Laserfiche WebLink
Technical Adequacy Review <br /> Two Rivers Application <br /> August 30,2022 <br /> Page 3 <br /> 7. Comment 12 — Please clarify the initial calibration process. Please discuss the validity of <br /> the model in the north study area. The model was not calibrated using model assigned <br /> monitoring wells. These wells (measuring points) were added after the model calibrations. <br /> The measured depth to bedrock and surveyed ground surface elevations, in combination <br /> with topographic map elevations, were input into Surfer - a three dimensional contouring <br /> software package. The bedrock and ground surface elevations were then imported into <br /> Visual Modflow (VMOD). Water table elevations measured in on-site piezometers over a <br /> four year period were averaged and these elevations were used in the calibration process. A <br /> site survey also provided surface water elevations of the South Platte and Big Thompson <br /> rivers. The model boundary was superimposed over a topographic map and the author <br /> generated hand drawn contours that best reflected measured groundwater and surface <br /> water elevations. In the extreme northwest model area the elevations of the intermittent <br /> stream beds were used as water table elevations. General head boundaries were assigned <br /> to the model perimeter and head elevations were assigned where hand drawn elevations <br /> intersected the model boundary. The model was then run and predicted heads were <br /> compared to measured values. This process took several iterations as water table gradients <br /> changed dramatically throughout the model boundary. After the calibration process was <br /> completed the "cell inspector" function was used to determine the predicted head <br /> elevations for model assigned wells. The head elevations at the model assigned wells prior <br /> to pumping or lining were used as a baseline to measure the effects of dewatering and mine <br /> wall lining. It also should be noted that the Big Thompson River is a groundwater divide and <br /> groundwater conditions north of the river have little to no effect on dewatering <br /> simulations. <br /> 8. Comment 13—Please simulate the dewatering of the full extent of the mined area. Please <br /> estimate the time to achieve steady state conditions. Dewatering simulations for the total <br /> area mined are provided in an updated report attached to this submittal. Bear 1979, <br /> presents solutions for the time required to reach steady state and can be estimated by the <br /> following equation. <br /> R(t) = 1.9(Tt/ne)1/2 <br /> Where: R(t) = radius of influence as a function of time (ft.) <br /> T=transmissivity(ft2/day) <br /> t=time(days) <br /> ne=effective porosity(dimensionless) <br /> Assuming a saturated thickness of 35 feet, a transmissivity of 4375 ftZ/day, an effective <br /> porosity of 0.27 and a radius of influence of 4000 feet the time to reach steady state is <br /> calculated to be 273 days. However dewatering will be progressive during mining and an <br /> estimated 1000 days will be required to achieve this radius of influence during early <br /> dewatering operations. <br /> �w�s 4SC9 Four Star Court, Fort Collins,CC SCJ24—91C-b9C-3SC1 <br />