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-_CIVAT A S S O C I A T E S I N C upgradient well, were used as background levels and were incorporated in the model's parameters. One BIOSCREEN model input parameter not collected at the site is methane (CH4). ATC applied typical CH4 values collected from 28 Air Force Center for Environmental Excellence (AFCEE) sites. These data were presented in BIOSCREEN's User's Manual, which accompanies the model. BIOSCREEN Modeling Summary • BIOSCREEN was used to try to reproduce the movement of the benzene plume from 1986 (when the release was stopped) to fourth quarter 1997, representative of site conditions. The soluble mass in the soil was estimated by integrating the benzene soil concentration map included as Figure 3. Approximately 13.7-Kg of benzene were estimated to be present at the site based on GC/MS analysis of soil samples collected from both the vadose and saturated zones. This value represented a half-life of 500 years with the instantaneous reaction model (the first value shown in the source half-life box in Figure 5), a relatively long half-life, so the 14-Kg estimated utilizing 1996 through 2002 data was assumed to be representative of 1986 conditions. `' • Based on prior observation and calculations (see ATC's Additional Site Evaluation and Request for No Further Action for Former Fayette Manufacturing Facility, Tracy, California, dated June 27, 2000) the first-order reaction model was used as the primary model in an attempt to reproduce the plume length (— 130 ft). • Using the instantaneous reaction model's array concentration output values (which exhibit the remaining soluble mass in the source), the 1986 source mass was estimated. Different source mass values were inputed into the model with a simulation time of eleven years, until a best-fit value matched the calculated 1997 mass of 13.68-Kg. The instantaneous model predicted a 1986 soluble source mass 13.9-Kg would result in a 1997 mass of 13.7- Kg of benzene. This reflects a very good match to 13.68-Kg value, estimated utilizing the benzene soil concentration contour map (Figure 3). • On account that a decaying source was used, the source concentrations on BIOSCREEN's input screen (representing concentrations in 1986) were adjusted so the closest monitored concentrations on the centerline output screen (representing concentrations found in MW2 and MW3 in 1997) reflected approximately 1.3 mg/L and 0.17 mg/L respectively. Because the source decay term is different for the first order decay and instantaneous reaction models, this simulation focused on matching the first order reaction model. The final result was a source concentration of 13 mg/L in the center of the source zone (note on the centerline output the closest monitored concentrations are 1.425 mg/L and 0.161 mg/L, Figure 6). w:\17861\reporfs\modeling.doc 6