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vials. Each vial was filled with no observable headspace or air bubbles to minimize the potential for volatilization, labeled for identification, and stored in an iced cooler. New screens and tubing were used for each monitoring well. The risers were decontaminated between boreholes to prevent cross-contamination. The groundwater samples collected from boring B1 was screened from 30 to 40 feet bgs and from boring B2 from 30 to 50 feet bgs. 3.7 Soil Gas Sampling 3.7.1 Soil Gas Probe Construction Soil gas probes screened at 5 and 15 feet bgs were constructed within borings 131 through B4 upon completion of soil sampling. Boreholes were backfilled with dry, granular bentonite to approximately 6 inches below the desired sampling depth. A new section of diameter Nylaflow tubing with a new 1/4-inch diameter polypropylene filter at the terminal end was inserted into the borehole to the desired sampling depth. One-inch diameter polyvinyl chloride (PVC) casing was used as a guide for the tubing to ensure that the desired sampling depth was achieved. Sand was poured into the boring annulus to form an approximately 1-foot long sand pack around the polypropylene filter, at which time the PVC piping was withdrawn. Approximately 1 foot of dry, granular bentonite was placed atop the sand pack and the remainder of the borehole was backfilled with hydrated bentonite to the next desired sampling depth where a soil gas probe was similarly constructed. Upon installation of the shallowest probe, the remainder of the borehole was backfilled with hydrated bentonite to the ground surface to form a seal.The sampling end of the tubing was fitted with a valve and the probe was labeled for identification. 3.7.2 Sub-Slab Soil Gas Probe Construction Each sub-slab soil gas probe, consisting of a pre-fabricated stainless steel screen and casing, was manually inserted into a '/z-inch diameter hole drilled within the concrete building slab using the rotary hammer drill. Sand was poured into the annulus to form a sand pack around the probe screen and approximately 2 inches of dry, granular bentonite was placed atop the sand pack. The annulus was backfilled with hydrated bentonite to the ground surface to form a seal and the barbed sampling end of the probe was fitted with a cap prior to sampling. 3.7.3 Soil Gas Sampling Methodology Soil gas and sub-slab soil gas samples were collected in general accordance with the July 2015 Department of Toxic Substances Control (DTSC) and Los Angeles Regional Water Quality Control Board (RWQCB) "Advisory—Active Soil Gas Investigations." Soil gas samples were collected using 1-liter, stainless-steel, cylindrical SUMMA canisters. The sampling containers were provided by SunStar, which subjected each canister to a rigorous cleaning process using a combination of dilution, heat, and high vacuum.After cleaning,the canisters were batch certified to be free of target contaminants to a specified reporting limit via gas chromatography/mass spectroscopy prior to delivery. Additional Subsurface Investigation Report PARTNER Project No.23-400162.14 July 31,2023 Page 7