2023-08-03_PERMIT FILE - M2023025 (32)

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Bernhardt Pit Groundwater Evaluation May 2023 Page 13 of 31 model, and display the results. We exported processed cell-by-cell flow terms from the model into ArcView GIS software to process prepare report figures. The model area is 11,000 feet high (north to south) and 15,000 feet wide (west to east) centered on the Bernhardt pit. The model consists of one layer, 55 rows and 75 columns using 200 foot square model cells. Model Boundary Conditions Bedrock outcropping on the west side of the model is modeled using no-flow model cell boundaries. The bottom bedrock surface was digitized in ESRI GIS software and imported into the model on a 200 ft grid. The south, north and east sides of the model consist of MODFLOW Constant Head(CH)cells that establish a fixed gradient within the stream alluvium. We assigned MODFLOW "River" cell stage elevations using 10m DEM data, and interpolating in between. The southern-most,up-gradient River cell elevation was 4750 ft(msl)and the northern most down gradient elevation was 4705 ft(msl). 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, our model creates more conservative (ie. more) modeled"change." 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 m is the streambed thickness (m) which we assume is 1 ft so that Ksb/m equals Ksb. CDM-Smith (2006, Task 34.3 Streambed Conductance Figure 9) evaluated the streambed permeability at two sites (SC-07 and SC-08 within 10 upstream and downstream of Bernhardt pit, and came up with Kh values ranging from 137 ft/day to 377 ft/day. We believe these rates are too high because they are too close to our horizontal K (Kh) value (600 ft/day). 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). 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 October 2017, we measured the river widths at the south (140 ft), middle (113 ft) an northwest (195 ft) corners of the model area (average = 149 ft). Representative MODFLOW River cell parameters are shown in the table below. MODFLOW River Cell Parameters Streambed COND *Width (ft) Length(ft) Conductance Ksb/m (ft^2/day) (day^-1) 150 207�1 36 1080000 All River cells were assigned a COND value of 1,080,000 ft^2/day. The flow from the river into the aquifer is determined by multiplying COND by the head difference in the river and the underlying model cell. With such a high COND value,water freely flows between the SPR and the aquifer. McGrane Water Engineering,LLC 1669 Apple Valley Rd. •Lyons,CO 80540•Phone:(303)917-1247;email:dennis@mcgranewater.com