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• contaminants through the borehole; limit groundwater production to a single aquifer; and prevent <br />the intermingling of groundwater from different sources through the borehole. Construction, <br />operation, and maintenance of the groundwater supply wells will comply with existing OSE rules. <br />By complying with OSE well construction requirements groundwater supply wells completed in <br />the Dakota-Cheyenne aquifer will be adequately sealed and cased. Adherence to well <br />construction requirements will minimize leakage from supply wells into the surrounding layers <br />and will prevent increase in groundwater flow, downgradient along an unnamed arroyo, toward <br />the Saint Charles River. Therefore, no effect on groundwater resources would be expected. <br />Shale covers an estimated 22 percent of the limestone area that would be disturbed during project <br />activities. Removal of this shale mantle would likely enhance permeability and infiltration of <br />meteoric water during the life of the mine. No subsequent effect on groundwater quality or inflow <br />to the Saint Charles River alluvial aquifer would be anticipated, as this groundwater would not be <br />likely to reach any surface drainage (as groundwater discharge). No corresponding, measurable <br />reduction in surface runoff or surface flows would accompany this enhanced infiltration. <br />Infiltration rates for the affected area still would be extremely low, and evaporation rates still <br />would be very high. <br />Bedrock that would be excavated during mining activities does not contain appreciable quantities <br />of groundwater. The fifty one test hales in the mine area drilled to depths of approximately 60 <br />feet in the limestone were dry. Fractured shale, sandstone, and limestone could be excavated <br />during mining activities. A wide range of hydraulic conductivities could occur in these fractured <br />rocks, depending upon the degree of fracturing present. Aiken et al. eds. (2000) presents <br />generalized hydraulic conductivities for fractured rocks containing these lithologies, which are <br />• used below to indicate the relative velocity of groundwater movement through these rock types. <br />Groundwater could move through fractured shales, sandstones, or limestones having hydraulic <br />conductivities ranging from 10~ to 10_Z ft/day, or a rate of one-half inch to four feet per year. In <br />100 years, groundwater in the affected bedrock units could potentially move 4 to 400 feet. It is <br />unlikely that any groundwater discharge from bedrock to the alluvial aquifer of the Saint Charles <br />River would be affected by the proposed project. <br />The withdrawal of groundwater by three supply wells that would be completed in the Dakota- <br />Cheyenne aquifer is not likely to deplete the natural flow of any stream or its alluvial aquifer. The <br />OSE has permitted the groundwater supply wells for this project. The OSE has the authority to <br />determine whether groundwater wells can be permitted and the conditions of approval for wells, <br />based on groundwater and surface water conditions in the affected area. <br />SURFACE WATER SUPPLEMENTAL INFORMATION <br />BACKGROUND <br />The Red Rock Plant and Limestone Mine is located within the Upper Arkansas Watershed (U.S. <br />Geological Survey (USGS) Hydrologic Unit Code (HUC) 11020002) that covers approximately <br />500 square miles in central Colorado, extending from the Continental Divide to the area <br />immediately east of the City of Pueblo. Surface water resources in the vicinity of the permit <br />boundary include the Edson Arroyo to the east, Greenhorn Creek to the west and several <br />intermittent arroyos within the permit area that flow mainly in response to significant storm <br />events. These drainages are tributary to the Saint Charles River that traverses the site along the <br />• northern boundary and flows northeasterly approximately 15 miles to the confluence with the <br />Arkansas River. The average annual precipitation is 12 inches based on a monthly climate <br />5 <br />