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Ms Diane Hinson Project 0805-118 03 February 13, 1995 Page 2 5 Provide a reference table of any values used in the calculations from published sources EMCON's response to each of the above concerns are as follows 1 There are two methods of estimating the concentration over an area based on an isoconcentration line, a) to calculate the linear average concentration over the area in consideration or b) consider the inner lune value to be constant over the entire area EMCON consistently used the outline value to calculate the mass of TPHG in soil for both sites The same procedure was used in calculations for both sites to make them comparable As the intent of the calculations was not to calculate the actual mass of TPHG, any consistent and valid procedure ought to be sufficient 2 The mass of TPHG in groundwater is presented in a separate section of this letter The procedures adopted in the calculation of the mass was sunilar for both the sites 3 The isoconcentration lines for TPHG in soil was developed based on sora • analytical results from soil borings The isoconcentration lines do not form perfect figures to calculate the area under two given lines In order to make the calculations simple, EMCON attempted to split the area into known figures such as circles, rectangles and triangles Measurement of distances for the calculation of an area has been conservative and probably more than the actual area For example, in Figure 1 (Isoconcentration Lines for TPH as Gasoline at 13 feet bgs) within the isoconcentration lune for 250 ppm, the area was calculated as a circle of 10-feet diameter (A=0 7854xD2=78 54 ft2) Calculating the area within the 250 ppm isoconcentration lune using a planimeter results in a value of 73 98 ft2 � 4 TPHG was used as a representative of petroleum hydrocarbons in soil Analysis of fuels using gas chromatography shows that TPHG consists of higher chain petroleum hydrocarbons (C2-C12), while benzene (C6), toluene (C7), ethylbenzene (C8) and toluene (C8) are the lower hydrocarbon compounds The analysis of TPHG does to a certain extent reflect the sum of the entire range between C2 and C 12 5 Table 5 presents all references used in the calculation of mass of TPHG in soil and groundwater MASS OF TPHG IN GROUNDWATER At the request of PHS/EHD, EMCON has calculated the mass of TPHG in groundwater To determine the mass of TPHG in groundwater at each site, EMCON first estimated the