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McGrane Water Engineering, LLC <br />1669 Apple Valley Rd. Lyons, CO 80540 Phone: (303) 917-1247 <br />E-Mail: dennis@mcgranewater.com <br />January 31, 2020 <br />Mr. JC York <br />J&T Consulting, Inc. <br />305 Denver Avenue, Suite D Via email at: jcyork@j-tconsulting.com <br />Ft. Lupton, CO 80621 <br />RE: Derr Pit –Hydrologic Assessment and Computer Modeling <br />Dear Mr. York: <br />The Derr pit is located east of Greeley, in Section , Township 5 North, Range 6 West of the 6th <br />principal meridian in Weld County, Co., and is being developed by Broken Arrow Investments, <br />LLC (BAI). The Derr pit is permitted under Colorado Division of Reclamation, Mining and <br />Safety (DRMS) permit no. DRMS M-2008-017. BAI has amended their mining permit to mine <br />north of the existing pit to C Street, west of Cherry to Ave (CR-43) and east of Balsam St. (CR- <br />41.5) which is referred to as the “amendment” area. BAI seeks to install a slurry walls around <br />the Derr pit in 2020 and the amendment area in 2022. <br />BAI is seeking approval for a Use by Special Review (USR) amendment from the Weld County <br />Planning Department and the Board of County Commissioners (Board). At a USR hearing on <br />September 11, 2019, several vicinity well owners expressed concerns about dewatering impacts <br />around the pit. BAI asked to continue the hearing so these issues could be evaluated. <br />BAI retained McGrane Water Engineering (MWE) to evaluate groundwater level changes <br />associated with the proposed slurry walls. MWE has experience in this area since we conducted <br />the Loloff Pit – Slurry Wall Assessment (MWE, July 13, 2015). For this study, MWE expanded <br />the study area, evaluated additional monitoring well data and constructed a new model to <br />evaluate the combined effects of the Loloff and Derr pits slurry walls, and the addition of the <br />amendment area slurry wall. <br />Study Area <br />The Derr pit is located approximately 1/2 mile north of the Cache la Poudre (Poudre) River <br />approximately 5 miles from the confluence with the S. Platte River (SPR). Figure 1 shows the <br />study area which centers on the existing Loloff pit, the Derr pit immediately to the east and the <br />amendment area to the north. The study area includes approximately 133 wells identified in the <br />Colorado Division of Water Resources’ (DWR) well database (CDWR, 2019). Most of the <br />DWR wells are used for domestic water supply, with a few for irrigation. <br />Slurry Walls <br />Slurry walls prevent groundwater from flowing into a pit. Once a slurry wall is installed, the <br />natural groundwater flow is impeded which causes the water table to back-up and rise on the