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Dewatering Evaluation Report <br />Varra Pit 122 <br />Weld County, Colorado <br />Page 2 <br />between one and three feet are common for this area; however, fluctuations of greater than ten <br />feet have been documented during drought conditions (Schneider, 1983). <br />The water table in the pit will be drawn down to bedrock over an estimated 100 acres. The mine is <br />dewatered by allowing groundwater to flow from the side walls of the excavation into ditches <br />excavated into the bedrock or pit bottom at the toe of the excavation walls. The ditches are sloped <br />so water drains to predetermined pump locations. The water is then pumped from the excavation <br />into the Seep Ditch, which eventually outfalls to the Saint Wain Creek. Varra Companies operates <br />an active gravel mine (Pit 116) immediately west of the proposed Pit 122. The pit is dewatered by <br />similar methods as described above and groundwater recovered by dewatering is pumped into the <br />Seep Ditch. The proposed Pit 122 has two cells (A and B), which may or may not be mined <br />concurrently. Pumping simulations are provided for multiple cells in Pits 116 and 122. <br />Project Assumptions <br />The following are assumptions made in estimating the effects of mine dewatering operations. <br />The aquifer within the model boundary is homogeneous and anisotropic. <br />The average saturated thickness of the aquifer prior to mine operations is approximately <br />33 feet (average soil depth of 39 feet and average water table six feet below ground <br />surface). <br />The average horizontal hydraulic conductivity (K) of the sand and gravel deposits is 125 <br />feet per day and the vertical K value is 12.5 feet per day. Silty sands, which are present <br />throughout the majority of the southern model boundary, were assigned a horizontal K <br />of 50 feet per day and a vertical K of 5 feet per day. <br />The vertical conductivity of the unlined ditch beds varies between 0.4 and 4 feet per day. <br />Other than dewatering associated with the Pit 116 and proposed Pit 122 mine operations <br />no other aquifer stresses such as drought and surrounding well use were modeled. <br />All groundwater solutions are steady state. <br />The bedrock which underlies the coarse alluvial deposits is an impermeable barrier.