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finite-difference grid containing 107 rows and 84 columns was used to model the area. Grid cell o er boundarieseoffrhe model.yO ~ model layer was used to sim late the shallow alluv0ial aqu fer1e 2.2 Baunda Conditions and Water Bodies The west 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 south and east of the South Platte River. Both in groundwater modeling theory and in the field, we have found that the large river acts as a boundary or barrier. That is, hydrologic stresses on one side of the river do not affect the groundwater on the other side of the river. Hence, all cells on the far side of the river from the site were turned not modeled. The southwest and north boundaries were set approximately one mile upstream and downstream from the Tucson South site so they would have minimal influence on calibration and simulations. General- head cells were specified to represent regional groundwater levels at the upstream (south) and downstream (north) 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 and 2000), revised as needed based on additional boring data. The additional data included approximately 53 Tetra Tech RMC borings or test holes within or near the Tucson South mine, 67 borings by others, and our records of the borings at the Rogers Pit and Walker Fit to the north. Existing mines surrounded by slurry walls or completed slope liners were represented with no-flow cells because the surrounding barriers are relatively impermeable (hydraulic conductivity several orders of magnitude less than the alluvial aquifers). Existing slurry walled mines in the area include the Walker Pit and Rogers Pit, north of the prey ssad no flow cells in someTs enarioss Formerlmines backfilled a h with a slope liner and thereby app fines were simulated as vertical barriers because fine-grained sediments are typically used to fill t em. These mines include the southern portion of the Rogers Pit ~ s lowered by 2 otrde s of magnitudein some scenarios). In those areas, hydraulic conductivity (K) Unlined water storage ponds were represented by setting the K value at a high, non-limiting level and the specific yield (S) equal to 1. Irrigation ditches 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 or existing plans were used ton stand in comer oases actualsurv ydata were used to estimate war r stage elevations topography mapp g in the various channels. The bottoms of all drainage and river channels were assumed to ave a one- foot thick bottom layer with a hydraulic conductivity being one and a half order of magnitude less than the alluvial aquifer. During calibration, and to represent seasonal differences, certain channels were turned on or off, or wetted channel geometries were modified, to simulate wet and dry season conditions (Section 3.2). August 2004 - 2 S.V9{9 019~TS GW Modc~TS Rcpotl\Twson SoutM1 R~ DraR.Qx