2025-03-14_PERMIT FILE - M2025016 (15)

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Mining and Water Storage Analysis Raptor Pit 125 Weld County, Colorado Page 2 greater than ten feet have been documented during drought conditions (Schneider, 1983). Project Assumptions The following are assumptions made in these analyses: • The aquifer within the model boundary is heterogeneous and anisotropic; • The average saturated thickness of the aquifer within the mine boundary is approximately 35 feet; • 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; • The hydraulic conductivity of unlined ditch beds was assigned a value of 0.4 feet per day and the Saint Vrain Creek bed was assigned a value of 4 feet per day; • The hydraulic conductivity of the lake bed material is 0.01 feet per day; • All groundwater solutions are steady state; and • The bedrock which underlies the alluvial deposits is an impermeable barrier. Model Parameters The effects of dewatering and pit lining 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. Plates 1 depicts model boundary conditions, which are described below. • The model boundary is 9420 feet (north-south) by 11986 feet (east-west). • The model grid is 180 rows by 240 columns or 43200 cells. • Pits A, B and C were assigned constant head boundaries for dewatering simulations and lake boundaries for water storage simulations. • River boundaries were assigned for the Last Chance Ditch and Saint Vrain Creek. • General head boundaries were assigned to the model limits (north, south, east and west). A uniform flow field was defined in the model with an unconfined aquifer. The natural groundwater flow direction varies between southeast (west of Saint Vrain Creek) and northwest (east of Saint Vrain Creek) within the model boundary.