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Mr. Infurna 04/03/2006 Page 3 of to prevent the potential for cross contamination between the A-,B-and C-zones the C-zone wells will be double-cased. The outer casing will be set and sealed at a depth of approximately 60-feet bgs. Once the casing is set,the borehole will be advanced to the 80-foot depth to allow for the installation of the C-zone monitoring wells. The newly installed groundwater monitoring wells will be developed and sampled a minimum of 48- hours following installation. Groundwater samples collected from the monitoring well network(existing wells and newly installed wells)will be analyzed for total petroleum hydrocarbons as gasoline (TPHg)by environmental protection agency(EPA) method 8015 and BTEX and methyl tertiary butyl ether(MtBE) by EPA method 8260. Task 2—Vapor Intrusion Pathway Evaluation The vapor inhalation pathway will be evaluated by collection and analysis of 13 soil gas samples. Figure 3 identifies the proposed soil gas sampling locations. A total of 13 soil gas samples will be collected from nine temporary sampling locations within the 40 foot by 40 foot grid cells as identified on Figure 3. Soil vapor samples will be collected from approximately 1 foot above the water table following the sampling methodology outlined in Chevron's Soil Vapor Sampling Technical Toolkit, presented in Attachment 1. At four locations, a second soil vapor sample will be collected at approximately one foot below the ground surface. Data from the near surface samples will be used, if necessary, to conduct a more detailed risk analysis if the Region IX Preliminary Remediation Goals(PRGs)are exceeded. The soil vapor data will be compared with USEPA Region IX PRGs for ambient air as an initial screening tool. These PRGs are conservative in nature and are based upon a life time cancer risk of 1/1,000,000 for carcinogens and a Hazard Index of 1 for non-carcinogens. The Region IX PRGs for BTEX compounds are listed below: • Benzene—0.25 µg/m3; • Ethyl benzene— 1100 gg/m3; • Toluene—400µghw; and • Total Xylenes— 110 µg/m3. Soil vapor samples exceeding these PRGs do not necessarily indicate that human health is adversely impacted. Exceedance of the PRGs will require additional risk evaluation to determine if human health is protected under existing or future development scenarios. The detailed risk assessment will be based on residential exposure scenarios for off-site receptors and commercial exposure scenarios for on-site receptors. Task 3—Report of Findings A technical report will be prepared following the completion of Tasks 1 and 2. The report will include analytical results and tables, boring logs with well completion details, cross sections with distribution of impacts,conclusions,and recommendations. 08262822.doc BLASLAND, BOUCK&LEE, INC. engineers,scientists,economists