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Revised Soil Gas Sampling .er VVorkplan September 8, 2017 <br />CA ARNG, FMS #24, Stocktoii, CA Page 6 <br />PROPOSED SOIL GAS INVESTIGATION <br />All field work will be overseen by an experienced field technician under the direction of a <br />professional geologist (PG) licensed in California. Soil gas probe installation, sampling, and <br />probe destruction will be subcontracted to a licensed (0-57) drilling contractor. Analytical <br />testing will be completed by a California-certified environmental testing laboratory. The <br />locations proposed herein (in green on Figure 3) were selected based on past remedial efforts <br />and typical groundwater flow direction and gradient at the site (Attachment 3). Three sample <br />locations were selected to best determine if there is vapor intrusion risk to building occupants, <br />and a fourth location was selected to determine potential risk to future workers conducting <br />subsurface work. Additional considerations are listed below. <br />Probe Location Reason for Selected Location <br />Near FMS-MW-4 Approximately downgradient from the former USTs, as close to the off-site <br />building as possible. <br />Near FMS-MW-3 Approximately downgradient from the former USTs, as close to the CSMS as <br />possible, in an open area, but not adjacent to the front door. <br />Near FMS-MW-5 At the original source area and hopefully away from a subsurface concrete slab. <br />Selected to determine if there would be soil vapor exposure risk to future workers. <br />Near FMS-MW-2 Crossgradient from the former USTs, close to the groundwater plume (depicted in <br />Attachment 3), and as close as possible to the warehouse that contains an <br />operational wash rack. <br />INSTALLATION OF TEMPORARY SOIL GAS PROBES <br />The licensed drilling contractor will install the temporary soil gas probes using DPT. The total <br />depth of the boreholes will be 10 feet, and the maximum diameter will be 3.5 inches. Six inches <br />of Monterey #3 sand will be placed at the bottom of the borehole to minimize the disruption of <br />airflow to the sampling tip and a temporary stainless-steel or ceramic soil vapor point will be <br />installed at 9.5 feet below ground surface (bgs). Six additional inches of sand will be placed on <br />top of the soil gas sample point (total sand filter pack interval is between 9 to 10 feet bgs). The <br />sample point will be attached to full 1/4-inch Teflon® tubing and shut off at the surface using a <br />luer lock valve (or similar). One foot (8 to 9 feet bgs) of dry bentonite granules will be placed <br />immediately above the filter pack. Hydrated bentonite will be used from 8 feet bgs to near <br />ground surface. A schematic is presented in Attachment 4, for reference. <br />If determined to be necessary, a down-hole rod will be used to support the well tubing in the <br />borehole. The support rod ensures that the probe tip is placed at the proper depth. The support <br />rod should be constructed to avoid possible cross contamination or ambient air intrusion.