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• • WORK PLAN FOR ADDITIONAL SOIL VAPOR SURVEY Proposed Assessment Activities May 30, 2014 the vadose zone at depths of 3 and 5 feet bgs, or in a smear zone if noticeable. Soil samples collected from the vadose zone or smear zone for analysis by EPA Method 8260 will be preserved using EPA method 5035, often at the request of the CRWQCB, which includes collecting the soil using EnCoreTM samplers. The sample container will be sealed, labeled and placed on ice pending submittal under chain-of-custody to a State of California certified testing laboratory. Following soil sampling, soil vapor probes will be completed within each boring, as described below. 3.5 SOIL VAPOR PROBE INSTALLATION Upon reaching the anticipated target depth of 5 feet bgs, the borings will be completed with a single-depth soil vapor probe.Stainless steel soil gas probes, attached to flexible Teflon or nylon tubing, will be installed in a permanent configuration at each of the proposed probe locations to the target depth specified. The soil gas probe tips are typically 6 inches long with a diameter of 0.25 inches. The depth-discrete vapor probes will be constructed as follows: ❑ After placement of the soil gas probe tip at the target depth, an annular one- foot-thick sand pack will be placed across the vapor probe tip, extending approximately 6 inches above the tip. One foot of dry bentonite will then be placed on top of the sand pack.The remainder of the annular space will be sealed with hydrated bentonite to approximately 1 foot bgs and overlain by a one-foot-thick cement slurry cap. The tubing connected to each sampling probe will extend through the cement slurry seal and the ends capped with a gas-tight Swagelok@ or equivalent valve and end cap within a flush mounted well box and marked with sample location and depth. See Figure 4. 3.6 SOIL VAPOR PROBE PURGING AND SAMPLING Vapor probe sampling will be performed after sufficient equilibration time has lapsed, which for probes installed using a hand auger is a minimum of 48 hours (DTSC 2012). Purge volume tests (1, 3 and 10 volumes) will be performed at one vapor well estimated to be in closest proximity to the historical source/release to determine the appropriate number of purge volumes to be removed prior to soil gas sample collection. In addition the purge volume test will be performed at each location with a noted change in lithology in accordance with DTSC guidance. Minimum purge volume will be determined by: Minimum Purge Volume = 4.46 ml/foot of 0.25 in. OD tubing x depth Purging and sampling will be conducted at a rate less than or equal to the sampling rate (less than 200 milliliters per minute [mL/min]) using a low-flow sampling pump or a 6- L purge canister (each 1-inch of mercury (in Hg) drop in canister vacuum equates to 3.7