2010-01-21_PERMIT FILE - M2009082 (11)

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c) Due to recharges from the Cache la Poudre River and nearby reservoirs groundwater modeling indicates drawdowns at the well location would be less than 2 feet. Four residences were identified adjacent to the proposed Parsons Mine. However, these homes receive municipal drinking water from the North Weld County Water District and do not have wells registered for other uses. Modeling A numerical groundwater flow model was constructed of the site and surrounding areas to assess the potential impacts that the mine operation may have on groundwater levels (Attachment G-2). The U.S. Geological Survey's groundwater simulation code MODFLOW (McDonald and Harbaugh, 1988) was used for the modeling. The thickness of saturated alluvial sediments within the proposed mine site ranges from approximately 10 to 30 feet. The typical groundwater flow direction is to the east-southeast toward the Cache la Poudre River system. The groundwater flow directions and gradients can be expected to vary locally and seasonally due to changing irrigation patterns and other mining operations. The water table generally represents a subdued replica of the ground surface. The natural terrace that runs generally east-west through the northern portion of the site has a significant effect on groundwater flow gradients. Gradients are relatively low north of the crest of the terrace and in the river valley below the terrace, but are steep across the terrace, consistent with the ground surface. As shown in Table 2 and Figure 2, the records search indicated that five permitted wells are located within 0.5 miles of the mine property. Results of the groundwater flow model indicate that the Parsons mining operation would not significantly affect (i.e., create a drawdown of more than 2 feet) any of the identified well locations. Groundwater modeling indicates that several feet of drawdown could occur in the vicinity of the Jones Ditch. However, based on groundwater measurements in nearby monitoring well MW-11, groundwater levels are well below the ditch and it does not appear that the ditch is in direct contact with the groundwater table. If direct hydraulic connection does occur due to leakage, calculations indicate that ditch losses induced by dewatering may be in range of approximately 0.1 cfs. Mitigation of any such losses is addressed in the mitigation section below. The modeling also indicates that minor drawdowns could occur beneath adjacent agricultural lands. Due to the depths to water, the only areas of potential concern for sub- irrigated lands include those within the Cache la Poudre River floodplain or bottom lands. Typical depths to water below ground surface (bgs) in groundwater monitoring wells located adjacent to the river (MW-07, MW-08 and MW-10) range from approximately 5 to 7 feet. Typical depths to water bgs in wells located in the river bottom but at greater distance from the river (MW-09, MW-11, and MW-12 range from 4 to 9 feet. All farmlands located adjacent to the mine and within the river bottom lands are currently Lafarge West, Inc. - Parsons Mine 4 Groundwater Monitoring and Mitigation Plan