SITE INFORMATION AND CORRESPONDENCE 1995-2004

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EVALUATION OF TPH AND BENZENE SOIL VERIFICATION CONCENTRATIONS,S70CKTONFORMER MGP SITE I = Water infiltration rate twace = Time for water to move through vadose zone to groundwater XYZ = Thickness of the vadose zone Tchem ad = Time for chemical to move throught the vadose zone The following describes the input parameters used for this model as shown in Table 1. Bulk density(1.37 g/cm3),soil porosity(0.263),and soil organic carbon (0.0016) are specific to the Stockton site and were used in all of the VLEACH modeling that supported the Feasibility Study and PAH verification target concentrations. The Log K.values were taken from the information presented in the Castle report.In the Castle report, the log Ko„.for gasoline was estimated using Log K.s for BETX and pentane, which range between 2.5 and 3.5.A value of 3 was selected for TPH-gasoline in the Stockton model. A conservative range of 3.5 to 4.5 was used in the Castle report for the heavier mixture of TPH-diesel. A value of 3.5 was selected for TPH-diesel in the Stockton model. The resulting leachability factor or retardation factor (R)using the site specific soil parameters and the Log K.„s are 17.6 for TPH-diesel and 6.26 for TPH-gasoline. These leachability factors are similar to those calculated in the Castle report. The infiltration rate used for developing the Stockton TPH concentrations was based on the assumption that the entire annual rainfall of 1.1 feet/year would infiltrate into the soil (similar assumption was used in the Castle report). This assumption is rather conservative given that the site is paved and will continue to be paved after soil remediation is completed. A lower rate was used in the VLEACH model for Stockton. A higher infiltration rate results in a lower cleanup level. A vadose zone thickness of 20 feet was used in the Stockton model. Depth to groundwater is about 23 feet. The same chemical half-lives were used for TPH-diesel and gasoline, 10 and 5 years,respectively as in the Castle report.For comparison, the half-life used in the VLEACH model for naphthalene in soil was 125 days. Therefore, these half-lives are conservative. The calculated Environmental Attenuation Factor for TPH-diesel was 439 and 80 for TPH- gasoline.As in the Castle report,0.1 mg/L or 100µg/L was assumed for the groundwater quality objective(Cg�qo)for both TPH-diesel and TPH-gasolineon .RWQCB's limit for TPH-diesel is 100µg/L. 100µg/L is used for TPH-gasolinebased be estimating a beneficial use standard comparable for protection of the individual benzene, toluene,ethyl benzene,and xylene fractions by dividing the MCL for each of the BTEX components by the fraction of the component in gasoline as listed in the LUFT manual (Castle, 1996). Using the input parameters, the resulting soil verfication target concentration for TPH-diesel is 770 mg/kg and 50 mg/kg for TPH-gasoline. References Air Force Base Conversion Agency(AFBCA), 1996,Response to California Regional Water Quality Control Board Staff Letter of 15 July 1996. [Proposed Cleanup Level Evaluation, TPHBBENZENEVERIFICATION MEMO.DOC 3