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and are approximately 11,300 cubic yards of soils remaining which are impacted with detectable levels of TPH/D The distribution of maximum levels of TPHID is indicated by confirmation sampling conducted subsequent to excavations (Dames & Moore, May 1992) This sampling indicates levels of TPH/D up to 9,200 mg/kg at a depth of 22 feet bgs in the east wall of the east excavation, and up to 8,300 mg/kg in the north wall of the west excavation at a depth of 13 feet Within the upper 10 feet bgs, impacted soils have been excavated, treated, and removed to a landfill 4 2 GROUNDWATER Depths to groundwater within the shallow (less than 150 feet bgs) unconfined aquifer beneath the site have varied over the period of investigations The shallowest levels, approximately 35 feet bgs, were measured in July 1987 when monitoring wells were first completed Water levels subsequently declined to a maximum depth of about 69 feet bgs in July 1991, then rose to approximately 61 feet bgs in May 1994 Comparison of on site water level measurements in the shallow aquifer to long term water level changes in wells penetrating the deeper regional aquifer indicates that water level trends in the shallow aquifer parallel those in the deeper aquifer Long term trends in the regional aquifer are downward as a result of overdraft pumping Thus, groundwater levels in the shallow aquifer are unlikely to recover to former levels within the foreseeable future, and if they did recover, the shallow levels would be unlikely to persist for more than a few years, based on historical trends (Dames & Moore, August 1994) No detectable levels of site constituents have been found in the shallow groundwater beneath the site, based on eight rounds of sampling from June 1987 through May 1994 Analyses included volatile organic compounds (VOCs) by EPA Method 602 and TPHID by EPA Method 8015 for all rounds Phenols by EPA Method 604, and polynuclear aromatic hydrocarbons (PAH) by EPA Method 610 were analyzed for during one or more sampling rounds In August 1994, Dames & Moore conducted an analytical model simulation of vadose zone leaching This model, which represented a period of 25 years, was used to predict what impact the residual soil TPH/D and its most mobile constituent, naphthalene, would have on groundwater (Dames & Moore, August 1994) One model scenario assumed a depth to groundwater of 54 feet bgs This depth was chosen to represent a long term average of the variable depth to groundwater SAC127 10 6 DAMES &MOORE