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HAHN WATER RESOURCES, LLC <br /> August 2, 2019 <br /> Angie Fowler, PE <br /> SGM <br /> 118 W Sixth St, Suite 200 <br /> Glenwood Springs, CO 81601 <br /> Letter Report: Evaluation of the Impacts of Dewatering at Rifle Pit No. 1 <br /> BACKGROUND AND OBJECTIVES <br /> This letter report provides a brief summary of my findings in an investigation of the impacts of <br /> dewatering of a proposed sand and gravel mine known as the Rifle Pit No. 1. The proposed mine <br /> is located in Garfield County, Colorado within the alluvial channel of the Colorado River. The <br /> effects of dewatering are of interest in terms of how they might impact several areas on the <br /> property that have been identified as wetlands. <br /> METHODS OF INVESTIGATION <br /> Basic Data <br /> Site-specific data available for this analysis was extremely limited. That data included drilling <br /> logs and water level measurements for 5 exploratory borings (HP Geotech, 2008), a site <br /> elevation survey, preliminary mapping of on-site wetlands, mapping of various ditches and <br /> laterals surrounding and traversing the property, a decree for water rights on an adjacent parcel <br /> that included a discussion of aquifer properties (SGM, 2019). and aerial imagery (Google Earth <br /> and NAIP imagery). <br /> Model Selection <br /> It was determined that a numerical model would be best suited for making the types of <br /> predictions required. Whereas analytical solutions can, in some instances, provide more precise <br /> solutions, they have difficulty representing the multiple boundary conditions that need to be <br /> addressed in this setting. It was further determined that given the very limited data available for <br /> the site, the model would need to be relatively simple in its representation of the hydrogeologic <br /> system. MODFLOW-2000 (Harbaugh, et. al., 2000) was selected as the modeling platform. <br /> MODFLOW is a three dimensional, finite-difference groundwater model developed by the U. S. <br /> Geological Survey. The model was constructed with the following attributes: <br /> Model Layering <br /> The model was constructed as a single layer model representing an aquifer consisting of coarse <br /> and highly permeable alluvial sands and gravels. The sands and gravels overlie <br /> claystone/siltstone bedrock. The bedrock is treated as an impermeable boundary. Considering the <br /> limited information on the underlying bedrock (depth, elevation), the aquifer was represented as <br /> being horizontal, with a flat bottom. <br /> 1 <br /> 6589 Elaine Road -Evergreen, Colorado 80439 - office 720.242.8639 - cell: 303.870.5757 <br /> hahnwaterresources@gmail.com - www.hahnwaterresources.com <br />