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Closure Report 20 Golden Grain Manteca Bean Facility,299 Moffat Boulevard,Manteca,Califomia July 16,19% University for the EPA, Is a numerical computer model based on the U S Geological Survey's Method of Characteristics solute transport code (Konikow and Bredehoeft 1978) A more detailed ' discussion of the BIOPLUME II model is included in appendix G This EPA model is widely recognized by others as an appropriate model for simulating natural attenuation and the movement ' and fate of dissolved chemicals under the effects of advection,dispersion, sorption, and aerobic or anaerobic biodegradation (Marquis 1994) 5.2 Model Input and Assumptions The site was modeled using a one-layered, unconfined hydrostratigraphic unit under steady state conditions with no active groundwater extraction system Observation pants were selected at the locations of the existing monitoring wells MW-1, MW-4, MW-7, MW-9, and MW-13 Initial dissolved benzene levels in groundwater at the beginning of the simulations were set at the currently observed ' concentrations based on the May 1996 sample results with a maximum concentration of 780µglL at well MW-7 Dissolved oxygen groundwater levels were estimated based on recent field measurements Measured hydraulic characteristics of the aquifer included transmissivity, saturated thickness, and hydraulic gradient Simulation of advection, dispersion, sorption and biodegradation by the BIOPLUME model are discussed below Table 9 summarizes the selected site-specific and ' estimated model parameters and rationale for the BIOPLUME modeling 5,2.1 Advection The primary mechanism in the migration of a dissolved contaminant or solute, such as benzene, ' through a porous medium is advection,the process by which solutes are transported by the bulk motion of flowing groundwater due to the hydraulic gradient (Mackay 1985) The amount of ' contaminant that is being transported is a function of its concentration in groundwater and the groundwater flow rate The groundwater flow rate is quantified by the average linear velocity or seepage velocity, vim„ expressed as vy„= K/ne I where the inherent capability of the medium to transmit water is expressed as the ratio of the medium's hydraulic conductivity, K, and the effective porosity, ng and the hydraulic gradient, i, is the ' slope of the potentiometnc surface The hydraulic conductivity value used In the model simulations . for this site, 4 8 x 10*4 ft/sec, was calculated from a transmissivity, T, of 1 2 x 102 ft2tsec determined ' 42OM213 CR FLUOR DANIEL GTI �5 1