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dimensions ranged from 100 by 100 feet in the Hazeltine resource area to 500 by 500 feet at the outer boundaries of the model. One model layer was used to simulate the shallow alluvial aquifer. 2.2 Boundary Conditions and Water Bodies The northwest boundary of the subject aquifer is formed along the edge of the historical South Platte River floodplain where the alluvium is bounded by claystone bedrock sediments or where the base of the alluvium is high enough in elevation to be unsaturated. Because the bedrock sediments are much less permeable than the alluvium, no-flow boundary conditions were used along the northwest edges of the aquifer. No-flow was also specified along part of the southeast boundary due to its distance from the mine, its approximate alignment with the east extent of the South Platte paleovalley (Robson, 1996), and the situation that groundwater flow appears to be largely down-valley (approximately parallel to the boundary). The southwest and northeast boundaries were set approximately one mile upstream and downstream from the Hazeltine site so they would have minimal influence on calibration and simulations. General- head cells were specified to represent regional groundwater levels at the upstream (southwest) and downstream (northeast) sides of the model area. The top of the model was set as the ground surface. The base of the alluvium was simulated as a no- flow boundary because as it coincides with the top of claystone, which has a hydraulic conductivity several orders of magnitude lower than the overlying alluvium. The bottom elevations were digitized from USGS mapping of the top of the bedrock (Robson, 1996), revised as needed based on data from approximately 42 borings or test holes within or near the Hazeltine mine. Existing lined water storage reservoirs (created from sand and gravel excavations) were represented with no-flow cells because the surrounding slurry walls or slope liners are relatively impermeable (hydraulic conductivity several orders of magnitude less than the alluvial aquifers). Existing lined water storage reservoirs in the area (Figure 2) include the Dunes Reservoir, an unnamed reservoir to the east, Cooley Reservoir, and the Howe-Haller Mine reservoir. Unlined water storage ponds were represented by setting the hydraulic conductivity (K) value at a high, non-limiting level and the specific yield (S) equal to one. Unlined ponds backfilled with fines were modeled with a hydraulic conductivity two orders of magnitude less than the surrounding alluvium. Irrigation ditches and drainage swales were designated as MODFLOW River Cells to allow flow into or out of the channels depending upon hydraulic head conditions and channel characteristics. Field reconnaissance was performed to estimate the dimensions and flow depths in the river, creek, and ditch channels. Surface topography mapping and in some cases actual survey data were used to estimate water stage elevations in the various channels. The bottoms of all drainage and river channels were assumed to have aone-foot thick bottom layer with a hydraulic conductivity being one order of magnitude less than the alluvial aquifer. During calibration, and to represent seasonal differences, certain channels were fumed on or off, or wetted channel geometries were modified, to simulate wet and dry season conditions (Section 3.2). 2.3 Aquifer Properties The model simulated the shallow unconfined aquifer within three geologic units (Figure 3). Information on the type of units found at the site was obtained from geologic maps prepared by the U.S. Geological Survey (Trimble and Machette, 1979). The valley fill deposits along the South Platte River and at the - 2 - July 2004 I:U919 018\GW ModeNtcpon\Hazdline Rpt J D.doc