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Attachment 6 TreadweII&RD110 <br /> A <br /> BioScreen Model LANSAN COMPANY <br /> 420 N Madison Street <br /> Stockton,California <br /> The BioScreen model Version 1.4 was run for a period of 9 years(2004 to 2013) under three modeling scenarios: <br /> 1. Instantaneous reaction; <br /> 2. First order decay; and <br /> 3. No degradation. <br /> Modeling Assumptions and Site Specific Parameters <br /> • The saturated source zone thickness was assumed to be 20 feet. This thickness was calculated as the distance from the water table in <br /> 2003 (approximately 16-18 feet bgs), to the depth of the deepest soil sample for which TPH was detected (36.5 feet.bgs in boring CSB- <br /> 2). <br /> • The total width of the,source area was assumed.to be 45 feet or approximately the width of the excavation conducted in June 2004. The <br /> source area concentration was assumed to be greatest in the center, and decreasing towards the edge of the source area.The <br /> maximum benzene concentration input for center of the source area was 75 lag/L. 46 lag/L was used as the next highest source area <br /> concentration; and the lowest source area.conce.ntrati.on was assumed to be 5 lag/L. A:simulation.time of 9 years was used, which is the <br /> time since sample CPT-EXC-W was collected. <br /> • The lithology was assumed to be silt or sandy silt, with an average hydraulic conductivity value of 1x10-3 centimeters per second (cm/s) <br /> (EPA, 1997) <br /> • The porosity value was assumed to be 0.25(EPA, 1997). <br /> • The gradient was assumed to be 0.002, based on gradients calculated for the adjacent 423 N Madison Street site(AGE, 2002). <br /> • The first-order degradation rate, 0.14 year', was based on the.analysis of time series data from 423 N Madison well MW-21A(described. <br /> previously). <br /> • Delta Dissolved oxygen = average background concentration minus source concentration = (2.47)+(1.77)—(0.66) =1.46 mg/L. <br /> Page 1 of 6 <br />