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<br />South ~,1 ~~undwaler Model Development Report <br /> <br />Ocl. 11, 2001 <br />Page 14 <br /> <br />due to the well generally is much larger than the vertical conductance due to the porous <br />medium. Differences in heads at nodes along the well calculated by the model, as a <br />result, are very small. This is why the heads calculated for the lower 8 layers for <br />simulation jac9b and for the lower 7 layers in simulations jac9c and jac9hvcO differ by so <br />little. Since multiaquifer wells are specified at such a large proportion of the cells in the <br />model grid, vertical head gradients should be diminished, especially in cells where <br />multiaquifer wells are specified. <br /> <br />') <br /> <br />Most important, in the field, an unplugged open well will behave as modeled by the <br />MODFLOW-SURF ACT fracture well package. As the South Metro region becomes <br />more and more developed, and more and more wells "pincushion" the area, we would <br />expect such wells to contribute to an attenuation of vertical head gradients that may <br />otherwise exist due to low-K interbeds within the aquifer units. <br /> <br />4.3 Summary and Conclusions of Regional Model Comparative Analysis <br /> <br />Simulations were performed on a 12-by-12 mile subregional domain extracted from the <br />SEO's SB-74 Denver Basin groundwater model to project drawdowns due to a pumping <br />schedule consistent with the state's allowance of 1% of stored volume extraction per year. <br />Both a single-layer and IS-layer model were developed to allow different approaches to <br />treat observed aquifer interbeds. The model results obtained indicate that: <br /> <br />) <br /> <br />. Modeling approaches which treat the aquifers as single vertically-homogeneous <br />layers and layered aquifers with intervening low-permeability interbeds lead to <br />similar predictions ofregional head over time in areas developed with pumping wells. <br /> <br />. Alternatively, the multi-layer models exhibit significant vertical gradients in areas <br />devoid of wells; this effect increases as VCONT decreases. <br /> <br />Some IS-layer model runs were performed to assess sensitivity of results by magnitude of <br />the vertical conductance between layers and to partial penetration of the pumping wells, <br />The model results obtained indicate that: <br /> <br />. The low-permeability interbeds do exert minor effects on the regional head <br />distribution, particularly in areas without wells that are completed across all <br />permeable writs within the aquifer. <br /> <br />J <br /> <br />Hydrosphere: Resource Consultants <br />1002 Walnut Suite 200, Boulder, CO 80302 <br />PO Box 445. Socorro. NM 87801 <br />