2022-04-04_PERMIT FILE - M2022013 (21)

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Dewatering Evaluation Report Varra Pit 124 Weld County,Colorado Page 2 past water resource investigation reports is on the order of 0.002 feet per foot. The average measured depth to groundwater within the mine area is 9 feet below ground surface.The natural groundwater flow direction varies from southeast to east, northeast within the model and mine area. Seasonal water table fluctuations of between one and three feet are common for this area; however, fluctuations of greater than ten feet have been documented during drought conditions (Schneider, 1983). The mine area is depicted on Figure 2.The water table in the pit will be drawn down to bedrock by allowing groundwater to flow from the side walls of the excavation into ditches excavated into the bedrock or pit bottom at the toe of the excavation walls.The ditches are sloped so water drains to predetermined pump locations. The water is then pumped from the excavation into irrigation ditches,which eventually outfall to the South Platte or Big Thompson rivers. Project Assumptions The following are assumptions made in estimating the effects of mine dewatering operations. • The aquifer within the model boundary is homogeneous and anisotropic. • The average water table altitude within the mine area varies between 4674 and 4681 feet above mean sea level. • The average horizontal hydraulic conductivity (K) of the sand and gravel deposits is 125 feet per day and the vertical K value is 12.5 feet per day. Silty sands, which are present throughout the majority of the northern model boundary, were assigned a horizontal K of 50 feet per day and a vertical K of 5 feet per day. • The vertical hydraulic conductivity of the river beds is 4 feet per day. • The hydraulic conductivity of the barrier wall material is 0.003 feet per day with a lining thickness of four feet. • Other than dewatering associated with the Pit 124 mine operations no other aquifer stresses such as drought and surrounding well use were modeled. • All groundwater solutions are steady state. • The bedrock which underlies the coarse alluvial deposits is an impermeable barrier.