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Derr Pit - Groundwater Modeling Report <br />January 31, 2020 Page 2 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 />upgradient side and decline on the downgradient side. Water level increases to within 10 feet of <br />the surface on the upgradient side of the pit could threaten flooding of basements, and in extreme <br />cases if water levels increase closer to ground surface encourage phreatophyte (such as cattails) <br />growth. A decline in water levels on the downgradient side could reduce the aquifer saturated <br />thickness and well yields if the decline is significant compared to the pump setting depth. <br />Available Data <br />We compiled hydrogeologic data from: <br />Existing reports from the U.S. Geological Survey and Colorado Division of Water <br />Resources (see Sources below); <br />Well permit completion reports from 133 registered alluvial wells (DWR wells) available <br />from the State’s Well database located within the study area; <br />Water level data from 18 monitoring wells located around the Derr pit; and <br />Water levels measured at 15 offsite private registered wells located within a mile of the <br />site. <br />Hydrogeology <br />The hydrogeology of the SPR alluvial aquifer is described by Lindsay and Others (1998 and <br />2005), CDM (2006 and 2013) and CSU (2013). Figure 2 shows the site surficial geology by <br />Tweto, 1979). The alluvium within the model areas consists of alluvial sand and gravel (Qa) <br />adjacent to the modern SPR and Poudre River flood plains, and older terrace alluvium (Qg) <br />outside the floodplain. The alluvium thins to the northeast where the Laramie shale (Kl) <br />outcrops just outside the northeast corner of the model area. <br />Table 1 (end of report) shows compiled pertinent well data that includes: location (Colorado <br />State Plane North coordinates) depth, yield (gpm), water level when drilled, and depth to <br />bedrock calculated from geologic logs (if available). The upper portion of the Table shows the 18 <br />Derr borehole/monitoring well data. The average well depth, water level when drilled and depth <br />to bedrock are 95, 13, and 85 feet respectively. <br />The center portion of the table include 15 private DWR wells that are currently being monitored <br />by BAI that are located within 1 mile of the Derr pit. The average well depth, water level when <br />drilled and depth to bedrock are 67, 28, and 60 feet respectively. The deeper depths reflect <br />ground surface elevations that increase to the north. Well yields range from 7 to 1200 gpm. <br />The lower portion of the table contain the remaining DWR wells within the model area. The <br />average well depth, water level when drilled and depth to bedrock are 51, 19, and 58 feet <br />respectively, whereas, well yields range from 0.75 to 1400 gpm. <br />Predevelopment Water Level Contours <br />Figure 2 shows the geology and reported water levels when drilled which best represent <br />“predevelopment” or pre-mining conditions. The predevelopment water table elevation at wells, <br />and for the Poudre River (at 10 ft increments), were calculated by subtracting the depth to water