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<br /> ` Adsorption: Retardation factor was calculated by the model using user-input values for soil bulk
<br /> �J density, partition coefficient, and the fraction of organic carbon(foe) in the soil at the site. Since
<br /> site-specific measurements of soil bulk density have not been performed, a bulk density of 1.7 1
<br /> kg/l (default value for the model based on User's Manual) was selected. A partition coefficient
<br /> for MTBE2 of 11.2 1/kg was used. Analytical results for foe in two soil samples collected in
<br /> January 2001 from boring GP-2 indicated foe levels of 1.3% and 1.9%. An averaged value of
<br /> 1.6% was used in the model. Adsorption values used were the same for both Sand A and Sand B
<br /> modeling.
<br /> Biodegradation: Since MTBEhas been clearly demonstrated to be naturally attenuating in
<br /> groundwater at the site, Stratus used BIOSCREEN to model solute transport with biodegradation
<br /> as a first-order decay process. The first-order decay coefficient was calculated by BIOSCREEN
<br /> using site-specific (and zone-specific) solute half-lives input for dissolved-phase MTBE in Sand
<br /> A and Sand B. Using lognormal graphs of MTBE over time, site-specific half lives for MTBE as
<br /> it degrades in the groundwater in Sand A and Sand B, were estimated at 1.91 and 2.20 years,
<br /> respectively. These values were based on the averages of half-lives observed in wells MW-2,
<br /> MW-3, MWO-2, MWO-3, MW-7, and MW-9 for Sand A, and in wells MWO-lx and MW-8 for
<br /> Sand B.
<br /> General: A modeled area length of 800 feet'for Sand A and 420 feet for Sand B were selected to
<br /> model the furthest extent of MTBE migration, and a modeled area width of 250 feet in both sand
<br /> units was selected.
<br /> Source Data: Based on boring logs at the site, source thickness in the Sand A saturated zone
<br /> i✓ was set at approximately 10 feet and approximately 15 feet for source thickness in.Sand B. The
<br /> plume in both zones was divided into an inner plume width of approximately 100 feet, flanked
<br /> on each side by a. 25 foot width of lower concentration area. To estimate source area
<br /> concentrations, the historical maximum MTBE concentration at well MW-1 (2,800 µg/l in April
<br /> 2000) was used.
<br /> Soluable Mass: Per the BIOSCREEN manual, the best estimate of dissolvable organics in the
<br /> source zone is obtained by adding the mass of dissolvable organics on soils, free-phase NAPL,
<br /> and residual NAPLs. The model then uses this quantity to estimate the rate that the source zone
<br /> concentration declines. As previously discussed in Section 3.3 of this document, no NAPL or
<br /> free product have been reported at the site in groundwater during the historic monitoring period,
<br /> nor has heavily MTBE-impacted soil been reported. At the time of maximum impact to the
<br /> groundwater in the source area in early 2000, maximum MTBE concentrations of 2,800 µg/L
<br /> were reported. Maximum MTBE concentrations in soil were 0.46 mg/Kg beneath the former
<br /> USTs when removed in 1998.
<br /> An extremely conservative estimation of the source soluble area mass assuming a source area of
<br /> 50 feet by 50 feet to a depth of 20 feet bgs (a total of 50,000 f13) at a concentration of the historic
<br /> maximum (0.46 mg/kg) across the entire volume of soil and bulk density of soil of 1.7 kg/L,
<br /> results in soluble source area mass of approximately 1.1 Kg. Based on model calibration efforts,
<br /> U 2 Howard,P.H„Michalenko,G.M.,Basu,K.D.,sage,G.W.,Meylan,W M.,Beauman,J.A.,Jarvis,W.F.,and Gray,D.A.,Eds.,Handbook of
<br /> Environmental Fate and Exposure Data far Organic Chemicals,Volume IV,Solvents,Lewis Publ.,Chelsea MI,578p,1993.
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