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The guidance for environmental attenuation factors used by the RWQCB suggests that attenuation factors up to 1,000 are appropriate depending on waste characteristics (LFs, etc ) and site conditions At this Site, the impacted soils are protected from incoming precipitation by an asphalt surface which reduces infiltration by 90% or more (Dames & Moore, 1994a) In addition to the low infiltration rate of water, soils in the unsaturated zone have low permeability resulting from the fine- textured soils with abundance of silts and clays, and groundwater is relatively deep (currently 56 feet bgs) The results of soil samples discussed above in Section 4 1 suggest that TPH-d concentrations decline from approximately 2,000 to 3,000 mg/kg at 21 to 26 feet to <5 mg/kg at 36 feet, a reduction of at least 98% every 10 to 15 feet Therefore, the environmental attenuation factor for TPH-d was fudged to be high, and a value of 500 was used Given the higher water solubility of semrvolatile constituents, an average environment attenuation factor of 100 was used for semivolatile organic compounds 4 3 3 Results of the DLM Evaluation Tables 3 and 4 show the Soluble and Total Designated Level calculations along with and the analytical results of total and WET constituent taken from the two most impacted samples obtained from the Site None of the analytical results exceed the total or soluble designated Ievels for either sample The total designated level for TPH-d (18,400 mg/kg) supports previous clean up values for soils employed during soil excavation and on-site treatment (Dames & Moore, 1990) Based on these results, the TPH-d-impacted soils at the Site do not present a potential threat to groundwater None of the other constituents of diesel oil, including napthalene exceeded total or soluble designated levels Therefore, these constituents are not a threat to groundwater at this Site Volatile halogenated organic compounds were not evaluated because no halogenated compound was detected in soils According to the DLM the soils at this Site do not represent a potential threat of any kind despite the conservative assumptions presented in Tables 3 and 4 4 4 GROUNDWATER CHEMISTRY None of the groundwater samples collected from monitoring wells on the site between 1987 and 1991 reported detections on any hydrocarbon constituents The round of sampling performed for this report reported only 1 detection of 0 18 mg/L for TPH-d analysis from monitoring well DMW-4 on May 2, 1995 D&M Lab reported that the hydrocarbons detected were in the range of • diesel, but did not match a diesel pattern Monitoring well DMW-4 also had a detection of toluene of 0 63 ug/L, slightly above the detection limit of 0 5 ug/L DMW-4 was resampled on May 26, SACT 56 14 11 DAMES &MOORE