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S. Melro GW M'5'4-'!1?'iremenls Memo January 8. 2001 Page 6 propagation and flow between the permeable beds (HRS Water Consultants, 1997). A key question is what impact these interbeds exert on drawdown distributions at the gridblock (one mile in the SB-74 model) scale. If actual (as opposed to simulated) regional drawdowns at scales on the order of one mile are significantly impacted by these interbeds, then modeling the aquifers as vertically homogeneous may not provide an adequate measure ofregional drawdown. Even though analytical well approximations and/or the radial flow MCW model is a robust tool for accounting for the effect of interbeds at the well-field scale (section 2.1), both these approaches use gridblock drawdowns obtained from the regional model as one of their input parameters. II is imporlanllO critically consider whether neglecting Ihe fine-grained inlerbeds wilhin Ihe aquifers in the regional model leads to subslanlially difJerenl prediclions Ihan if Ihey were somehowaccounledfor. '. , ) 4. SUMMARY AND MODEL DEVELOPMENT APPROACH Hydrosphere proposes that the South Metro regional groundwater flow model be based on the SED SB-74 Denver Basin model. This recommendation is based on: I. The SED model addresses nearly all of the South Metro model requirements that have been highlighted with ila/ics in the above discussion. 2. The SED model is built using the MDDFLDW code, which provides a solid foundation of portability and transferability of the model. ) 3. The MDDFLDW platform will facilitate model extensions and/or enhancements using well- documented and broadly accepted hydrologic process modules. 4. The SED model incorporates the required data and information, and is well-documented and has gained general acceptance as a analysis tool by the SED. 5. Project budget and schedule constraints. As described in sections 3.2 and 3.3, there are a couple issues that merit further investigation before one can determine whether the SED model can be adopted entirely as is without modification or enhancement. Those issues are: 1. New data being utilized as part of the South Metro regional groundwater will need to be screened and compared to data and information used in developing the SED 8B-74 Denver Basin Model. If significant deviations are found, it may be necessary to re-calibrate the model. II. It is important to critically consider whether neglecting the fine-grained interbeds within the aquifers in the regional model leads to substantially different predictions than if they were somehow accounted for. . J Hydrosphere Resource Consultants 1002 Walnut Suite 200, Boulder, CO 80302 PO Box 445, Socorro. NM 87801