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<br />. <br /> <br />cross-bedded silty claystones and shales interspersed with andesitic and basaltic <br />sandstones and conglomerates. The thickness of the Denver aquifer ranges from 400 <br />to 800 feet throughout most of the basin. The bulk of the Dawson aquijer consists of a <br />thick sequence of conglomerate, sandstone, and clayey shale with local beds of shale <br />and lenses of Iignitic coal. The Dawson aquijer ranges in thickness from 800 to 1000 <br />feet in the deeper parts of the Denver Basin. <br /> <br />. <br /> <br />A large volume of ground water is in storage in the Denver Basin. Estimates of <br />total and recoverable storage in each aquijer are presented in Table 2.1, A portion of <br />this water, estimated by Robson as 300,000 acre-feet (af), (less than 0.1 % of the totai), <br />is stored in confined conditions. The remainder of the water would be released alter <br />the aquifers become unconfined and the aquifer material is drained. The largest <br />ground water use from the bedrock aquifers is in the Denver metropolitan area, <br />extending south to Castle Rock and in the Colorado Springs area. Ground water use is <br />not uniform over the basin and is concentrated along the Front Range of the Rocky <br />Mountains. <br /> <br />3.0 Previous Work <br /> <br />Robson (1987) developed a three-dimensional numerical model of Denver Basin <br />for use in prediction of changes in the system over time. A steady state calibration to <br />1958 water levels was followed by a 20-year history matching period through 1978 and <br />a 72-year prediction period ending in 2050. The major features of the hydrogeologic <br />system were included in this simulation model including: natural recharge. leakage <br />between the aquifers, interaction with the surtace water system, and pumping <br />withdrawals. Predictions of future water levels were based on several water demand <br />projections and possible wellfietd locations. Development of the model and the <br />assumptions inherent in the analysis are discussed in detail in the referenced report. <br /> <br />Banta (1989) developed a more detailed three-dimensional numerical model of <br />. the Denver Basin aquijers based on Robson's work, but with smaller cell size to provide <br /> <br />6 <br />