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Evaluation of the Potential Impact of Ponded C-Pit Water <br />on Groundwater Quality in the Dakota Sandstone <br />Cemex USA Lyons Plant <br />Lyons, Colorado <br />Executive Summary <br />The potential impact of ponded C-Pit water on groundwater quality in the Dakota Group <br />sandstones beneath the Cemex USA Lyons Plant, Lyons, Colorado was evaluated to determine <br />the need for accelerated removal or treatment of the pit water to protect groundwater quality in <br />the Dakota sandstone. Water accumulated in the C-Pit has contacted the disposed cement kiln <br />dust (CKD) and contains elevated concentrations of some of the regulated constituents, total <br />dissolved solids (TDS), selenium, chloride, sulfate, thallium, and pH, specificed in the mining <br />permit and a recent inspection report. <br />Geologically, C-Pit is excavated in the Fort Hays Limestone Member of the Upper Cretaceous <br />Niobraza Formation. The Niobrara Formation is underlain by a thick sequence (about 500 feet <br />[ft]) of interbedded, low permeability claystones, siltstones, shales, and limestones of the Carlile <br />Shale, Greenhorn Limestone, Graneros Shale, and Mowry Shale formations. The Dakota Group, <br />comprised of sandstones, siltstones, and shales, underlies the Mowry Shale. Groundwater present <br />in sandstones of the Dakota Group is locally used for water supply. <br />The potential impact of C-Pit water on groundwater quality in the underlying Dakota sandstone <br />aquifer was evaluated using a groundwater flow and transport model for the six groundwater <br />pazameters (total dissolved solids (TDS), selenium, thallium, chloride, sulfate, and pH). The <br />results of the analyses indicate that none of the constituents will adversely impact Dakota <br />sandstone groundwater quality. Therefore, the ponded C-Pit water does not require accelerated <br />removal or treatment from a groundwater quality perspective. <br />Introduction <br />Although no viable groundwater pathway appears to exist between the C-Pit and underlying <br />Dakota sandstone aquifer because of a thick sequence of ]ow permeability claystones, siltstones, <br />and shales underlying C-Pit, the potential impact of ponded water in the C-Pit on groundwater <br />quality in the Dakota sandstone was evaluated for the selected groundwater parameters currently <br />specified in the mining permit and a June 24, 2004 Colorado Division of Mines and Geology <br />(DMG) inspection report. Potential groundwater quality impacts on the adjacent St. Vrain Creek <br />October 29, 2004 1 Revision 0 <br />