SITE INFORMATION AND CORRESPONDENCE

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RE-MANUFACTURING -2- 28 February 1991 CLOSURE PLAN According to Kleinfelder, there is roughly 400 cubic yards of soil that can be removed from the SI without disturbing existing on-site facilities. The SI is adjacent to Mormon Slough, which is to the north. A storm drain line and surface water drainage channel are just east of the SI and drain to Mormon Slough. Excavation of the SI is also limited on the south and west sides by a sanitary sewer pipeline (see figure) . On page 16 of the closure plan, Kleinfelder states that "Excavation of the constrained soil will result in the removal of significant lead contamination from the SI. However, it is not known at what concentrations lead is present in the soil outside of the constrained soil area. " This is not a sufficient argument to show that further excavation is unreasonable or infeasible. The extent of the contamination must be determined, in order to satisfy the requirements of the HAR and to determine the feasibility of further excavation. The soils remaining in the SI do not appear to be native soils. In fact, waste sludges may still remain in the floor and sidewalls of the SI . On page 15 of the plan, Kleinfelder states that the sidewalls and floor of the excavated pit visually appears to contain remaining disposal sediments. This could account for the extremely high levels of lead found in the sidewalls. Kleinfelder proposes to close the SI without further excavation and backfill with on- site soils. They also propose a 2-inch AC cap, to be placed on top of the backfill material . The recommendation to leave the existing hazardous wastes in place is based on the results of two computer models: the Hydrologic Evaluation of Landfill Leaching Potential (HELP) and Seasonal ized Soil Compartment Model (SESOIL) . Kleinfelder used the two models to assess the potential for contaminant migration through the vadose zone, towards ground water. Ground water is roughly 60 feet below ground surface, or roughly 50 feet below the floor of the SI. The HELP model provides an indication of the amount of water that would pass through a landfill cap and the SESOIL model models the migration of contaminants through the vadose zone. The DI-WET results for lead were used in the SESOIL model to estimate the maximum concentration of lead that infiltrating rain water would pick up when passing through the soil . Since all the DI-WET results were non-detectable, the detection limit of 0.5 mg/l was used as the maximum concentration of soluble lead. The migration of lead was simulated over a period of thirty years. Permeability values for the vadose zone were estimates based on boring logs, rather than field or laboratory data. No permeability value was given for the AC cap. According to Kleinfelder's report, results of the models indicate that soluble lead will move through the vadose zone at a rate of 16 cm in 30 years if the SI is backfilled with on-site soils. This is presuming all the assumptions put into the models are correct and the cap remains intact. Extensive compaction effort of the fill below two feet of grade is not proposed. The design includes an assumption that the AC cap will remain as open space and will not be used for vehicle traffic or material storage, which would cause a failure of the cap. There is no guarantee that this will happen. How will Re-Manufacturing assure that the area will remain open in the future? Section 2593(b) of Chapter 15 requires that a closure and post closure maintenance plan per Section 2597 be submitted to discuss these issues. The models only considered the threat to ground water due to the migration of rain water through the proposed cap. However, the protection of ground water is not the only concern. It is also important to determine the extent of the lead contamination