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disposable plastic bailers and transferred to 1-liter amber bottles, then labeled and placed into a • cooled ice chest for later transport to the laboratory All samples were analyzed for TPH-diesel The laboratory report is contained in Appendix D and a summary of these data is in Table 3 2.3 Mass Balance Calculations The soil analytical data were transferred to Table 4 and used to construct new contour maps of diesel concentrations The maps were constructed by using the mean concentration for each 5-foot sample interval The mean was approximated by averaging the concentrations at the top and bottom of each interval For example, the mean concentration for the 35-40 depth interval in boring GT-22 determined from Table 4 is (3,480 mg/kg + 1,560 mg/kg)—2=5,040 mg/kg--2 =2,520 mg/kg i In some borings, samples were collected at 10-foot intervals In these cases, the upper/and lower sample were averaged to estimate the concentration at the mid-point between samples, and then this calculated concentration was then used to calculate the mean concentration lin the intervals above and below it Boring GT-23 provides an example The concentration at 60 feet is estimated from the concentrations at 55 and 65 feet (1 3 mg/kg+ 9 25 mg/kg)— 2 = 10 55 mg/kg T 2 = 5 mg/kg Hence, the mean concentration between 55 and 60 feet is • (13 mg/kg+ 52m g) -2 = 3mg/kg In those cases where the sample at either the top or bottom of the interval was reported as Non- detect (below the 1 mg/kg detection limit), the mean concentration for the interval was determined by adding the other concentration to 0 and dividing by 2 Mean concentration data were then contoured to map the extent of soil contamination All maps plotted the 10, 500, 1,000, and 5,000 mg/kg contours, and for some maps the 10,000 and 15,000 contours were also drawn However, because the area within the 10,000 and 15,000-foot contours was small, these contours are not shown on the final drafted maps in this report After the maps were generated, the volume of gasoline within each depth interval was computed AutoCADQ automatically measured the area between contour lines at each depth honzon (35 feet, 40 feet, etc) For example, in Table 4, the 5,000 mg/kg contour line at a depth of 35 feet encompasses approximately 95 square feet Because the hydrocarbon plume spreads outward with depth, this same contour line encompasses 540 square feet at a depth of 40 feet Hence, an approximate estimate of the volume of soil between the top and bottom of this depth interval is Vsoil = [(95 ft2+ 540 ft2)— 2] x 5 ft= 1,588 ft' 3 f