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Lodi Ready Mix Page 8 Site Closure Report Project No. 93.1 May 12, 1995 ITS will tend to preferentially travel along the finer grain units before transitioning to a coarser grained unit (at saturation). Therefore because no diesel Y range petroleum hydrocarbons were detected in P the silt at 25-27 ft we conclude that this is layer represents the vertical limit of ' the contamination. The horizontal limits have been interpolated to be approximately five feet inward from the walls of the excavation. These limits are supported by laboratory analytical data which show that all of the confirmation samples tested from SB-1 are free of detectable diesel and gasoline range petroleum hydrocarbons. The depth to groundwater at this site (approximately 53 feet- see attached Figure 3 Water Level Map) suggest that any remaining hydrocarbons above 27 feet should not pose a threat to groundwater. Even at the "wet" year high water mark of 40 feet (bgs) an approximate 13 to 28 foot buffer may exist between the last detectable contamination in the excavation and the water table. No evidence of recent contact with the water table was observed in the soil contained in the split spoon sampler from the 40 -42 foot interval sample. Manifestation of recent groundwater contact may include oxidized iron laminae and elevated moisture content. Although no groundwater samples were collected or tested, several factors ' suggest that the groundwater has not been impacted. The degraded nature of the contaminant, diesels' tendency to not migrate far, a 13 to 28 foot buffer between high water and the deepest observed contamination, and the ' attenuating action of the silt layer at 25 feet strongly suggest that the remaining diesel above 25 feet is not in a position to threaten groundwater. In time these low concentrations will attenuate to negligible levels. Also ' because the former excavation is capped with concrete we do not expect percolating rainwater from coming into contact with the remnant hydrocarbons. ' Therefore Geological Technics Inc. concludes that the evidence gathered during this investigation suggest that a groundwater study is not warranted ' and we recommend that no further action take place. We feel that it is reasonable for the PHS/EHD to corroborate with the staff of the CRWQCBCVR and consider the candidacy of this site for closure.