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Derr Pit - Groundwater Modeling Report <br />January 31, 2020 Page 3 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 />from U.S. Geological Survey10m Digital Elevation Model (DEM) elevation data. We did not <br />use Derr monitoring well levels in Figure 2 because they are currently below pre-mining <br />conditions. We contoured the data using the US Geological Survey (Hurr and Schneider (H&S), <br />1972) water table map as a guide. The H&S maps were created in 1972 and are widely accepted <br />by hydrogeologists to represent “predevelopment” conditions. <br />Groundwater flows perpendicular to the contours and general toward the river. The contours <br />show that the water table generally flows from the northwest to the southeast across the model <br />area at a gradient controlled by the river. Overall, the water table drops approximately 30 feet <br />from northwest to southeast. A steeper gradient (closer contours) comes from the north as the <br />aquifer thins towards outcropping bedrock (Laramie Formation). <br />Bedrock Elevation Contours <br />Figure 3 shows the well permits and reported well depths. Figure 4 shows the calculated <br />bedrock elevation based on the ground elevation minus the reported depth to bedrock from a <br />“driller’s” log. Wells with a less than (<) sign represent wells that did not encounter bedrock <br />based on their driller’s logs. The data was used to update the bedrock elevation contours (aka. <br />structure map) published by the USGS (Hurr and Schneider, 1972). The map shows a deep (over <br />100 foot deep) erosional “paleochannel” extending from the northwest to southeast across the <br />study area. The depression was eroded into the bedrock by the Poudre River thousands of years <br />ago before the river established its current depositional character. The bedrock elevation within <br />the paleochannel drops from approximately 4600 feet to approximately 4530 feet in elevation. <br />The Derr pit is on the southern flank of the paleochannel where the alluvium is deepest which is <br />conducive to gravel mining. <br />Saturated Thickness <br />The well saturated thickness is an important factor for evaluating impacts caused by changes in <br />water levels that could affect well yield. Figure 5 shows the borehole saturated aquifer <br />thicknesses calculated by subtracting the depth to bedrock minus the depth to water data. <br />Greater than (>) signs are used as prefixes for shallow wells that did not encounter bedrock based <br />on the driller’s logs. The saturated thickness contours were obtained by subtracting the bedrock <br />surface elevation of model cells (Figure 3) from the modeled derived predevelopment water table <br />elevation (Figure 11). The contours show that the saturated aquifer thickness at the Derr pit <br />ranges from 55 to approximately 75 feet deep, and is 75 to over 80 feet thick in the amendment <br />area. The aquifer is thickest in the paleochannel and thins away from the river. <br />Reported Well Yield <br />Figure 6 shows reported well yields (gpm) range from under 15 gpm for domestic wells to 1,400 <br />gpm for agricultural wells. <br />Aquifer Transmissivity <br />Figure 7 shows the contoured aquifer transmissivity (T) which is the product of the aquifer <br />hydraulic conductivity (a measure of permeability) multiplied by the saturated thickness (Figure