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Derr Pit - Groundwater Modeling Report <br />January 31, 2020 Page 9 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 />approximately 1.6 ft at the Parker domestic well. The greatest downgradient decrease is - <br />0.9 ft at the Taylor and Baab domestic wells. <br />Table 3 shows that if the Loloff, Derr and amendment area slurry walls are installed, then <br />thirteen (13) wells could be impacted. The maximum upgradient increase is <br />approximately 3.2 ft at the Davis well, and maximum downgradient decrease is -2.1 ft at <br />the Murata well. <br />The amount of current dewatering ranges from a positive 1.2 ft at the Harrell irrigation <br />well to -9.7 ft at the Murata domestic well (Table 3). <br />The modeled drawdown caused by slurry walls is less than current drawdown conditions. <br />Based on our evaluation and modeling, we strongly believe that installing a slurry wall around <br />the Derr pit will improve existing groundwater conditions for all nearby private well owners <br />because water levels will recover from existing conditions. After recovery, water levels will <br />likely mound on the upgradient sides of the pits, but not enough to flood existing basements. <br />Downgradient, existing wells will likely see lower water levels compared to preexisting <br />conditions, but higher levels than today. We do not believe any downgradient wells will <br />experience significant adverse impacts such as a measurable decline in well yield. <br />Recommendations <br />We therefore recommend that BAI should install a slurry wall around the Derr pit as soon as <br />possible. This would allow ongoing dewatering to cease and water levels to recover. Full <br />recovery could take 1 to 2 years. During the meantime, we recommend that BAI: <br />Continue to monitoring the 18 Derr monitoring wells and vicinity private wells on a <br />monthly basis until the slurry wall is installed, and less frequent thereafter until recovery <br />is complete; and <br />Although not anticipated, if upgradient mounding does cause any flooding, mitigate the <br />situation by the use of sump pumps or drains. <br />Sources <br />AgPROfessionals, , 20. , 20 Email from <br />, 2019. <br />CDM-Smith, April, 2013. South Platte Decision Support System Alluvial Groundwater Model Report. <br />Colorado Division of Water Resources (DWR) Well Data Base, 2019. <br />https://dwr.state.co.us/Tools/WellPermits <br />Hurr, R.T and Schneider, P., 1972. Hydrogeologic Characteristics of the Valley Fill Aquifer in the <br />Greeley Reach of the South Platte River Valley, Colorado. USGS Open File Report 73-124. <br />Langer, W. H., and Lindsey, D. A., 1999, Preliminary deposit models for sand and gravel in the Cache la <br />Poudre River valley: U.S. Geological Survey Open-File Report 99- 587, 27 p.