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MEMORANDUMa BBA <br />water consultants <br />BISHOP-BROGDEN ASSOCIATES, INC. <br />To: Stephanie Fancher, File 9641.12 <br />From: Timothy A. Crawford <br />Subject: Loveland Ready Mix- Hydrogeologic Investigation of the Proposed Cell 21 Gravel Pit at the <br />Larimer Pit Site <br />Job: 9641.12 <br />Date: May 2, 2014 <br />This memorandum presents a hydrogeologic investigation of the impact of the potential mining and lining <br />of the proposed Cell 21 Gravel Pit at the Larimer Pit site generally located in Section 16, Township 5 <br />North, Range 69 West of the 6`h P.M. Specifically, the hydrogeologic investigation focuses on providing <br />information regarding the impact to ground water level elevations in the alluvial aquifer of the Big <br />Thompson River as a result of dewatering the pit during gravel mining and the lining of the pit after <br />mining has been completed to assist the their decision making process regarding mining, dewatering and <br />the installation of a low -permeability barrier. To analyze the potential water level impacts a MODFLOW <br />model was prepared, as described below. The purpose of this modeling effort is to understand potential <br />impacts of the proposed mining, dewatering and liner installation on ground water elevations and to <br />address any concerns the Colorado Division of Reclamation, Mining and Safety (DRMS) may have <br />associated with the proposed pit regarding impacts to water levels. The general location of the model and <br />proposed gravel pit has been presented in the attached Figures 1 and 2. <br />Background <br />The proposed Cell 21 gravel pit at the Loveland Ready Mix Larimer Pit site will mine sand and gravel <br />material from the Big Thompson River alluvium. Dewatering of the pit during mining will be required <br />due to saturated conditions at the proposed pit location. The dewatering of the proposed pit will lower <br />water levels in the alluvium near the pit. Instantaneous dewatering rates up to approximately 300 gallons <br />per minute (gpm) are expected based on dewatering pump operations at other nearby pits, however, <br />pumping rates are expected to decrease as dewatering progresses. <br />Available geologic mapping indicates that the alluvium at the Larimer Pits site consists of the Post Piney <br />and Broadway alluvial terraces. The location of the proposed Cell 21 pit, specifically, consists of the <br />Broadway alluvial terrace. Based on the geologic mapping, the bedrock below the alluvial aquifer at the <br />site location consists mainly of Pierre Shale. <br />Available USGS geologic mapping, State Engineer well permit database information, test drilling data <br />and South Platte Decision Support System (SPDSS) mapping were utilized to identify the extent and <br />occurrence of the Big Thompson River alluvium for this effort. The available SPDSS mapping included <br />mapping of bedrock elevations, water level elevations and transmissivity for the general area that was <br />reviewed as part of the investigation of impacts of mining. The digital elevation model (DEM) data were <br />utilized as part of the impact investigation as well. <br />Sufficient data was not available for the area to allow for a full calibration of the model. The model was <br />refined to simulate the saturated thicknesses indicated by the limited data, the expected dewatering rates <br />and the expected underflow.