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5.0 FATE AND TRANSPORT MODELING Using the Site data available, estimates of detectable benzene travel times in Site groundwater were calculated Two different soil samples collected from the sandy zone were analyzed for hydraulic✓ ' conductivity, porosity and specific gravity These ASTM geotechnical laboratory methods were performed by Wallace-Kuhl & Associates Their results are provided in Appendix D The results ' between the samples were comparable indicating a vertical hydraulic conductivity of 1 x 10-4 cm/sec, a porosity of 0 40 to 0 45 and a specific gravity of 2 6 Considering the fact that the alluvial deposits at the Site are stratified, the measured vertical hydraulic conductivity value was adjusted to an approximate horizontal value by multiplication by a factor of 10 This multiplication factor represents ' a typical K,/Kh ratio for "medium level' stratification The resulting value is typical of the horizontal conductivity of silty sands and clean, fine sands (Groundwater Manual, 198 1) I Both soil samples were also analyzed for total organic carbon indicating 0 03% to 0% in the samples Using these data and the Log K"H, (octanol/water partition coefficient) for benzene, a retardation factor(Rf) for benzene in the sandy zone was calculated to be 1 11 This factor is relatively low for benzene which commonly has an Rf of 2 to 3, and may be indicative of the low total organic carbon content of the sandy zone Using these data, a simplistic flow model was used to estimate adsorption retarded benzene travel times in the sandy zone The modeling was conducted using the RES SQC model code by Javandel and Tsang, 1984 as contained in the Well Head Protection Area (WHPA)program package (Blanford and Huyakorn, March, 1991) This model is a two dimensional analytic model which solves the groundwater flow equation The model parameters used are detailed in Table\� Modeling calculations indicate that for a gradient ranging from 0 001 to 0 01, the retarded transport rate of benzene in Site groundwater could range from less than 5 feet to 25 feet per year based on a Icontinuous and uniform hydraulic gradient Given that the gradient magnitude and direction is documented to vary significantly throughout the year, the actual lateral travel distance of detectable benzene from the Site during a typical year would be much less than these estimates suggest . q SEACOR• N0026-001-01 RDF 1 1 August 1 1994