2022-11-07_PERMIT FILE - M2022001 (11)

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degree of soil saturation (Das, 1998). The alluvium, underlying the region, is generally a clean to silty sand and gravel, as described above with an approximate depth of 50 to 95 feet in the upper terrace and shallower than 10 feet along the South Platte River floodplain. Aquifer properties used in a previous study of the area performed by Bishop-Brogden Associates, Inc.(BBA)for the Town of Gilcrest were incorporated as this report utilized two long-term aquifer pumping tests in the vicinity of Gilcrest:the B4-66-11 adc well 3 miles north of Gilcrest and the Lorenz Well approximately'/z mile to the east of Gilcrest. ➢ The B4-66-011 adc well approximately 3 miles north of Gilcrest is the nearest controlled long-term aquifer pumping and showed a hydraulic conductivity of 1,270 ft/day. ➢ BBA performed an analysis of water level data from observation wells located near the Lorenz Well approximately'/2 mile to the east of Gilcrest. Their analysis indicated a hydraulic conductivity of approximately 150 ft/day. The hydraulic conductivity was set at the lower end of the ranges provided by BBA and were increased until the model heads converged. The model is sensitive to horizontal hydraulic conductivity and operates without errors in a narrow range of values at the high end of the ranges provided by BBA. Horizontal hydraulic conductivity for the calibrated model range from a low of 65 ft/day for the hills to the east and a high of 805 ft/day underlying the Site. An area of high horizontal hydraulic conductivity. A map with the horizontal hydraulic conductivity values utilized in the model is shown on Figure 3. 5.4 Specific Yield Specific yield for the aquifer was set at 20%which corresponds to the value determined by BBA and in support of numerous water court decrees. 5.5 Boundary Conditions Boundary conditions were set based on geologic features on the east and west sides of the model. The constant head boundaries on the south and north sides were varied to match observed groundwater data sets. ➢ Boundary conditions in a groundwater model are one tool used to approximate existing conditions at the outer extent of the model. The boundary conditions are set sufficiently far away from the area of interest to allow the aquifer to respond to local stresses and are based on hydraulic, hydrogeologic, hydrologic,geological and geographical assumptions. ➢ The model extends 3.3 miles southwest where constant head nodes were placed approximately 14 feet below the ground surface to represent existing groundwater levels taken in the area. ➢ Another constant head boundary was place approximately 3.7 miles northeast Site and constant head nodes were placed on the southeastern flank at water surface elevations approximately ten feet below the ground. ➢ To the southeast of the model surface topography and bedrock slope up in elevation creating a natural barrier to flow. The eastern limit of the alluvial aquifer was confirmed by researching the states database of permitted wells in the area and mapping where the permitted aquifer switches from the alluvial aquifer to a deep confined aquifer. The approximate location of the shallow and deep aquifer divide is approximately'/4 mile east the location of the Platte Valley Ditch. ➢ The South Platte River forms a natural hydrologic boundary to the west of the model. Model inputs for the MODFLOW river package are bottom elevation, head-stage and conductivity. The bed elevation was estimated using the USGS topographic quadrangle and Google Earth top of bed elevation and subtracting 6 feet to Groundwater Model Report—Section 20 Gravel Mine Site -4- August 2022