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Vapor Intrusion Assessment Work Plan Project number:60653972 samples will be co-located with sub-slab samples and in Buildings 2, 3,4, 6 and 9.Three ambient air (AA) samples will be collected upwind,down wind and crosswind side of the buildings. Sampling locations are shown on Figure 2. Sample locations will be determined by accessibility to building interiors prior to sampling and the following rationale per DTSC VI Guidance(DTSC,2011 &2023): • Indoor air samples will be located,when possible, at the center of the space away from doors 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. • Sampling will occur with windows closed unless the sampling occurs during summer months in a building with no air conditioning.If sampling is conducted in the winter months,building heating systems will be run for at least 24-hours prior to sampling to maintain normal indoor air temperatures (approximately 65'Fahrenheit). • Heating,ventilation, and air conditioning(HVAC) systems will be operated normally for the season and time of day. • The AA(outdoor background) samples will also be collected over an 8-hour sampling period, corresponding to the commercial primary sample locations and be placed on the upwind side of the building at a distance twice the height of the building. 4.4.1 Field Activities Each of the indoor air samples will be collected as follows and in accordance with the July 2015 DTSC/RWQCB Advisory for Active Soil Gas Investigations(DTSC,2015): • A laboratory-supplied flow controller will be connected to a laboratory-supplied, individually certified 6-liter SUMMA canister.The flow controllers will be set for an approximately 8-hour sampling period(as required for commercial locations).The canisters will be evacuated to more than 25 inches of mercury gauge pressure by the laboratory prior to sampling. • The SUMMA canisters will be placed at the interior locations at a height of 3 to 5 feet, or the average height of breathing zone for adults. • The valve on the SUMMA canister will be opened to begin sample collection.The initial vacuum pressure indicated by the canister pressure gauge will be recorded on the chain-of-custody(COC) form. • After approximately 8 hours,when the vacuum gauge pressure is between 1 and 10 inches of mercury,the valve on the SUMMA canister will be closed to end sample collection.The final vacuum pressure indicated by the canister pressure gauge will be recorded on the COC form. • The SUMMA canister will be shipped to a qualified environmental laboratory for analysis of VOCs using USEPA Method TO-15 selective ion monitoring(SIM). • Duplicate samples will be collected at a rate of at least I per day. Upon completion of sampling, indoor air sampling equipment will be removed. 4.4.2 Laboratory Analysis Samples will be sent under chain of custody to a certified laboratory for analysis. Samples will be analyzed for VOCs by USEPA Method TO-15 SIM in accordance with Table 2. 4.5 Soil Gas Sampling Soil gas sampling will be conducted to assess vertical and lateral extent of VOCs in soil gas in the areas surrounding Buildings 2, 3,4, 6, and 9.Up to 18 soil gas samples will be collected at 9 locations Prepared for: California Regional Water Quality Control Board,Central Valley Region AECOM