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graphical format that illustrates the mass rate of contaminants loading to the groundwater versus time for each polygon. 4.0 MASS BALANCE CALCULATION In order to perform the modeling simulation, it is necessary to calculate or estimate the mass of the contaminants that remain in place in the vadose zone. Normally, this is done by drilling a series of borings through the contaminant plume, analyzing a number of samples, and producing a series of contour.maps showing the concentration and extent of the contaminants at various depths. This technique is difficult to apply at the ST&E Railroad site, because contaminants were detected in only one boring, making it difficult to delineate the impacted area and to,estimate the average concentration of diesel throughout that area. In this case,the size of the contaminant plume was estimated by constructing a cross section through three of the borings(Figure 4�. In cross section,the plume is shown as a cone-shaped mass with a maximum surface area at the base of approximately 200 square, y' feet and a height of 10 feet. The height was determined from boring B-I' where the upper limit of contaminated soil is above 15 feet and the lower limit is below 20 feet. For modeling purposes, a somewhat more conservative approach is used. The plume is envisioned as two concentric cylinders with different average diesel concentrations. The " outer cylinder is modeled as a polygon with a surface area of 160 square feet, and a concentration that increases from non-detect at the outer margin to perhaps a few hundred . parts per million along the inner margin. Hence, we estimated an average concentration of 200 ppm for this outer cylinder. The core of the plume occupies the inner cylinder, and its average concentration is somewhere between the highest measured concentration(9,100 ppm) and the 400 ppm concentration at the boundary with the outer cylinder. With no other means of estimating this average concentration, we selected 3,000 ppm as being representative of the average concentration in the core of the plume. An estimate of the total mass of diesel'in place was then calculated using the following formula: (Aerial eAcnt in fl)*(hei&in 8)*(ave.contamination cmcentration in mg/kg)*(average density of soil in kgffl3 yk(graml1000mg) thus; Polygon#1= 40 ftz * 10 ft * 3000 mg* 50.5 kg * I g = 60,600 grams kg W 1000 mg Polygon#2=,160fta * 10 ft 200 ms * 50.5 kg* 1 g = 16,160 grams kg ft, 1000 mg or in gallons; 76,760 g kg * 22 lbs * Gals. diesel W 23.8 gallons diesel 1000 g kg 7.1 lbs 6