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DUCKWORTH PIT GROUNDWATERMODEL <br />The three-dimensional finite difference groundwater model developed by the LJSGS, <br />MODFLOW, was used to model the groundwater flow in the vicinity of the Duckworth Pit <br />for current conditions, conditions doting the mining operation, and conditions upon <br />completion of reclamation of the site. The modeling was used to evaluate the proposed <br />mitigation efforts at the site, and identify any additional mitigation efforts that may need to <br />be undertaken to protect nearby well owners. The modeling was completed in accordance <br />with the American Society for Testing and Materials' (ASTM) Standard D5447, where <br />applicable.' <br />Model Dezrlopment -Assumptions <br />In order to develop the model, the following general assumptions were made: <br />The area that was explicitly modeled extended from approximately one hall mile <br />south of the site to about 1,500 feet north of the site. The eastern boundary of the <br />site was set approximately 1,000 feet east of the site. This area was selected based on <br />the geographic extent of current monitoring well data. Modeling outside of this area <br />would have been speculative due to the limit of the available data. <br />Groundwater flow through the bedrock was included in the model, but was given a <br />relatively low rate of conductance. The hydraulic conductivity of the bedrock in the <br />area is estimated to be about three orders of magnitude less than that of the <br />alluvium For the bedrock layer, a horizontal hydraulic conductivity of 0.05 ft/day <br />was assumed. Vertical hydraulic conductivity was assumed to be one-tenth of the <br />horizontal hydraulic conductivity. This is a typical value for vertical hydraulic <br />conductivity relative to horizontal hydraulic conductivity. <br />The effects of Idaho Creek and the Smith & Emmons Ditch on the groundwater <br />flow in the area were simulated by assuming a constant recharge rate of 0.4 ft/day. <br />Information on the average permeability of soils in the area was not available, <br />therefore this value was assumed based on previous experience in alluvial deposits <br />along the Front Range. A sensitivity analysis was completed on this variable, the <br />results of which are discussed in a following section. <br />The horizontal hydraulic conductivity of the alluvium was assumed to be 334 ft/day <br />and was assumed to be representative of the hydraulic conductivity throughout the <br />area that was included in the model. The vertical hydraulic conductivity was <br />assumed to be one-tenth of the horizontal hydraulic conductivity. Site specific data <br />on the hydraulic conductivity was not available, therefore an average value was <br />detem~irted based on the formula: <br />~ ASTM Standard D5447 - 93: Standard Guide for Application of aGround-Water Flow Model to a Site- <br />Specific Problem, revised (1998) <br />Lafarge West, Inc. -3- Applegate Group, Inc. <br />Duckworth Pit Groundwater Study March, 2004 <br />