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American Water Engineering Services, LLC <br /> August 30, 2022 <br /> Raptor Materials, LLC <br /> 8120 Gage Street <br /> Frederick, Colorado 80516 <br /> Attn: Garrett Varra <br /> RE: Comments on Dewatering Analysis <br /> 112C Permit Application Adequacy Review <br /> DRMS Letter Date August 5, 2022 <br /> Dear Mr. Varra: <br /> This letter report presents responses to a review of a dewatering analysis performed by American <br /> Water Engineering Services, LLC (AWES) for the Raptor Materials, LLC (Raptor) mine located near <br /> Evans, Colorado. This report is intended to address the issues raised in the Division of Reclamation, <br /> Mining and Safety (DRMS) letter dated August 5, 2022. The review letter is provided as Attachment <br /> A. <br /> The following summarize DRMS comments by item. <br /> 1. Comment 6— Please justify the assumption of anisotropy and the chosen K values for the <br /> sand and gravel aquifer. Westest, LLC of Lakewood Colorado conducted a geotechnical <br /> investigation, which consisted of drilling 12 soil borings by hollow stem auger techniques <br /> and obtaining representative soil samples to determine soil strength and grain size <br /> distribution. The borings were also completed as groundwater monitoring wells. The grain <br /> size distribution curves show the 90% retained (d10) to vary between 0.2 and 0.3 millimeters <br /> (mm). The Allen Hazen equation as presented by Freeze and Cherry, Raghunath and others <br /> provides rough estimates of hydraulic conductivity. Use of the equation K=Ad2 <br /> 10, where K is <br /> hydraulic conductivity in cm/s, d10 is in mm and A is a constant equal to one. Based on grain <br /> size curves the hydraulic conductivity is estimated to vary between 113 and 255 feet per <br /> day. The author has also been involved with Varra Inc., mining operations since 1989 and <br /> numerous evaluations at operating mines have indicated K values ranging from 50 feet to <br /> 300 feet per day. Schneider 1983, reported K values ranging between 15 and 300 feet per <br /> day in the alluvial valley deposits,which support past analyses. <br /> The clay, sand and gravel deposits are alluvial and have bedding planes. Hydraulic <br /> conductivities are smaller in directions normal to bedding than in directions parallel to <br /> bedding. This by definition is anisotropic. As different horizontal K values were assigned for <br /> the unconsolidated deposits the aquifer within the model area is more correctly described as <br /> heterogeneous and anisotropic. <br /> 2. Comment 7 — Please describe how these wells were used for pre-mining aquifer <br /> characterization (besides the collections of water level data). The wells are small diameter <br /> 4SC9 Four Star Court, Fort Collins,CC SCJ24—97Q C-3SC1 <br />