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PDDR • -2- • 31 March 1995 DDRW', Tracy water flow and contaminant transport models. In particular, the PDDR does not provide adequate responses to Board staff comments, dated 22 September 1993, and 29 December 1993, on the Draft Ground Water Modeling Work Plan, and the Ground Water Modeling InterimTechnical Memorandum (Ground Water ITM), respectively. These documents were submitted during the preliminary stages of conceptual model development. As a result, model development, calibration, and simulation application (verification) data gaps were identified. The Ground Water ITM also included a project schedule which proposed the submittal of a report in March 1994 to sumarize model calibration results. This schedule also indicated that model simulation applications (the model verification period) would begin in February 1994 and be completed by the end of June 1994. In addition, Board staff requested a stand-alone report presenting modeling development and verification to expedite review time required during the conceptual design stages. However, it is our understanding that these reports were not submitted and only one status report was provided during a mid-1994 Project Managers Meeting. The PDDR must clarify how data gaps identified in the Ground Water ITM were addressed. The PDDR must also be revised to include more detailed and specific information to adequately present model development components and how the model was calibrated (See 22 September 1993 Board staff comment letter- specific comments 3, 4, and 6; 29 December 1993 Board staff comment letter). Deficiencies in the PDDR with respect to particular model development components are discussed in more detail in the specific comments below. In addition, we concur that additional modeling efforts are necessary, as discussed in 10.1.5 of the PDDR, prior to determining an acceptable remedial alternative. GROUND WATER MODELING 1. Model Specifications. The Board previously indicated that MODFLOW and MT313 models are appropriate for use in determining compliance with the substantive requirements of Chapter 15 and State Board Resolution 92-49 which require DDRW Tracy to demonstrate the technical and economical feasibility of cleaning up the OU-1 plume to background. However, the model specifications for the MODFLOW and MT3D models were not included in prior submittals. This information is imperitive in evaluating the acceptability of the modeling approach with respect to the DDRW Tracy aquifer system. This information must be related to assumptions made and the capability of the model to meet specific objectives to evaluate the adequacy of model development. The specifications, including a description of capabilities and limitations, of the MODFLOW and MT3D models, or a reference to such specifications, must be provided in the PDDR. 2. Model Assumptions. The PDDR refers to "necessary simplifying assumptions" (Section 5.2.1.10) that must be considered in interpreting model results. However, the assumptions and simplifications used in developing the ground water flow and contaminant transport models, and the extent to which these assumptions effect the applicability of the models, are unclear. The PDDR should clearly describe and substantiate all assumptions, modifications, and simplifications made in developing and applying these models. 3. Model Grid and Boundary Conditions. The PDDR generally describes model configuration and boundary conditions. The PDDR depicts the model domain and grid on a large scale (Figure 4-2)