SITE INFORMATION AND CORRESPONDENCE FILE 2

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Former Georges Service Page 2 of 3 1600 Durham Ferry Road 16 January 2008 Tracy, CA 95376 degradation of the petroleum hydrocarbons in groundwater in an offsite area where other remedial options cannot be readily applied. The EHD is concerned with a few technical issues regarding the recommended remedial approaches, the greater concern being with the air sparging proposal. The EHD notes that the site geology consists primarily of fine-grained soil from surface to approximately 20 feet bsg and is coarse-grained from approximately 20 feet to 40 feet bsg, and consists of alternating coarse- and fine-grained units to total depth explored (approximately 95 feet bsg). The depth to water on the site is generally on the order of 12 feet bsg, thus air sparged into the saturated zone at either 22 feet bsg or 38 feet bsg must migrate vertically through low permeability soil to reach the vadose zone in either natural soil or the excavation backfill. As the vertical permeability may be one to two orders of magnitude less than the horizontal permeability, the EHD is concerned that sparged air will tend to migrate horizontally and not necessarily remove stripped hydrocarbons into the excavation backfill vadose zone for recovery. This potential problem becomes more likely as the distance between the sparging point and the former excavation increases; one proposed sparge point is nearly 50 feet from the former excavation. The EHD is concerned that air sparged such great distances from the proposed recovery point simply will not be recovered, and may therefore act to spread the plume rather than remediate it. The EHD will approve a phased approach toward testing SVE and IAS on your site and recommends the following approach: • Conduct a limited SVE test on the existing SVE wells in the excavation backfill to characterize recovered air and vapor without benefit of IAS; • Install and utilize IAS wells AS-5 through AS-8 and test with SVE system, note differences in SVE response, operate the IAS well(s) with a tracer to document potential recovery of the sparged air; • If the air injected in the initial IAS wells is recovered, step outward to the more remote proposed IAS wells and test in like manner; • Include a monitoring design to detect any plume migration caused by unrecovered sparged air. Submit a work plan to the EHD by 17 March 2008 for pilot testing SVE and IAS that addresses the concerns and recommendations noted above. The EHD recognizes the limited options available to address and remediate impacted groundwater that has migrated off-site. Submit a work plan for installing and monitoring the ORC installation. Much of this has been included in the RAP, but the EHD believes a method for closely monitoring the effectiveness of the system should be included. The proposed ORC installation points in the roadway are in a high traffic area and a traffic control plan accepted by the appropriate traffic agency for that location must be properly implemented for worker and motorist safety. WP Comment Letter 0108.doc