2023-01-10_PERMIT FILE - M2023001 (12)

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Fair Meadows Pit Groundwater Evaluation November 19, 2022 Page 7 of 28 Model Boundary Conditions Model boundary conditions include the Poudre River on the south side, the top of bedrock defines the model bottom, and constant head cells establish a fixed gradient within the stream alluvium from the west and north to the east. We assigned MODFLOW "River"cell stage elevations using IOm DEM data, and interpolating in between. The western-most, up-gradient elevation was 4631 ft (msl) and the eastern-most down gradient elevation was 4605 ft(msl). The water level gradient from west to east is approximately 0.0017 ft/ft [(4631-4605)/15000)]. We modeled the aquifer's connection to the river using the MODFLOW "River" package which uses a streambed conductance term(COND)to calculated flow from one to the other in proportion to the hydraulic gradient of adjacent. A high level of connection mitigates impacts by allowing water to freely flow between the river and aquifer. Therefore,by using the lower end COND value that the data supports,our model creates more conservative (ie. more)modeled impacts. COND is calculated as the product of the streambed unit conductance (Ksb/m) times the wetted river area (length * width). Ksb is the streambed vertical permeability and in is the streambed thickness (m) which we assume is 1 ft so that Ksb/m equals Ksb. CDM-Smith (2006, Figure 9) evaluated the streambed permeability at three sites (SC-8, SC-13, and SC-14) within 10 miles of the FAIR MEADOWS pit, and came up with Ksb values ranging from 362 ft/day to 404 ft/day. We believe these rates are too high because they are too close to horizontal K(kh) values. Tests conducted in 2009 by Leonard Rice Engineers, Inc. (Denver, Co.) in Twn. 2N, Rng. 66W, Sec. 18, arrived at a Ksb value of 36 ft/day (Miller, 2009). We believe 36 ft/day is more accurate because it was determined through rigorous aquifer testing and is approximately 10 times less than the Kh value (300 ft/day in that area). A 10:1 Kh/Kv ratio is a common ratio used for alluvial aquifers. Therefore,to be conservative, we decided to reduce the connection to the river by using a Ksb of 36 ft/day. Using Google Earth imagery taken in September,2021,we measured the river widths at the 4610, 4620, and 4630 ft elevation contour locations shown on Figure A2, which resulted in an average river width of 61 feet. Representative MODFLOW River cell parameters are shown in the table below. River Length(L)= 200 ft River Width(W)= 61 ft Streambed Conductance(Ksb/m)= 36 day-1 COND= 439,200 ft^2/day Model Results We conducted two model runs to evaluate groundwater level changes caused by the Fair Meadows pit slurry wall. Pre-Fair Meadows Slurry Wall Run The initial run <FMpit3> is meant to simulate the water table prior to slurry wall. Figure A8 shows the modeled water table contours compared to "target" contours shown in Figure A3. In McGrane Water Engineering,LLC 1669 Apple Valley Rd. •Lyons,CO 80540•Phone:(303)917-1247 E-Mail:dennis@mcgranewater.com Web:ttp://www.mcgranewaterengineering.com