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Soul)4f4:rJ GroundwQter Model Development Report Ocr. ii, 2001 EXECUTIVE SUMMARY This report provides a detailed review of tasks undertaken over the past several months to provide a solid foundation and plan for development of a South Metro regional groundwater modeling approach. ) To address the modeling objectives and requirements, Hydrosphere recommends that the South Metro regional groundwater flow model be based on the SEO SB-74 Denver Basin model. To provide a robust tool for assessing impacts of groundwater development in the South Metro region, the South Metro groundwater model needs to extend the capabilities of the existing SB-74 model in at least three important ways: . It should incorporate the best available up-to-date data and information from the South Metro region, . It should be capable of explicitly addressing the model limitations identified and discussed in Section 2, and . It should be capable of projecting drawdowns at the local (well and well-field) scale to support estimating the costs of adding supplemental and alternate points of diversion as needed to maintain production through the aquifers' life. The report describes a comparative modeling analysis that indicates that low-permeability interbeds within the Denver Basin aquifer units exert only minor effects on prediction of regional drawdowns. We recognize, however, that these interbeds may strongly affect localized drawdowns in the well bore vicinity. The report presents a detailed screening of hydrogeologic data recently acquired from three sources: the South Metro water providers, the SEO Denver Basin well log database, and the USGS NWIS. This screening indicates that original parameterizations in the SB- 74 model deviate slightly from the most up-to-date available data. ) Based on the findings of data screening and preliminary model analyses, we present a recommended model development approach. That approach involves: . incorporating the newly acquired South Metro data into the current SB-74 model as described in section 7.1, . performing model runs using two different streambed conductances to represent alluvial aquifer - bedrock aquifer interactions: one run would use the values as specified in the SB-74 model, and the other would employ values IO-times lower, Hydrosphere Resource Consultanls 1002 Walmtt Suile 200, Boulder, CO 80302 PO Box 44S, Socorro. NM 87801 .