,up to 12 permanent sub-slab soil gas probes will be installed, including two each in Buildings 6 and 9, one each in Buildings 2 and 3, and six in Building 4,near possible preferential pathways when accessible. • Sub-slab drilling locations will be marked and cleared of utilities (public and building specific).If the building has a tensioned slab the tension cables within the slab will be located prior to drilling either through visual observation or through remote sensing with ground penetrating radar. • The sub-slab probes placed immediately below the slab will be installed by removing any existing floor covering and drilling an approximate 1-inch-diameter hole through the building floor slab using an electric hand drill or concrete corer.The hole for the probe installation will be advanced 3 to 4 inches into the engineered fill beneath the building slab. The slab thickness and total depth of the probe drill hole will be recorded in the field notebook. • For the deeper probes,two screened intervals will be installed within one borehole with screens set immediately below the slab, and at 5 and 15 feet bgs. • Each sampling probe will be constructed using a 1/8-or 1/4-inch diameter tubing with a permeable probe tip at the end.After each probe is installed in the hole,the annulus around the probe tip will be filled with a sand pack centering the probe tip with at least 6 inches of sand above and below the level of the probe tip.After placing the sand pack,a dry granular bentonite transition seal will be added on top of each sand pack, followed by a hydrated bentonite annual seal(see Figure 3 showing typical sub-slab vapor probe design). The remainder of the hole will be filled with neat cement mixed with bentonite for a permanent probe. The concrete surface in the borehole will be cleaned Prepared for: California Regional Water Quality Control Board,Central Valley Region AECOM