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saturated thickness of 95 feet was given a value of 35 percent; the smallest saturated <br />thickness of 11 feet was given a value of 10 percent to reflect the diminished value of <br />this resource for supplying large capacity wells for irrigation. Ground -water in storage <br />was computed by multiplying the saturated portion of the geologic formation by the <br />estimated percent sand/silt thickness, and by the specific yield estimated from map <br />information. <br />Assumptions <br />Several assumptions were used to estimate the recoverable ground -water resources in <br />the Basin in 2001, which are summarized in Table 7-1, and detailed in Table 7-2. <br />Previous studies have used 0.15 as the specific yield of the High Plains aquifer (a value <br />used in the Denver Basin for the comparatively fine-grained Laramie -Fox Hills aquifer). <br />A specific yield value of 0.20 was used in Table 7-1 for the High Plains aquifer because <br />this aquifer is largely comprised of unconsolidated to partially consolidated coarse sand <br />and gravel of alluvial origin. Also as established in the MWE Phase 1 report, a specific <br />yield value of 0.15 was assumed for both the Dakota and Dockum aquifers, because the <br />respective aquifer sands tend to be fine-grained. The percent sand and silt ratios were <br />derived from several sources by tabulating the reported footage intervals of sand and silt <br />in drilling reports or measured stratigraphic sections, and dividing by the total footage of <br />that particular formation and then averaging the results for each formation. Because the <br />vast majority of records do not provide adequate lithologic separation, only those that <br />appeared to distinguish the lithologies, were used. <br />Table 7-1. Summary of Recoverable Ground -water by Aquifer Unit <br />Aquifer <br />Averaged <br />Saturated <br />Thickness <br />In feet <br />Saturated Volume <br />in acre-feet <br />Recoverable <br />Ground -Water <br />in acre-feet <br />Proven and <br />Possible <br />Ground -water <br />In acre-feet <br />High Plains <br />35 <br />3,102,300 <br />1,002,000 <br />1,002,000 <br />Dakota <br />52 <br />3,740,700 <br />1,496,300 <br />1,496,300 <br />Cheyenne <br />95 <br />8,660,500 <br />3,464,200 <br />3,464,200 <br />Dockum, proven <br />176 <br />12,844,400 <br />6,422,200 <br />6,422,200 <br />Dockum, possible <br />300 <br />19,266,500 <br />- <br />9,633,300 <br />Total <br />658 <br />47,614,400 <br />12,384,700 <br />22,018,000 <br />VI 1-2 <br />99-028.003\Phase 2 Report\SHP GW Resources <br />