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<br />Souch ^!5cmormdwQCer Model Development Report <br /> <br />Oct. //,2001 <br />Page 7 <br /> <br />"I <br /> <br />4.0 <br /> <br />COMPARATIVE ANALYSIS FOR HANDLING AQUIFER <br />INTERBEDS IN REGIONAL GROUNDWATER MODEL <br /> <br />To address concerns raised by some members of the South Metro Water Supply Study <br />Board, a comparative modeling analysis was undertaken to assess the need to explicitly <br />account for fine-grained interbeds within the Denver basin aquifers when performing <br />regional flow analyses (Hydrosphere, 200 I). The SB-74 Denver Basin groundwater <br />model represents each of the six major aquifers (Upper Dawson, Lower Dawson, Denver, <br />Upper Arapahoe, Lower Arapahoe, and Laramie-Fox Hills) as vertically homogeneous <br />layers. This is despite the fact that geophysical and well-drillers' logs indicate a <br />substantial anlount of layering and vertical heterogeneity within each of the aquifer units. <br />For example, the Castle Pines Arapahoe well A-3 (HRS, 1997) illustrates typical <br />layering characteristics observed in wells completed in the Upper Arapaho aquifer (Fig. <br />4.\). <br /> <br />To address this question, we undertook the comparative modeling analysis described <br />here. The objective of this analysis was to determine whether one obtains significantly <br />different water levels in response to long-term pumping for two distinct aquifer <br />) conceptualizations: <br />. an explicitly layered aquifer, or <br />. an equi valent homogeneous single-layer aquifer. <br /> <br />4.1 Regional Groundwater Model Comparative Analysis Approach <br /> <br />The comparative analysis involved simulating a subregion (centered on the South Metro <br />area) of the Denver Basin's Arapahoe Aquifer as both a vertically homogeneous single <br />layer, and as a heterogeneous IS-layer aquifer containing low permeability interbeds <br />between permeable sandstone units. The subregion selected was comprised of the 144 <br />grid blocks within Townships 6 and 7 South (T6S - T7S) and Ranges 66 and 67 West <br />(R66W - R67W) in the Upper Arapahoe layer (Figure 4.2). This region is located in the <br />northeastern portion of Douglas County and encompasses parts of the Highlands Ranch, <br />Parker, Castle Rock, and Lone Tree municipalities. <br /> <br />As detailed below, the hydraulic property distributions for the single-layer case were <br />directly extracted from the SED's SB-74 model, and the fifteen-layer case incorporated <br />layering characteristics consistent with field observations of the Arapahoe aquifer in the <br />) Castle Pines area (HRS, 1997). To ensure parsimony between the two models, the <br />./ <br /> <br />Hydrosphere ResouW: Consultants <br />1002 Walnut Suite 200, Boulder, CO 80302 <br />PO Bo)( 445, Socorro. NM 87801 <br />