2024-11-04_PERMIT FILE - M2024056 (6)

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Dewatering Evaluation Report Asphalt Specialties Company,Inc. Weld County,Colorado Page 2 Based on domestic well test records the hydraulic conductivity of the sand and gravel deposits varied between 52 and 468 feet per day with an average conductivity of 163 feet per day. A hydraulic conductivity of 200 feet per day was used for model simulations. The average effective porosity of the local sand and gravel deposits is estimated at 0.27.The natural hydraulic gradient as documented by past water resource investigation reports is on the order of 0.002 feet per foot within the coarse alluvial deposits. The average depth to groundwater measured in the ASCI monitoring wells was 5.42 feet below ground surface (annual high and low average). The natural groundwater flow direction within the mine area is to the northeast. Within the model area the groundwater flow direction varies from northeast to southeast.Seasonal water table fluctuations of between one and three feet were documented by ASCI; however,fluctuations of greater than ten feet have been documented during drought conditions (Schneider, 1983). The mine area and model boundary are depicted on Figure 1. Slurry walls will be installed at the extents of the mining areas as depicted on Figure 2. Mining will consist of two phases and the water table in the pits 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 discharge trench,which eventually outfall to the South Platte River. It is our understanding that the property just to the west of the mine is being used as part of a surface water augmentation project and that water is being injected into the subsurface. Should groundwater mounding become an issue ASCI is committed to installing a drain parallel to the western most slurry wall. A drain boundary condition was used to model the predicted influence of the drain. An infiltration gallery was also simulated to estimate the effects of reinjecting groundwater captured by the French drain. Project Assumptions The following are assumptions made in estimating the slurry wall and pit dewatering effects. • The aquifer within the model boundary is heterogeneous and anisotropic. • The average water table altitude within the mine area varies between 4659 and 4662 feet above mean sea level. • The average horizontal hydraulic conductivity (K) of the sand and gravel deposits is 200 feet per day and the vertical K value is 20 feet per day. A 1.5 inch recharge rate from precipitation was assigned to the entire model area. • The vertical hydraulic conductivity of the South Platte River bed is 4 feet per day.