2018-10-16_PERMIT FILE - M2018039 (2)

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Given input values of h,,,. W, Kh, and initial simulate several types of hydrologic sources and sinks (premining)saturated thickness above the base of the including aquifer recharge,evapotranspiration, wells, mine (h=h,,), the distance of influence(x;)can be drains, and rivers, and it can simulate either steady- calculated directly by setting x equal to xi and re- state or transient conditions. arranging equation 7. Once x;is determined,h can be calculated for any distance upgradient from the mine wall, and drawdown can be calculated as hV—h. In SIMULATION OF THE HYDROLOGIC addition,the inflow rate per unit length of mine, Q EFFECTS OF MINING AGGREGATE [L2/T].can be calculated as: Q = Wx; (8) Two hydrogeologic settings in the Colorado Front Range area were simulated using analytical and The analytical solution for a linear mine wall is numerical methods. The first set of simulations used conceptualizations of aggregate mining in sand-and- valid for ground-water flow systems that meet the gravel aquifers,and the second set of simulations used following assumptions: conceptualizations of aggregate mining in fractured crystalline-rock aquifers. Analytical and numerical • The geologic materials are homogeneous and simulations were used to estimate the steady-state isotropic; hydrologic effects of mining. Under steady-state conditions, discharge to a mine reaches equilibrium • Ground-water flow is steady state, unconfined, hori- with the surrounding ground-water system, and the zontal,and perpendicular to the mine wall; extent of drawdown caused by dewatering a mine • Recharge is uniformly distributed at the water table, ceases to increase.Therefore, steady-state simulations and all recharge within the distance of influence predict the maximum potential effects of mining over is captured by the mine; time. To predict short-term effects, transient(time- varying) simulations are necessary. Steady-state • The uphill mine wall is approximated as a straight simulations of pits in sand-and-gravel aquifers may line, overpredict the effects of mining if active dewatering of the pit ceases before steady-state conditions are • The static premining water table is approximately reached. The hydrologic effects of pits in sand-and- horizontal; and gravel aquifers after active dewatering ceases(pits lined with slurry walls or refilled pits undergoing • The base of the pit is coincident with the base of the evaporative losses)likely reach steady-state conditions aquifer, and there is no flow through the mine because such pits may be left open indefinitely.The bottom. hydrologic effects of quarries in fractured crystalline- rock aquifers also likely reach steady-state conditions MODFLOW-2000(Harbaugh and others, 2000) because quarries commonly drain without the aid of was used to estimate the steady-state extent of draw- active dewatering measures (Knepper, 2002) and may down near a mine and ground-water inflow to a mine be left open indefinitely. Predicting the transient under conditions that consider heterogeneity, anisot- hydrologic effects of mining is beyond the scope of ropy, and boundaries. MODFLOW-2000 solves the this report. transient ground-water flow equation by using implicit finite-difference methods and is based on a three- dimensional, block-centered, finite-difference grid. Simulation Of Pits in Sand-and- Aquifer properties can be heterogeneous and aniso- Gravel Aquifers tropic provided the principal axes of hydraulic conduc- tivity are aligned with the coordinate directions Definitions of input parameters for simulations (Harbaugh and others, 2000: McDonald and of aggregate mining in sand-and-gravel aquifers Harbaugh, 1988),and aquifer layers can be simulated were based on data reported in the literature as confined, unconfined,or a combination of both (see"Hydrogeologic Settings"). Definitions of mining (Harbaugh and others, 2000). MODFLOW-2000 can extents (area and depth) were defined based on mine SIMULATION OF THE HYDROLOGIC EFFECTS OF MINING AGGREGATE g