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Paul Banks May 13, 2004 Page 2 Arkansas River lies generally along the southern edge of the alluvial valley, and passes through the southern portion of the mine property. The Excelsior ditch, a relatively large and unlined irrigation canal, lies along the northern alluvial valley boundary, turning northward as it nears the Chico Creek drainage. Numerous irrigation wells supplying large center pivot sprinklers are found along the alluvial valley. One specific set of wells, the Evans wells, are oriented in a north-south line just off the northeast corner of the mine permit boundary. Both the Excelsior ditch and the Evans wells were considered with respect to this study, and as addressed further below. The goal of this analysis is to determine the potential impacts on the local alluvial water table resulting from the dewatering operations that will accompany the mining of the St. Barbara mine. Specifically, how any water level changes would impact the Excelsior ditch and the ability of the Evans wells to continue to operate are considered. To make these determinations, a numerical ground water flow model has been constructed incorporating the alluvial valley, the Arkansas River, and the mine, with an initial level of mining simulated along the entire northern boundary of the property. While little hard data is available regarding the aquifer characteristics, the model was set up to first allow for a calibration to the estimated general alluvial water table, assumed to be strongly controlled by the river stage across the model domain, and then to develop a satisfactory set of parameters that could be used for the predictive simulations. Figure 2, the Model Grid Configuration, presents the model domain, the boundary conditions, the location of the river cells associated with the live channel of the Arkansas River and the mine area. The model input parameters for hydraulic conductivity and specific yield were initially set at approximately 568 gpd/ft2 and 0.20, respectively. The hydraulic conductivity value, coupled with the saturated thickness of 25 feet, was derived from a reported transmissivity value of 100,000 gpd/ft applied to these materials for another mining operation relatively nearby. One prominent feature in the model is the fact that the north and south alluvial boundaries are simulated as "no flow" boundaries. It is unknown if this represents a true representation of actual conditions, as we are not aware of any field investigations that could confirm this having been carved out to date by Trans Colorado. The potential impacts of this boundary are most important on the north side, as the presence of the live stream channel of the Arkansas River on the south side will act as an effective recharge boundary on that side of the active model domain. Our review of the State Engineer's Office well Master List database failed to indicate any wells in the disintegration residuum materials, as mapped to the north. Whether this indicates that the materials are not capable of supporting well production and are thus either essentially desaturated or are lacking the needed hydraulic conductivity/storativity, or that there simply has not been demand for well production in this area, is unknown. We have thus modeled this interface as a no-flow boundary and consider this to be a very conservative assumption that results in higher predicted mining induced drawdowns along this northern boundary than would otherwise be the case. The initial model runs aimed at establishing the water table gradient across the model domain indicated that the original hydraulic conductivity value input was too high. Subsequent Mania and Wood h7ater Coesu(tants, Inc.