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4� - Ms. Lori Duncan June 12, 2007 _ CONESTOGA-ROVERS &ASSOCIATES Historical groundwater monitoring data is included as Attachment A. - - MTBE MASS REMAINING IN GROUNDWATER The MTBE mass remaining in groundwater was'estimated using the following equation: ,. Mass(lbs)=Aquifer Volume(gal)x Porosity(%)x Avg Concentration(µgfl X110?kglgg x 3.785(Ugal)x 2.205(lbs/kg) CRA used MTBE concentrations from the January 22; 2007'groundwater monitoring event for these ; calculations. The impacted,water bearing: zone was taken from.10 fbg to 25' fbg. ..The resulting groundwater thickness is assumed to be 15 feet. t ' An isoconcentration contour map of MTBE is presented as Figure 4. The.porosity was assumed to be 40. percent for silts and clays identified at this site; based on typical porosity values for soil types published in Groundwater, by Freeze and Cherry, .1979. F,The average contaminant concentration per+contoured interval was assumed half the.value of the higher contour interval. For the center contour interval, we assumed that the average contaminant concentration was the median of the highest detected value'and the contour interval. Table 2presents theestimatesand calculations for the MTBE mass remaining in groundwater. Based on the approach and assumptions presented above,.the estimated MTBE massin. groundwater is approximately 1,85 pounds(030.gallons). These contaminant mass remaining estimates:are based on available site data and an assumed static environment versus the true dynamic environment' The error associated.with.these estimates should be: considered with any decision-making. a MTBE MASS FLUX TO SUPPLY WEL'L'' '. Potential impacts on groundwater receptors and the need.for and relative benefits of alternative remedial { measures may be evaluated on the basis of the mass flux'of contaminants"from the source area to the receptor. This mass-based approach to site assessment and remediation has been identified by.the United - States Environmental Protection Agency(USEPA) as .a .key consideration in the evaluation of natural' = attenuation remedies(USEPA,Monitored Natural°Attenuation for Groundwater, 1998). _ r To assess the potential MTBE`impact to local drinking water for this site, CRA assessed the mass.flux of MTBE.to a virtual supply well using the following equation(Buscheek,'June 2002):.: 1667 _ 4. l .. _ Worldwide Engineering,Environmental,Construction,and IT Services