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far MTBE is predicted to migrate and 2) to estimate the amount of time predicted for MTBE L, levels to reach the secondary MCL. To accomplish these goals, Stratus employed the use of USEPA's BIOSCREEN (version 1.4; July 1997). BIOSCREEN is a screening model which simulates remediation through natural attenuation of dissolved-phase hydrocarbons or fuel additives. The software, programmed in the Microsoft Excel spreadsheet environment and based on the Domenico analytical solute transport model, has the ability to simulate advection, dispersion, adsorption, and aerobic decay as well as anaerobic reactions that have been shown to be the dominant biodegradation processes at many petroleum release sites. BIOSCREEN includes three different model types: (1) solute transport without decay, (2) solute transport with biodegradation modeled as a first-order decay process (simple, lumped-parameter approach), and (3) solute transport with biodegradation modeled as an instantaneous biodegradation reaction with multiple soluble electron acceptors including dissolved oxygen, nitrate, and sulfate. Since MTBE has been clearly demonstrated to be naturally attenuating in groundwater at the site and site-specific half-lives can be estimated from graphs, Stratus used BIOSCREEN to model solute transport with biodegradation as a first-order decay process. The model was run for both Sand A and Sand B, while making the conservative assumption that the two sand units are hydrologically separate (although likely not the case). The model results were validated using quarterly groundwater sampling data and comparing it to the modeled conditions over time. 5.2.1 Model Input Parameters The input parameters for the BIOSCREEN model were a combination of site-specific data, standard values, textbook values, and recommended model values. Solute concentrations in groundwater were converted from µg/L (as reported by laboratories) to mg/1 for use in the model. Key input parameters into the model are discussed below. Hydrogeology: Sand A- and Sand B-specific values for hydraulic gradient, porosity, and hydraulic conductivity were used to calculate seepage velocity. In both Sand A and Sand B, an average historic hydraulic gradient of 0.0005 ft/ft was used. For both Sands A and B, a porosity value of 0.30 (30%) was selected based on typical values provided in the BIOSCREEN User's Manual. Hydraulic conductivity values were estimated for each unit based on typical values provided in the BIOSCREEN User's Manual for sand and silty sands, then fine-tuned during the model calibration for each unit. Hydraulic conductivity values of 2.9x10-2 cm/sec for Sand A and 1.3x10-Z cm/sec for Sand B were used (see additional discussion in Section 5.2.2 below). Dispersion: The estimated plume lengths in the A and B zones were estimated to be the distance between the well MW-1 and the estimated 5.0 µg/1 MTBE isoconcentration contour line beneath Interstate 5 (approximately 400 feet in Sand A and 300 feet in Sand B). Longitudinal, transverse, and vertical dispersivity values were calculated by BIOSCREEN using the plume lengths. Page 19 STRATUS