SITE INFORMATION AND CORRESPONDENCE FILE 3

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ARCADIS BBL Mr. Michael J . Infurna April 5, 2007 Amended Task 2 — Vapor Intrusion Pathway Evaluation The vapor inhalation pathway will be evaluated by collection and analysis of nine soil gas samples from nine permanent sampling locations within 40-feet by 40-feet grid cells as identified on Figure 1 . Nine soil vapor samples will be collected from approximately 1 foot above the water table. Vapor probe installation and sampling will be performed in accordance with Chevron 's Soil Vapor Sampling Technical Toolkit (Version 1 .6) and DTSC guidelines. ARCADIS BBL has selected the direct-push soil vapor sampling probe (implant type) as detailed in Section II , Subsection B of Chevron's Soil Vapor Sampling Technical Toolkit for the proposed nine vapor probes. Deep Vapor Probes Installation To install the deep probes, a direct-push rig will be used to advance a series of drive rods to within 1 foot of the water table (approximately 7 feet bgs) in each of the nine proposed locations. A round of water-level measurements will be collected from the on-site water-table monitoring wells to determine the actual depth-to-water immediately prior to initiating the drilling activities. When the targeted depth is reached , the probe will be slid down the bore of the drive rods. As the drive rods are removed, a sand pack will be placed around the probe (extending from approximately 7-6 feet bgs) followed by a dry granular bentonite seal (extending from approximately 6-5 feet bgs), hydrated granular bentonite (extending from 5-1 feet bgs), and a 1 -foot cement cap. The soil vapor tube will be fitted with a gas-tight cap (such as Swagelok ® cap) to eliminate the potential for barometric pressure fluctuations to induce vapor transport between the subsurface and the atmosphere. An 8-inch wail box will be installed at surface grade to protect the soil vapor probe. Vapor Probe Sampling and Analysis Procedure The soil gas sampling system will be set-up according the DTSC guidelines. To ensure stagnant or ambient air is removed from the sampling system , a low-flow pump will be utilized to purge approximately three volumes of dead soil gas at a flow rate of >200 milliliters per minute (mL/min) . The purge volume of dead space volume will be calculated based on the internal volume of the tubing and annular space surrounding the probe tip. A leak test will be conducted to ensure the integrity of the sampling system . A tracer check compound (e.g . , Freon or helium) will be placed at locations where ambient air can enter the sampling system . The soil gas samples will Page: PmnxnsenrznMmdm.mroCAP Rommex 2/4