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

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Dewatering Evaluation Report Asphalt Specialties Company,Inc. Weld County,Colorado Page 3 • The vertical hydraulic conductivity of Godfry irrigation ditch bed is 0.4 feet per day. • The hydraulic conductivity of the slurry wall material is 0.0025 feet per day with a wall thickness of fivefeet. • The conductance of the drain boundary is 1,000 feet per day,which correlates to capture rate of 127 gallon per minute. • The infiltration gallery covers 1.43 acres and was assigned a recharge rate of 2005 inches per year. • Other than dewatering associated with the pits no other aquifer stresses such as drought and surrounding well use were modeled. • Only one pit is dewatered at any given time. • All groundwater solutions are steady state. • The bedrock which underlies the coarse alluvial deposits is an impermeable barrier. Model Parameters The effects of dewatering on groundwater flow within the study area were evaluated by using the three dimensional groundwater flow model Visual ModFlow Pro(VMOD).The general parameters used in the model are presented below.The model grid is depicted on Plate 1.A model grid cross section is presented as Plate 2. • The model boundary is 8100 feet(east-west)by 7400 feet(north-south); • The model grid is 162 rows by 148 columns or 23976 cells; • Two layers were used in the model with the upper layer representing unconsolidated alluvial deposits and the lower layer representing bedrock; • The thickness of the upper layer varied between 35 and 45 feet within the pit areas; • Constant head boundaries were assigned for the dewatering line sinks.; • River boundaries were assigned for the South Platte River and Godfrey ditch; • A drain boundary was assigned for the French drain(parallel to the western most Phase I slurry wall); • A second 1.43 acre recharge zone was assigned to simulate an infiltration gallery; • Barrier walls were assigned to the slurry walls for shadow and mounding simulations; and, • General head boundaries were assigned to the model perimeter. A uniform flow field was defined in the model with an unconfined aquifer. Water levels obtained from published water level data and existing monitoring well data were used to generate water level contours unaffected by any pumping influences.Water level data within the mine boundaries were used to calibrate the model based on observed conditions. Ground surface and bedrock elevations were obtained from site surveys, drill hole data, water well records and USGS maps. The ground surface and bedrock elevations were input into the geo-statistical model Surfer®, which created surface and bedrock contour maps.These surface and bedrock elevation data were imported into VMOD to define the ground surface and bedrock elevations within the flow model. The river stage elevations were extrapolated from survey data obtained from an adjacent mine property survey and available groundwater data. Water table elevations measured in on-site piezometers over a 14 month period were averaged and these elevations were used in the calibration process. Water table contours generated from