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MWSI Project Phase" Conjunctive Use Summary Report - DRAFT August 22, 1995 of the Basin, ground water pumping has captured some of the ground water discharge or changed the situation where ground water flow is now from the streams. The majority of the precipitation recharge occurs to the Dawson aquifer. The Denver, Arapahoe and Laramie-Fox Hills aquifers receive precipitation recharge through their outcrop areas located on the margins of the Basin, Precipitation recharge is estimated by Robson (1987) to be 29,400 aflyr in the Dawson aquifer, 4,000 af/yr in the Denver, 2,050 af/yr in the Arapahoe and 4,200 aflyr in the Laramie-Fox Hills. Groundwater flow between the Dawson, Denver and Arapahoe aquifers occurs, Generally, flow occurs from the Dawson to the Denver and from the Denver to the Arapahoe aquifer. The Laramie-Fox Hills aquifer is isolated from interaquifer flow by the Laramie Formation. Groundwater pumping has altered the steady-state water budget for the Basin. Approximately 5200 aflyr flows from the Dawson to the Denver aquifer and 3850 aflyr flows from the Denver to the Arapahoe aquifer, Robson (1987). The aquifer properties of most interest in wellfield design and groundwater injection projects are transmissivity, hydraulic conductivity, storage coefficient and specific yield. These parameters are obtained from pumping tests and are generally well known over the developed areas of the Denver Basin, Robson (1987) reports that hydraulic conductivity in the Dawson, Denver and Arapahoe aquifers ranges from 0,05 to 7,0 feet per day, Three feet per day is a common value for hydraulic conductivity. The higher hydraulic conductivity values exist in the Arapahoe and Dawson aquifers, Transmissivity values are highest in the west central portion of the Basin in southern Arapahoe and northern Douglas Counties. Transmissivity generally ranges from 100 to 1200 ft2/day in the Dawson aquifer, JOO to 400 ft2/day in the Denver, 100 to 2100 ft2/day in the Arapahoe, and less than 100 to 1000 ft2/day in the Laramie-Fox Hills, Confined aquifer storage coefficient normally ranges from less than 2 x 10-4 to more than 8 x 10-4, Then Denver Basin Aquifer Rules (1985) establish specific yield values for the Denver Basin aquifers. The specific yield value for the Upper and Lower Dawson aquifers is 20 percent, the Denver, Upper Arapahoe, Lower Arapahoe, and Arapahoe aquifers are 17 percent, and the Laramie-Fox Hills aquifer is 15 percent. Specific yield laboratory analysis demonstrates that a range of specific yield exists within each aquifer, This range of specific yield values is generally greater than 10 and less than 35 percent for each of the Denver Basin aquifers. Estimates a/Water Availability There is an enormous amount of water in storage in the Denver Basin aquifers, approximately 467 million acre-feet. Of the total amount of water in storage approximately 56 percent of the water is theoretically recoverable. Table I lists the amounts of water stored and theoretically recoverable from the Denver Basin aquifers (Robson, 1987). All values are in acre- feet. 2