2014-01-06_REVISION - M2000041 - Laserfiche WebLink

Home Browse Search

Given input values of h,,,. W, Kh, and initial (premining) saturated thickness above the base of the mine (h = h,,), the distance of influence (x;) can be calculated directly by setting x equal to xi and re- arranging equation 7. Once xi is determined, h can be calculated for any distance upgradient from the mine wall, and drawdown can be calculated as h. — h. In addition, the inflow rate per unit length of mine. Q [L2/T1. can be calculated as: Q = Wx; (8) The analytical solution for a linear mine wall is valid for ground -water flow systems that meet the following assumptions: • The geologic materials are homogeneous and isotropic; • Ground -water flow is steady state, unconfined, hori- zontal, and perpendicular to the mine wall; • Recharge is uniformly distributed at the water table, and all recharge within the distance of influence is captured by the mine; • The uphill mine wall is approximated as a straight line; • The static premining water table is approximately horizontal; and • The base of the pit is coincident with the base of the aquifer, and there is no flow through the mine bottom. MODFLOW -2000 (Harbaugh and others, 2000) was used to estimate the steady -state extent of draw - down near a mine and ground -water inflow to a mine under conditions that consider heterogeneity, anisot- ropy, and boundaries. MODFLOW -2000 solves the transient ground -water flow equation by using implicit finite- difference methods and is based on a three - dimensional, block - centered, finite- difference grid. Aquifer properties can be heterogeneous and aniso- tropic provided the principal axes of hydraulic conduc- tivity are aligned with the coordinate directions (Harbaugh and others, 2000: McDonald and Harbaugh, 1988), and aquifer layers can be simulated as confined, unconfined, or a combination of both (Harbaugh and others, 2000). MODFLOW -2000 can simulate several types of hydrologic sources and sinks including aquifer recharge, evapotranspiration, wells, drains, and rivers, and it can simulate either steady - state or transient conditions. SIMULATION OF THE HYDROLOGIC EFFECTS OF MINING AGGREGATE Two hydrogeologic settings in the Colorado Front Range area were simulated using analytical and numerical methods. The first set of simulations used conceptualizations of aggregate mining in sand -and- gravel aquifers, and the second set of simulations used conceptualizations of aggregate mining in fractured crystalline -rock aquifers. Analytical and numerical simulations were used to estimate the steady -state hydrologic effects of mining. Under steady -state conditions, discharge to a mine reaches equilibrium with the surrounding ground -water system, and the extent of drawdown caused by dewatering a mine ceases to increase. Therefore, steady -state simulations predict the maximum potential effects of mining over time. To predict short-term effects, transient (time - varying) simulations are necessary. Steady -state simulations of pits in sand - and - gravel aquifers may overpredict the effects of mining if active dewatering of the pit ceases before steady -state conditions are reached. The hydrologic effects of pits in sand -and- gravel aquifers after active dewatering ceases (pits lined with slurry walls or refilled pits undergoing evaporative losses) likely reach steady -state conditions because such pits may be left open indefinitely. The hydrologic effects of quarries in fractured crystalline- rock aquifers also likely reach steady -state conditions because quarries commonly drain without the aid of active dewatering measures (Knepper, 2002) and may be left open indefinitely. Predicting the transient hydrologic effects of mining is beyond the scope of this report. Simulation of Pits in Sand -and- Gravel Aquifers Definitions of input parameters for simulations of aggregate mining in sand- and - gravel aquifers were based on data reported in the literature (see "Hydrogeologic Settings "). Definitions of mining extents (area and depth) were defined based on mine SIMULATION OF THE HYDROLOGIC EFFECTS OF MINING AGGREGATE