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square miles and ranges in thickness from 800 to 1000 fect. Distincti\"e characteristics of this aquifer arc its tine-grained nature and the fact that sandstone Icnses do not extend over large areas. At the base of the Denver Aquifer. an extensive clay shale unit of approximately 50 feet thickness separates the upper layer from the underlying Arapahoe Aquifer. 1.3.13 Arapahoe Aquifer - The Arapahoe Aquifer consists of a series of interbedded conglomerates. 5..1.ndstones. siltstones and clay shales. The aquifer covers an area of almost 4.700 square miles and is generally from ~oo (0 400 feet in thickness. In the Study Area. the Arapahoe Aquifer is typically characterized as a single aquifer although in the northern parts. a 50 to 100 foot shale layer separates it into an upper and lower zone. The Arapahoe Aquifer contains many high productivity wclls and is used extensively throughout the Study Area for municipal purposes. Water quality in this aquifer is t).pically \"ery high and meets state and federal drinking water standards. 1.3.1..t laramie-Fox lIi11s formation - Approximately 400 feet of clay shales with minor interbeds of sandstone and siltstone separate the Arapahoe Aquifer from the underlying Laramie- Fox Hills Aquifer. Only minor amounts of water are found in the Laramie Fonnarion and water quality is degraded with concentrations of hydrogen sui/ide and methane gases. Basal sandstone beds are isolated from the rest of the fomlation and are used in combination \I,ith the underlying Fox Hills sandstones for municipal purposes. The lower sandstones of the Laramie Fomlation and the Fox Hills sandstone combine to form the Laramie.Fox Hills Aquifer. This aquifer underlies approximately 6.700 square miles and represents the areal cxtent of economically developable ground water in the Denver Basin. The Laramie.Fox Hills Aquifer is between 250 and 300 feet in thickness with about 150 to 200 feet of water in line to medium-grained sandstones. Well yields can exceed 300 gallons per minute and this formation is used for municipal purposes in certain areas of the Stud:. Area. Low water quality plus high costs for development limit the current use of this aquifer to a few water districts. 1.3.1.5 Pierre Shale - The 4.500 foot thick Pierre Shale underlies the Fox Hills Fomlation and contains minor sandstone beds. This formation is not used for municipal purposes and represents the vertical boundary for economic developmcnt uf water from the Denver Basin. 1.3.2 \Vater AvailabiJit), - The USGS has estimated that the Denver Basin aquifers store 467 million acre-feet of ground water of which :!59 million acre. feet is theoretically recoverable by gravity drainage. Approximately one third of the Denver Basin is found in the five county Denver Metropolitan area. In Douglas County. the USGS estimates that 49 million acre-feet of water are theoretically recoverable although practical development levels have not been established. The actual availability of ground \vater for municipal purposes is restrained b:' legal and physical factors. The State of Colorado has classitied ground water as being either tributary or non- Page ]-6 I I I 4 4 4 4 4 4 4 4 4 . . . . . . . t t t t . . . . . . . ..