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Mr. Philip S. Isorele <br />March 10, 1999 <br />Page 4 <br />codes as well as pre- and post -processors, will be used to facilitate model development, <br />analysis, and graphical presentation of the results. The MODFLOW package will be <br />used to generate an appropriate calibrated groundwater flow model, which will be <br />incorporated into the MT3D transport model. MT3D uses advection, dispersion, <br />sorption, and biodegradation to simulate contaminant migration and degradation. The <br />solution routines are based on the Method of Characteristics (MOC) solute transport <br />model as developed by Konikow and Bredehoeft (1978) for the U.S. Geological Survey <br />(USGS) two-dimensional MOC model code. MT3D was modified by Zheng (1990) to <br />allow three-dimensional solutions and the use of a modified MOC method, which reduces <br />numerical dispersion. <br />Groundwater Flow Model - The existing model hydraulic parameters will be used as the <br />initial flow parameters and modified for groundwater flow calibration. The purpose of <br />the model calibration process is to duplicate groundwater conditions at the site as closely <br />as possible. The model will be calibrated by altering hydraulic conductivity and <br />specified -head boundaries in a trial -and -error fashion until simulated groundwater <br />elevations approximate measured field values within a prescribed accuracy. <br />Groundwater elevation data will be used as calibration targets for the flow model. <br />Groundwater elevations will be simulated for non -pumping and pumping conditions <br />(from existing extraction wells). <br />Contaminant Transport Model - Model input parameters affecting the distribution and <br />concentration of the simulated contaminant plume will be modified so that model results <br />are similar to groundwater concentrations measured in 1998 and 1999. Data collected <br />from the concurrent offsite sampling program will be incorporated in the model. <br />Source terms :will be evaluated using the approach described by Spitz and Moreno <br />(1996). The source is represented as a "black box" that produces appropriate <br />concentrations at selected points in the model domain. Under this approach, the effects <br />of a "black box" source on plume geometry are evaluated, rather than directly deriving <br />source characteristics from site data. <br />Transport model calibration will be achieved by varying contaminant source term <br />concentrations, time periods, weathering rates (i.e., source concentration decreases), and <br />contaminant transport parameters in the model in a trial and error fashion until simulated <br />concentrations are similar to observed concentrations. Since there is little evidence that <br />biodegradation is occurring at the site, the biodegradation rate will be assumed to be zero. <br />Feasibility of Offsite Remediation. Per the RWQCB request remedial alternatives for the <br />offsite areas will be evaluated. Remedial alternatives will include containment of <br />chemical migration by means of groundwater extraction wells and treatment of the <br />extracted water by air stripping (pump and treat) to achieve chemical concentrations that <br />are protective of groundwater beneficial uses. Present worth capital cost of the <br />installations and long term system operation and maintenance will be evaluated. <br />RESP_RWQ <br />