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_ .7 Lone Palm Avenue, Suite 201B Modesto, CA 95351 Telephone 209-579-2221 Fax 209-579-2225 www.atcgroupservices.com ENVIRONMENTAL • GEOTECHNICAL BUILDING SCIENCES • MATERIALS TESTING types encountered and sample collection intervals will be recorded on boring logs. Soil samples will be collected continuously, field screened with a Photoionization Detection (PID) meter, and two soil samples from each of the borings will be submitted for laboratory analyses. Following the advancement of each soil boring to the maximum depth, the direct push rods will be removed and the annulus will be measured to verify an open hole to the specified depth. If an open hole to the specified depth can be verified, approximately six inches of sand pack will be placed into the base of the annulus and a one-inch long filter screen attached to approximately seven feet of 1/4-inch outside diameter Teflon or Nylaflow® tubing will be placed inside the annulus. The remaining annulus will be filled according to DTSC guidelines and RWQCB specifications. A traffic rated well box at least four inches in diameter will then be installed flush with the paved ground surface over the soil vapor probe. Soil Vapor Sampling Methodology In an effort to appropriately conduct soil vapor sampling on site, ATC will follow the guidelines established by the Los Angeles Regional Water Quality Control Board (LA RWQCB) and the Department of Toxic Substances Control (DTSC) in documents entitled Advisory — Active Soil Gas Investigations - FINAL, dated July 2015, and Final — Guidance for the Evaluation and Mitigation of Subsurface Vapor Intrusion to Indoor Air, dated October 2011. Each soil vapor point will be sampled after no less than 48 hours following installation to allow grout to set completely. Consideration of rain, snow, or mixed precipitation will also affect the sampling schedule, as the pavement at the site is generally old and somewhat porous. Soil Vapor Sample Collection In preparation for sampling, the above ground sample equipment, provided by the analytical laboratory and fitted with a 200 ml/minute flow restrictor and pressure gauge, will be assembled with separate purging and sampling ports and tested for leaks using a five-minute shut-in test of each sampling manifold. All external valves are closed during the shut-in test and the purge syringe is drawn until 10- 15 inches of mercury (inHg) are showing on the integral gauge. The valve to the syringe is then closed and the pressure gauge observed for several minutes to determine if the vacuum in the sampling manifold is maintained. If signs of a leak are observed, adjustments are made and a new shut-in test conducted. Three purge volumes, using a 60-millimeter (m1) syringe, will be removed from the vapor probe assembly at each location. The purge volume will be calculated based on the internal volume of tubing and annular space around the probe tip and the tubing volume of the above ground assembly. Because an on-site lab is not being employed, a purge volume test will not be conducted. Once purging is complete, a 400 ml Summa canister will be connected to the vapor probe assembly and samples collected. Prior to sampling, a leak check compound derived from commercial computer "dust- off' spray (such as 1,1-Difluoroethane [1,1 DFA], or similar) will be applied to clean cloths and used to monitor leaks within the above ground vapor probe assembly. The "dust-off' spray will be sprayed into a plastic bag containing several cotton cloths. The cloths will be placed adjacent to, but not touching, S: \62596 \ reports \SVIS_WP_ADD-2016.doc 2 ATC