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Derr Pit - Groundwater Modeling Report <br />January 31, 2020 Page 4 of 24 <br />McGrane Water Engineering, LLC <br />1669 Apple Valley Rd. Lyons, CO 80540 Phone: (303) 917-1247 <br />E-Mail: dennis@mcgranewater.com Web: ttp://www.mcgranewaterengineering.com <br />A5). We used a constant hydraulic conductivity of 625 ft/day that best matches the published <br />H&S transmissivity map (H&S, 1972). The transmissivity is highest (300,000 to 400,000 gpd/ft) <br />within the paleochannel and decreases to the northwest and away from the channel. <br />Modeling <br />We used the USGS (McDonald and Harbaugh, 1988) MODFLOW modeling program to <br />evaluate the effect of the existing Loloff and Derr Pit(s). We included the effect of the Loloff <br />pit in our analysis because we feel that it is important to determine the effects of all mining <br />activities on vicinity wells even though the Loloff slurry wall is already installed. We used the <br />Visual MODFLOW (VM) classic interface (version 4.6.0.167) to construct, run and display <br />model results. The model area is approximately 2 miles high and 3 miles wide (west to east) <br />centered on the pits, and consists of 55 rows and 73 columns using 200-foot square model cells. <br />We conducted two “steady state” runs. The first representing predevelopment conditions that <br />establishes the water table flow direction and aquifer thickness prior to pit development. We <br />then use the same run with the pit cells off to simulate post slurry wall conditions. Because the <br />model cells do not allow flow through them, the upgradient water will mound up and flow <br />around the pit(s). By subtracting the post slurry wall water table elevations from the <br />predevelopment water table elevations, we are able to calculate the change in water levels caused <br />by the slurry walls. <br />Model Boundary Conditions <br />Model boundary conditions include the Poudre River, and constant head cells on the west and <br />north side of the model and a few constant head cells on the east side of the model that were set <br />at predevelopment water table elevations to allow water to freely flow in and out of the model at <br />gradients tied to the river elevations. (Figure 2). <br />We assigned model river cell stage elevations every 10 feet using 10m DEM data, and then used <br />the VM interface to interpolate values linearly between the points. The western-most, upgradient <br />elevation was 4618 ft (msl) and the eastern-most downgradient elevation was 4581 ft (msl). <br />We modeled the aquifer’s hydraulic “connection” to the aquifer (ie. the ability of the river to <br />buffer effects of mining) using the MODFLOW “River” package which uses a streambed <br />conductance term (COND) to calculate flow between the river and aquifer. A high level of <br />connection mitigates impacts by allowing water to freely flow between the river and aquifer. <br />COND is calculated as the product of the streambed unit conductance (Ksb/m) times the wetted <br />river area (length * width). Ksb is the streambed vertical permeability and m is the streambed <br />thickness which we assume is 1 ft so that Ksb/m equals Ksb. CDM-Smith (2006, Figure 9) <br />evaluated the streambed permeability at three sites (SC-8, SC-13, and SC-14) within 10 miles of <br />the Derr pit, and came up with Ksb values ranging from 362 ft/day to 404 ft/day. We believe <br />these rates are too high because they are too close to horizontal K (kh) values. Tests conducted <br />in 2009 by Leonard Rice Engineers, Inc. (Denver, Co.) in Twn. 2N., Rng. 66W., Sec. 18, arrived <br />at a Ksb value of 36 ft/day (Miller, 2009). We believe 36 ft/day is more accurate because it was <br />determined through rigorous aquifer testing and is approximately 10 times less than the Kh