ARCHIVED REPORTS_2011 REVISED FEASABILITY STUDY

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Appendix A Application of Ripon Study Area MODFLOW/RT3D Model to Feasibility Study Alternatives 1 Ripon Study Area Flow and Transport Model The MODFLOW/RT3D model developed by Environmental Cost Management, Inc. (ECM) (Final Groundwater Model Report, 2007), and approved by Regional Water Quality Control Board (RWQCB), was used to simulate future TCE transport and concentrations under the various scenarios investigated for the Revised Feasibility Study'. Initial conditions for these model runs were established using output from the calibrated model for December 2007 (as predicted for the last time step of the updated 1957 through 2007 model). Unless otherwise noted, boundary conditions (i.e., well pumping rates, recharge rates, TCE source area concentrations, TCE biodegradation rates, etc.) for each future modeling run are established based on end of year 2007 conditions. For the purpose of estimating future groundwater and dissolved-phase TCE movement, pumping and recharge rates from calendar year 2008 were assumed to be representative of future conditions for the study area. 2 Application of Model to Feasibility Study Alternatives 2.1 Overview of Modeling Scenarios For each alternative, boundary conditions were adjusted as described below in order to approximate the changes to groundwater pumping and/or Site treatment conditions proposed for each alternative assessed in the Revised Feasibility Study Report. Unless indicated, all other aquifer properties and boundary conditions remain consistent with the conditions as described in the Final Groundwater Model Report for the final time step of the 1957 through 2007 base period model run'. 2.2 Alternative 1 — No Action The No Action alternative exists as the baseline for assessing the anticipated effects of other feasibility study alternatives. Alternative 1 does not involve changes to 2008 average quarterly pumping rates or the application of any Site reduction technologies. The current operation of groundwater extraction and treatment systems at the Site and the Stockton Avenue location cease under the No Action alternative. The assessment of this alternative assumes no other significant changes in local groundwater use relative to the 2008 average quarterly pumping rates. As no active remediation or groundwater extraction controls are simulated as part of this future scenario run, dispersion and biodegradation within the saturated zone primarily remove mass from the Site. Parameters influencing these degradation phenomena (partition coefficients, dispersion coefficients, 1