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Mr. Jeffery g Won 7h3adweII&RoIIo San Joaquin County Environmental Division 5 January 2004 Page 3 PROPOSED REMEDIATION We propose to conduct an enhanced bio-remediation program at the Site using high pressure injections of ORC. During our 5 December meeting with you, you expressed concern about the effectiveness of ORC. In order to be effective, the ORC must be distributed as widely as possible in the subsurface to increase the area of influence, the subsurface soil and groundwater conditions must be favorable to the diffusive and advective dispersion of ORC, and there should not be an extremely high natural oxygen demand. Additionally, ORC is not a very efficient remedial method at sites with free product and extremely high dissolved-phase contaminant concentrations. ORC should be effective in both the A and B zones of shallow aquifer at the Site for the following reasons: 1. High pressure injection of an ORC slurry using direct push technology will provide optimal distribution of the ORC. This method distributes ORC several feet away from each injection point by hydrofracturing the soil, and has been shown to be much more effective than previous,passive, delivery methods such as using adding ORC to existing wells. 2. Subsurface conditions at the Site are acceptable for ORC use. The A and B zones at the Site are comprised of silty sands with a good transmissivity, as shown in previous investigations, including the aquifer pumping test performed by AGE in May 2001. Additionally, soils with very high oxygen demands such as peaty, highly organic clays are not present. 3. ORC has been shown to be quite effective at sites with similar concentrations of petroleum hydrocarbons as the Site [0.1 to 10 milligrams per liter (mg/L) of Total Petroleum Hydrocarbons (TPH)]. 4. The ORC will be injected in both the A and B zones of the shallow aquifer. We propose to inject ORC at up to 20 points in the grid pattern as shown on Figure 1. The points would be spaced at 12-foot centers, and one additional point would be driven in the center of the former UST No 8 location. ORC would be injected in the entire saturated zone from approximately 57 to 17 feet bgs. Attached is a remedial design worksheet that was used to help design the ORC injection program. The worksheet also includes a preliminary cost estimate to perform the work. While ORC injections can cost more than the initial installation cost of a simple groundwater extraction system, they are usually less expensive overall, as there are no ongoing operation or maintenance costs (ORC is generally effective in the subsurface for 9 to 12 moths). Since the proposed demolition and redevelopment at the Site will prevent installation or operation of an active remediation for at least one year, the proposed ORC remediation program has the added advantage