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5.0 MODEL SIMULATION �s The computer model was used to determine the impact (if any)that the mass of diesel may have on groundwater. Appendix A contains the VLEACH model data sheet used to - summarize the data input into the model. Some of the data were available from the previous investigation or published field retards and reference material. However, reference material for the organic distribution coefficient, Henry's constant and free air i. distribution coefficients are not available for such a complex chemical as diesel fuel. Therefore,values for these parameters were taken from published information on specific chemicals that are contained in diesel. These determinations are discussed below: • Organic Carbon Distribution Coefficient (K.). This parameter has a large impact on the results,but was the most difficult factor to estimate for the simulation. c Available tabulated data for compounds found in diesel fuel have a K.ranging from 54 to 3,000 ml/g. Chemicals with low Kr values tend to be more volatile and can escape more rapidly to the atmosphere. They may also be more soluble and able to leach downward through the unsaturated zone to the saturated zone. Forexample,the K� of benzene is 64 and the K�of xylene is 692. However, these light compounds constitute a very small percentage of diesel. The predominant components in diesel are heavier chemicals with higher K,,,. In general, these components are less volatile,have low solubility values, and have Ka values in the 2,000 to 3,000+ranges. Hence, a Ka of 2000 was used in the simulation. • Henry's Constant (Kh); calculated data ranged from 7 to 24. • Free Air Distribution Coefficient; available data ranged from 0.5 to 1.5.- This range was rather narrow and tests using extreme values had only a slight impact on the final calculations. 6.0 MODEL RESULTS The results of the simulation are shown in tabular form in Appendix B. The simulation predicts that approximately 12,528 grams will impact groundwater over a 500-year time. period. Relative to the calculated diesel mass in place,this equals approximately 4.9 gallons of diesel (19% of the total). Figure 5 illustrates the calculated impact to groundwater in graphical form. It shows that the impact would occur over a long period, of time, and at low concentrations. Appendix C is a printout of hydrocarbon concentrations in gas,liquid, and solid phases for polygon#1. Included are profile predictions of the soil at 15 year intervals for 45 years. After 15 years the soil concentration at a depth of 25 feet was predicted to reach 8.7 mg/kg. At 30 years the concentration was predicted to be 16 mg/kg and at 45 years 21.7 mg/kg. 7