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Casey Sanders - 2 - 25 September 2023 Samples will be taken in accordance with the July 2015 DTSC/RWQCB Advisory for Active Soil Gas. Each sample take will be collected in a laboratory supplied, batch certified 1-liter SUMMA canisters, with new tubing between each sampling to avoid cross contamination. Duplicate samples will also be collected at a rate of at least 1 per sample event or 10 percent, whichever one being greater. Lastly, the SUMMA canisters will be shipped off to a laboratory for analysis of helium leak check compound and VOCs using ASTM Modified Method D-1946 and U.S. Environmental Protection Agency (USEPA) Method TO-15. Indoor and Outdoor Air Sampling To assess indoor air quality, 12 samples will be co-located with the sub-slab samples in Buildings 2, 3, 4, 6, and 9. Windows will be closed when indoor air samples are being taken, unless there is no air conditioning in the buildings during summer months. In the winter, indoor air temperatures will be maintained to around 65' F when samples are being collected. Three ambient air samples will be taken in total, upwind, downwind, and crosswind of the buildings. Ambient air samples will be collected over an 8-hour period and corresponding to the indoor air sample locations. Indoor air samples will be taken in accordance with the July 2015 DTSC/RWQCB Advisory for Active Soil Gas Investigations. 6-liter SUMMA canisters will be inside buildings at the height of 3 to 5 feet, and flow controllers will be set for approximately 8 hours for the sample period. Prior to sampling, canisters will have more than 25 inches of mercury gauge pressure and down to 1 to 10 inches at the end of the 8-hour sampling period. These SUMMA canisters will also be shipped off to the laboratory to be analyzed for VOCs using USEPA Method TO-15 selective ion monitoring (SIM). Soil Gas Sampling Up to 18 soil gas samples will also be collected across 9 locations in the areas surrounding Buildings 2, 3, 4, 6, and 9. To reach the subsurface, concrete coring will be conducted on each sampling location as needed. Soil gas probes will be installed as dual nested wells at the depths of 5 feet and 15 feet below ground surface and sampling probes will be created with steps similar to the sub-slab soil gas probes. Similar to other samplings, soil gas samples will be collected in accordance with the July 2015 DTSC/RWQCB Advisory for Active Soil Gas Investigations. Samples will be shipped off in 1-liter SUMMA canisters and will be analyzed for helium and VOCs by ASTM Modified Method D-1946 and USEPA Methods TO-1 5. Comments Central Valley Water Board staff concurs with the proposed sampling methods and locations, and sampling should be conducted as proposed. Please provide us with the field sampling schedule when it becomes available to provide us with the opportunity to observe the work. A Vapor Intrusion Assessment Report should be submitted to this office no later than 1 March 2024. In addition, please upload the relevant information