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ATC Associates Inc. 1117 Lone Palm Avenue,Suite 201 Modesto,California 95351 two 209-579-2221 fax:209-579-2225 ASSOCIATES INC. pounds per gallon, this represents removal of the equivalent of approximately 1,000 gallons of gasoline. V METHODS AND PROCEDURES Three soil vapor sampling probes were installed adjacent to the Diesel Performance facility to evaluate health risks to occupants of the business resulting from potential vapor intrusion from the subsurface into indoor airspace. One additional soil vapor sampling probe was installed w0 along the eastern fence line to evaluate the potential effects on the residences that are adjacent to the site. The subsurface installation and soil gas sampling activities were conducted in general accordance with ATC's Soil Gas Survey Workplan for Diesel Performance Inc, dated December 2, 2010. Prior to conducting the subsurface activities, ATC notified Underground 4"' Service Alert (USA) to locate possible subsurface utilities in the vicinity of the proposed soil borings. ti. Subsurface Installation Activities On January 17, 2011, ATC supervised the advancement of four vapor point borings identified as SGA, SG-2, SG-3, and SG-4. The vapor point borings were advanced to a depth of four-and-a- half to five feet using a Geoprobe® drilling rig employing direct push technology. Drilling was conducted by Precision Sampling, C57 License No. 636387. The locations of SGA through SG-4 are shown on Figure 3. w. A 2-inch diameter steel core barrel equipped with an acetate sleeve was driven into the ground using the direct push drilling rig. When the probe had been advanced to the desired depth, it was extracted along with a continuous soil core. Approximately three to six inches of sand were placed in the bottom of each bore hole and a stainless steel wire screen tip implant attached to 0.25-inch outer diameter Teflon tubing was placed on the sand pack. An air-tight valve was installed at the opposite end of the tubing. Approximately three to six inches of sand were 6" added to the bore hole, covering the implant. Approximately one foot of hydrated granular bentonite was placed above the sand pack and the remainder of the boring annulus was grouted to the surface. The portion of the tubing attached to the valve was contained under a 60 small, flush-mounted, steel plate. Soil Vapor Sampling Activities 60 On January 20, 2011, an ATC field geologist collected soil vapor samples from soil vapor points SGA, SG-2, SG-3, and SG-4. The soil vapor samples were collected following the guidelines established by the Los Angeles Regional Water Quality Control Board (LARWQCB) and the Department of Toxic Substances Control (DTSC) in documents titled Advisory—Active Soil Gas Investigations dated January 28, 2003 and Interim Final — Guidance For the Evaluation and Mitigation of Subsurface Vapor Intrusion to Indoor Air dated December 15, 2004, revised February 7, 2005. A sampling manifold equipped with a vacuum/pressure gauge was connected to the Teflon vapor probe tubing and valve at the ground surface. Purge volumes were calculated for each soil gas sampling point based on the inside diameter and length of the tubing, the diameter of the boring (approximately two inches), and the height of the sand pack. A 60-millimeter (ml) S:1Environmenta11625771Reports\2011 Soil Gas.doc 3 ATC Associates Inc. �r 1r