PERMFILE47013 - Laserfiche WebLink

Home Browse Search

Mr. Alex Schatz March 30, 2007 Page 3 of 7 The model was initially run as a transient model for 720 days with no mining simulated to develop the equilibrium starting heads to be input for the prediction model runs. Each predictive simulation was run for 360 days to reach an approximately steady-state. Two hundred time steps were used on the transient model, and 100 time steps were used for each predictive simulation. The time step multiplier was set to 1.05. Physical Characteristics Though the material of the alluvial aquifer varies throughout the model domain, it was modeled as a homogeneous, isotropic aquifer. Based on the nature of the material described in the boring data we reviewed and the information presented in Jenkins, 1964, a hydraulic conductivity of 869 feet per day (6,500 gallons per day/square foot) and specific yield of 22 percent were applied throughout the model domain, except when the gravel pits were converted to lakes or silt ponds following mining (see "Modeling of Pits" below). Sensitivity analyses showed that varying the specific yield within the range of 18 to 26 percent did not appear to have any noticeable effect on the model results. However, varying the hydraulic conductivity of the ground water aquifer did have an effect on drawdown surrounding the pits. The model was run with hydraulic conductivity ranging from 500 to 5,000 feet per day. Generally, higher aquifer hydraulic conductivities allow the aquifer to reach equilibrium or steady-state quicker, and higher hydraulic conductivities extend the resulting drawdown contours. The drawdown contours in Pit 1 simulations with hydraulic conductivities of 1,250 and 5,000 feet per day, the drawdown extends further east and affects more ground water across Fountain Creek than when the hydraulic conductivity is 869 feet per day. Modeling the drawdown due to dewatering and recharge at Pit 7 with various hydraulic conductivities, changes the impact to offsite wells because of the changes in the drawdown contours. A hydraulic conductivity of 500 feet per day only slightly shifts or modifies the drawdown contours from those predicted based a hydraulic conductivity of 869 feet per day. Higher conductivities of 1,250 and 5,000 feet per day increase and extend the drawdown contours further away from Cell 7. These results indicate that the aquifer hydraulic conductivity is an important input value which effects the predicted results. Because we had published data supporting a hydraulic conductivity of 869 feet per day, we decided to use that value in the modeling. The model was also run with varying conductance inputs for the river. These values ranged from 500 to 200,000 square feet per day. The contours across (east) the river were less affected at higher river conductances. We note that the river conductance values do appear to be a sensitive input to the model. We currently have not strong empirical support for any specific values or range of values. We strongly recommend Feld testing to assist in developing a better definition of this important parameter. Calibration runs of the model showed that in order for the river to maintain the water table as expected, the river conductance had to be set to 20,000 square feet per day. Varying the conductance of the river did have an effect on drawdown within and surrounding the Sundance site. If field data later show that site dewatering activities do effect the ground water east of 5:536 -Banks and Gesso, LLC~536.4 -Sundance Pi[\Report\Final Letter Report.doc Martin and Wood Water Consultants, Inc.