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CONESTOGA-ROVERS &ASSOCIATES isoconcentration lines showing the estimated 'areas enclosed"by these '.lines are shown on Figure 6. MTBE chemical data in groundwater and isoconcentration.lines showing the estimated " areas enclosed by these lines are presented .-on r Figure 7. The areas enclosed by each k isoconcentration"line are also listed in the tables in Attachment D. The impacted,soil or groundwater volume for each-interval was estimated by multiplying.the areas enclosed by the isoconcentration lines (the :impacted surface area) by,the impacted thickness of the soil or groundwater.. A porosity of 30% was.used for calculating actual.volume T of impacted groundwater. The remaining mass calculations are_based on available. site data, and an assumedstatic environment versus the true dynamic:environment. The error (±20%) associated with these - ' estimates should be considered with any decision-making. - - Assumptions and Formulas ■ A depth interval�4of 6.5 to 21.5 fbg(the screened interval for monitoring,well MW-1 A)was P _ thickness of the impacted 'water-bearing zone:due to water level . assumed for the p g . E variability. The impacted water-bearing zone is therefore approximately.l5 feet thick. ■ For each center;contour interval, we assumed that the applicable chemical concentration _ c was the median.of the-highest detected'.value and the contour interval. -For all other contours, the,average concentration was -assui`ned,to be one-half of the value of the - higher contoui,interval. - ■ Soil contaminant mass.in kilograms (Kg) = soil volumeincubic feet (cu-ft) x bulk density (89.9 lbs/cu-ft)x.1 Kg/2.2046 lbs x average concentration(mg Cg)x-10-6 Kg/mg. ■ Groundwater contaminant mass in. kilograms += groundwater -volume (cu-ft) x 30% estimated-pore volume x '28.32 liters (L)/cu-fl x average concentration (pg/L) x 10-9 kg/pg._ Contaminant Mass in Soil TPHd Mass in SoU. The average concentrations of TPHd in oil enclosed within the 1-mg/Kg, .. 10-nig/Kg, and 100 mg/Kg isoconcentration lines were conservatively estimated.as 5 mg/Kg, 50 mg/Kg, and 500 mg/Kg, respectively. The average concentrations of TPHd in soil enclosed 4 within the-1,000-mg/Kg isoconcentration line was conservatively estimated as 2,000 mg/Kg. Thee f -0813 9 Worldwide Engineering,-Environmental,Construction,and-IT"Services