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<br />f)~~~tro Groundwater Model Development Report <br /> <br />Oct. /1,2001 <br />Page II <br /> <br />"- <br />) MODFLOW-SURF ACT automatically allocated extraction rates from the layers based on <br /> <br />the layers' transmissivity distribution. <br /> <br />4.1. 5 Vertical Conductance Sensitivity Analysis <br /> <br />From aquifer testing conducted at Castle Pines well A3, Robson and Banta (1993) <br />inferred vertical conductance values (VCONT) for the low-K interbeds, and these values <br />were used for our base case VCONT values in the 15-layer model. There is concern <br />expressed by some that these values may be unrealistically high (i.e., 8/27/01 letter from <br />Halepaska and Bredehoeft to Binney), and note core penneameter analysis results that <br />indicate core-seale vertical conductivities of the interbeds that are 4 to 5 orders of <br />magnitude lower than the field-scale values presented by Robson and Banta (1993). <br />While one might expect core-scale values to be significantly lower than field-scale <br />values, there remains a large degree of uncertainty about the appropriate value to use for <br />VCONT parameter. <br /> <br />) <br /> <br />To address uncertainty in the appropriate vertical conductance to use in the regional <br />model, we perfonned a sensitivity analysis as part of the comparative modeling. This <br />sensitivity analysis involved setting VCONT IOO-times lower than the Robson-Banta <br />values, as well as a run with VCONT set equal to zero. <br /> <br />4.2 Results of Comparative Analysis <br /> <br />To illustrate and compare model results, we selected four summary performance <br />measures: <br /> <br />. Areal head distribution over entire model domain at end of simulation <br />. Vertical head distribution at simulation end at heavily pumped and unpumped <br />locations <br />. Hydrographs (head versus time) at two selected locations, for both regional and local <br />conditions <br /> <br />J <br /> <br />Hydrosphere Resource Consultants <br />1002 Walnut Suil< 200, Boulder, CO 80)02 <br />PO Box 445, Socono, NM 87801 <br />