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• <br /> • In the deep zones, the model consistently over-predicted groundwater elevations. <br /> This needs to be investigated further to determine if adjustments to the model <br /> need to be made to better match site conditions. <br /> Any or all of these factors could influence the capture zone estimates. Since future <br /> decisions on installation, re-start or shutdown of extraction wells will be based at least in <br /> part on the modeling results, these issues need to be investigated before it can determined <br /> how these data gaps could affect the capture zone estimates. <br /> 2. Appendix D describes a fairly robust calibration process that was completed to calibrate a <br /> steady state flow model and a transient contaminant transport model. However, the <br /> calibration process did not include any figures showing how well the flow model was <br /> able to reproduce observed groundwater gradients and groundwater flow directions or <br /> how well the transport model was able to reproduce the plume geometry over the 3-year <br /> transport calibration simulation. Please include figures in Appendix D that superimpose <br /> the model-derived groundwater elevation and iso-concentration contours with observed <br /> groundwater elevation and iso-concentration contours. <br /> 3. It is not clear from Appendix D if the flow portion of the model has ever been calibrated <br /> to transient conditions. Testing a groundwater flow model that has been calibrated to <br /> steady state conditions against a transient response is one of the best ways to verify a <br /> groundwater flow model. Please describe in Appendix A if the groundwater flow model <br /> has ever been tested against a transient data set and if not, please perform a transient now <br /> model calibration or explain why such calibration was not performed. <br /> SPECIFIC COMMENTS <br /> 1. Appendix D, Transport Parameters, Page DIS: This section describes dispersivity <br /> values that were used in the model. The range of values for longitudinal dispersivity used <br /> in the model included the following: 1.32 ft, 0.66 ft and 0.0 ft. Overall, based on field <br /> observations of macrodispersion at other sites, these numbers are low. A typical <br /> approximation used in most groundwater modeling studies is the longitudinal dispersion <br /> at the field scale is roughly equal to 10 percent of the total flow distance of the <br /> contamination. This approximation is included in the documentation for Visual <br /> MODFLOW/MT3DMS, developed by the University of Waterloo. Although this <br /> approximation is certainly an oversimplification of a physical process that is still not <br /> completely understood, it appears to be roughly consistent with the field data collected at <br /> some sites. Considering these factors, it is recommended that higher levels of longitudinal <br /> 2 <br />