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1 <br /> Wilbur-Ellis Transport Modeling <br /> September 15, 2005 <br />' Page 2 of 3 <br />' Parameter „Value _ Explanation <br /> Gradient 0 01 ft/ft to NE site specific <br />' Hydraulic conductivity E-4— E-3 cm/sec silt & clay w/some sand <br /> Effective porosity 5-20% silt & clay w/some sand <br /> Organic carbon content 05 - 1% silt & clay with organics <br /> First order decay yes default values from Howard <br /> Source zone concentration 5000-10,000+ppb MW I, B-2, P5W max values <br /> Source zone width 100 feet approximate <br />' Mixing zone thickness 30' AD I, high water to below deepest hit <br /> Travel time 18 -.30 years tank use and removal <br /> Current conc in MW3 50 - 250 ppb 2004 & 2005 data <br />' Current conc in SBB <0 5 ppb 2005 data <br /> Results <br />' Source Zone Concentration of 10,000 parts per billion (ppb) <br /> With a source zone concentration of 10,000 ppb, the model calibrated fairly well with the <br />' concentrations measured in well MW3 and boring SBB at 18 years by using a hydraulic <br /> conductivity of 2 E-4 cm/sec, effective porosity of 15% and organic carbon content of <br /> 065% The predicated benzene concentration in MW3 was 93 ppb and the predicted <br /> I • benzene concentration in SBB was 0 86 ppb (which is slightly higher than the detection <br /> limit of 0 5 ppb SBB was non-detect) <br />' Running the model with these parameters, steady state was reached by 30 years The <br /> steady state concentration at MW3 would be 96 ppb and at SBB would be 1 4 ppb At a <br /> point 200 feet downgradient of the source the concentration after 30 years would be <br />' 0 025 ppb (not detectable) <br /> Source Zone Concentration of 60,000 ppb <br />' Somewhat better calibration was obtained utilizing a source zone concentration of 60,000 <br /> ppb and increasing the organic carbon content to I% The predicted concentrations at 18 <br /> years in MW3 (120 ppb) and in SBB (0 23 ppb) more closely approximate the measured <br /> values This model predicts that the steady state concentration in MW3 would be 140 <br /> ppb and in SBB would be 0 79 ppb Steady state was reached by 30 years The <br /> concentration after 30 years of transport 200 feet downgradient of the source would be <br /> I 0 0048 ppb (not detectable) <br /> I <br /> Conclusions <br /> Good calibration was obtained with the model using site-specific hydrogeologic and <br /> contaminant parameters or reasonable estimates The model predicts that detectable <br /> concentrations of benzene will never be present 200 feet downgradient of the source area <br /> and that the maximum concentration at the downgradient property line will be below or <br /> only slightly above the MCL of 1 ppb <br /> I <br />