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ti +1 i KEI-P91-0202 .R7 March 18 , 1993 Page 3 The highest concentrations of soil contamination at the subject site were detected near the south end of the eastern most former underground fuel storage tank. MW1 is the closest well to this location. Therefore, the Boring Log for MW1 was used to create the soil data. Various physical and chemical properties of the soil types were obtained from standard geological and engineering references. The location of the former underground fuel storage tanks, monitoring wells, and exploratory borings are shown on Figure 1 . The pollutant characteristics data was created to simulate the migration of benzene through the unsaturated zone. Various physical and chemical properties of benzene were obtained from standard chemical references . These included molecular weight, solubility, an air diffusion coefficient, Henry ' s law constant, an organic carbon/water partitioning coefficient, and a soil adsorption coefficient. Biodegradation was not considered and was assigned a zero value for Simulation #1 . Benzene was used because the maximum contaminant level (MCL) for benzene is the most stringent of the contaminants being considered. Therefore , using benzene as the chemical pollut- ant yields the most conservative simulation that can be performed. The application parameters used were also extremely conserva- tive. In an effort to remove as much of the contaminated soil as possible, the underground fuel storage tank pit was overexcavated in the area of highest contamination to a depth of approximately 26 feet below the surface . Therefore, the results of soil samples collected at a depth of 26 feet in the area of highest contamination were used as a contamination source for the SESOIL model . Beginning at a depth of 26 feet, four soil layers with varying properties and thicknesses were used in the model, based on soils encountered in the boring for well MW1 . The horizontal area of the soil column was assumed to be 20 feet by 20 feet. This is an extremely conservative estimate because SESOIL assumes that the contamination introduced to the subsurface is uniform in concentration throughout the entire area of simulation. Therefore, SESOIL assumed a greater amount of contamination than was actually present. The area of the SESOIL model for Simulation #1 is shown of Figure 1. Contamination of the subsurface system was simulated by entering an instantaneous spill corresponding to the time of . the tank excavation (which was in March of 1991) . This source was assumed to be present only in the first month of the simulation. The simulation was run for a period of ten years