2016-01-12_REVISION - M1985112 (7)

Home Browse Search

Mr.J.C. York July 13, 2015 Page 4 well yields range from 10 to 1500 gpm and averages 244 gpm. The calculated saturated alluvial thicknesses ranges from 7 to 82 feet and averages 31 feet. We used the 31 foot average thickness to define the aquifer bottom and gradient (21 feet west to east) across the model. The average aquifer thickness is consistent with the 37 foot thickness reported for the Loloff pit (Well permit no. 77467 F) which was used in the radius of influence calculation (JT Construction, August 27, 2007). Appendix B includes the following data plots: Figure B1 - Well Permit Numbers; Figure B2 — Well Owner; Figure B3 — Well Depth; Figure B4 — Depth to Water; and Figure B5 — Saturated Aquifer Thickness which support our model conceptualization and assumptions. Aquifer Permeability The aquifer hydraulic conductivity(K) is a measure of its permeability. Contoured K's obtained from pumping tests reported for the State's South Platte Decision Support System (SPDSS Task 43.3, Figure 5C) groundwater modeling project (CDM-Smith, 2006) range from less than 250 ft/day to over 680 ft/day. Once the model was calibrated, the model area K averaged 600 to 650 ft/day(CDM-Smith, 2013, Figure 4-2). We assume an average model K of 625 ft/day. The assume an average aquifer transmissivity in the model (145,000 gpd/ft) which is the product of the hydraulic conductivity (625 ft/day) multiplied by the saturated thickness (31 ft). This is consistent with published transmissivities for the Loloff pit model area by CDM-Smith (2006, Figure 7a) and Hurr and Schneider, 1972. Model Boundary Conditions Model boundary conditions includes the Poudre River, aquifer subflow, and aquifer bottom. We assigned the elevation of the Poudre River as 1 foot above the interpolated ground elevations which are 4647 feet (msl) on the west end and 4626 feet (msl) on the east end of the model. The river stage was assigned a foot above where the topographic elevation contour cross the Poudre River and interpolated in between. We calculated model cell conductance (COND) as the product of the streambed unit conductance (Ksb/m) times the wetted river area (length x width). The average unit conductance obtained from SPDSS vertical permeability tests on the South Platte tests is approximately 37 ft/day/ft. We assume that the average width of the Poudre River is 50 feet. Therefore, model cell conductance is 185,000 ft^2/day(37 ft/day/ft * 50 ft * 100 ft). Aquifer subflow in and out of the model were simulated using constant heads on the west and east ends of the model set at the ground elevation contours at the Poudre river (4647 ft (msl) on the west and 4626 ft (msl) on the east side. Constant head cells were spaced every 1000 feet in the column 1 and every 100 feet along the east side of the model. Model Mass Balance Aquifer subflow in and out of the model was solved determined for the pre-pit steady state run. Table 2 shows that river and aquifer inflows and outflows total 7.8 cfs and there is a net gain in streamflow of 1.4 cfs within the model domain.