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1 RESIDUAL MASS BALANCE CALCULATION The residual mass balance in soil was calculated for 1,2-DCA only (0 0017 mglkg @ VB-2-25), based on the analytical results of the soil samples collected by Apex on August 29, 1997 (Table 1) Other petroleum constituents were not detected in these soil samples The residual mass balances in groundwater for MTBE, 1,2 DCA and TPHd were calculated based on the results of groundwater sampling performed by Apex on November 21, 1997 (Table 2) Residual mass balance was not calculated for other petroleum constituents since they were not detected or negligible in the groundwater The calculation approach below is conservative in nature, and it is likely that the actual mass quantities of the constituents are less than the calculated amount r ASSUMPTIONS Because of the uncertainty in estimating the concentration distributions m groundwater and subsurface soil, following assumptions were made ' 1 The areas of the impacted groundwater are shown on Figures A, B, and C The vertical extent of the impacted groundwater is from 20 to 50 feet bgs The saturated soil contains lie 25% water by volume 2 The area of the 1,2-DCA impacted soil is shown on Figure D The impacted soil is assumed ' to extend from 25 to 50 feet bgs The density of the soil is 120 lb/ft3 3 Mean concentrations were used in the mass balance calculations, and considered uniform ' throughout their respective calculation columns Concentrations at below the standard laboratory detection lumts are considered zero CALCULATION 1 impacted Groundwater Residual Mass Balances (calculation spreadsheet attached) ' The estimated MTBE remained in the groundwater is 0 034 kg or 0 068 lbs The estimated 1,2DCA remained in the groundwater is 0 016 kg or 0 032 lbs ' The estimated TPHd remained m the groundwater is 0 23 kg or 0 56 lbs --- ' 2 Residual 1,2-DCA mass balance in soil is ' 4,400 (ft)x 25 (ft) x (0 0017/2) (ppm)x 10'6 x 120 (lb/ft') =0 01 lbs 01 1 i