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i 1 1 • Soil results from 1998 at MW-2 indicated the presence of 14,000 mg/kg TPH at a depth of 20 5 ' feet bgs and 1,500 mg/kg TPH at a depth of 30 feet bgs Appendix A provides cross sections of the areas to be removed, calculations of mass removal, and detailed cost estimates ' The concentration of 1,500 mg/kg was the highest recorded concentration at a depth of 28 feet and this location coincided with the tank cavity location Much of the area would only need excavation to 25 feet to remove the TPH concentrations targeted for this option Depth to groundwater is approximately 12 ft bgs This excavation alternative would extend approximately 14 feet into the water table, and shonng would be required for the site Given the ' proximity of the excavation to utility lines, railroad tracks, and the depth of the excavation, shoring becomes a necessity for this option and is estimated to cost an additional $85,000 to $95,000 An additional impediment to excavation is the 14 ft by 20 ft concrete pad, estimated to be 6 inches in thickness that is partially over the excavation area Half of this pad will have to be removed to allow excavation Costs for removing the concrete pad are estimated to be between $1,000 and $2,000 The power line and power pole that lay adjacent to the excavation area will also need to be removed for safety purposes The cost of removing the power line and power ' poll is estimated to be between $2,000 and$2,500 The top 10 feet of soil is considered clean fill and would be stockpiled and re-used on site Thus,of the 1,350 cubic yards removed, approximately 505 cubic yards are re-used and 850 cubic yards would be disposed of at an appropriate landfill facility Ancillary operations needed to address soils below the water table include dewatering the excavation at a rate of 15 gpm, treating the water, and discharging to the City of Stockton sewer system By ' extending excavation into the water table, additional costs will be incurred to dewater the site It is estimated that dewatering will cast between $21,800 and $37,500 Extracted water will be piped to a treatment system consisting of one 2,000-pound carbon unit arranged in series, pumps, sand filter, holding tanks, water level contiollers, piping and manifold The treated water is to be discharged to a City sewer with appropriate permits The costs for this alternative are estimated between$254,000 and$299,000 Shoring costs of$85,000 to $90,000 represent almost 30 percent of the cost of this option This alternative would result in the removal of approximately 11,500 pounds of TPH material from the tank excavation area, which ' translates to a cost of between$22 44 and$26.23 per pound of TPH removed 52 Alternative A2 Excavation with Limited Vertical Extent(5 Feet into the Water Table3 ' A second alternative considered removing the contaminated soils occurring at the top of the water table and extending approximately five feet into the water table This option extended to a larger area and included excavation from HA-8, HA-9, the utility pole, and then towards MW-3 Appendix B provides cross sections of the area to be removed, calculations of mass removed, and detailed cost estimates This represents an area of 1,540 ft2 compared to Alternative Al 17 1 R 1CONTGRAMSImkton\CAPICAP ADDENDUM