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• <br />• Information on the stream and canal configuration and initial property estimates will be <br />available from the Phase 3 model. These will be updated as necessary to include changes to <br />represent conditions during the calibration period. <br />• Information on alluvial well screening intervals are not available, thus all alluvial wells will <br />be estimated to be screened throughout the entire saturated thickness of the alluvial aquifer. <br />Since the alluvial aquifer model is currently conceptualized as a one-layer model, this lack of <br />information on screen intervals should not affect the calibration. <br />• Boundary inflows will be estimated for alluvial groundwater flux from tributary valleys not <br />explicitly modeled, including most of the drainages from the western foothills and smaller <br />drainages in the basin. In addition, inflows on the mainstem above Chatfield Reservoir and <br />outflows on the mainstem near the state line will be computed and included under this task. <br />These inflows and outflows will be estimated using Darcy's Law calculations and engineering <br />judgment, based on computed groundwater hydraulic gradients using data from Tasks 39 and <br />44, estimates of aquifer properties using data from Task 43, and aquifer geometry using data <br />from Task 42. Pre-development estimates of boundary inflows, available from the Phase 3 <br />Task 48 pre-development model, will be modified as necessary to account for the changes due <br />to development, using information provided by the CU/Water Balance Contractor and based <br />on review of land use over time as obtained from the GIS Contractor. The boundary locations <br />and flow estimates will be uploaded into the `boundary condition feature' in the modeling GIS <br />repository that was designed in Phase 3 under Task 50. These boundary conditions will be <br />exported from the modeling GIS repository for processing in AGG and StatePP for generation <br />of model package input files. The definition of boundary conditions will conform to ASTM D <br />5609-94, Standard Guide for Defining Boundary Conditions in Groundwater Flow Modeling. <br />• Recharge and discharge between the alluvial aquifer and the Denver Basin bedrock aquifers is <br />another boundary condition included in the alluvial model. It is anticipated that these bedrock <br />fluxes will be available from the model being developed by the USGS. The location and <br />timing of these bedrock fluxes will be incorporated in the modeling GIS repository for use in <br />the data centered modeling process. Differences in the model grids and stress period lengths <br />and time frames between the USGS and SPDSS models will be addressed using a tool to be <br />developed in Phase 4, Task 50, so that the appropriate bedrock flux data will be extracted <br />from the USGS model. These boundary fluxes will be coded within the `boundary condition <br />feature' in the modeling GIS repository designed in Phase 3 under Task 50 in order to be <br />compatible with the data centered modeling process. These boundary conditions will be <br />exported from the modeling GIS repository for processing in AGG and StatePP for generation <br />of model package files. If the USGS model results are not available, then bedrock fluxes will <br />be obtained from the State's SB-74 groundwater model instead. <br />Most of the alluvium downstream of the Denver Basin is underlain by the Pierre Shale, a <br />relatively impermeable deposit, so flow to or from the bedrock in this reach is estimated to be <br />negligible and will be coded as a no-flow boundary. Near the state line, the Ogallala aquifer <br />is present beneath the alluvial South Platte aquifer. The interaction between the South Platte <br />alluvium and Ogallala aquifer will be assessed by comparing available water level data in <br />each aquifer using the published sources identified in Task 44. If significant differences in <br />groundwater levels are identified, then estimates of groundwater flow between the alluvial and <br />SPDSS Phase4 gw SOW_09-] 5-06.doc 9 <br />