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I I I I I I I I I I I I I I I I I I I DRAFT 8/24/92, Page 28 2. The cost of purchasing excess storage capacity from Corps flood control facilities is based on federal regulations that aim to maximize federal revenues by reallocating the cost of the project on a pro rata basis. This could result in exorbitant costs to water suppliers. The purchase price for such storage would have to be negotiated in an auspicious political setting. 3. The efficient use of reallocated flood control capacity for water supply will probably involve several changes of water rights. These changes would have to approached from a systems integration approach to water supply pranning. 4. Operation of water supply pools in flood control reservoirs could have both recreational and environmental impacts. Operations would have to be developed to avoid or minimize these impacts. 5. Operation of flood control reservoir for storage of winter flows and other supplies could adversely affect instream flows in Bear Creek and the South Platte through the metro Denver area. 4. Conjunctive Use of Surface Water and Groundwater Supplies and Systems a. Use of Nontributary Sources as Dry Year Reserves Because of the mismanagement of groundwater resources in the Ogallala aquifer and Arizona there has been a stigma associated with development and reliance upon nontributary groundwater for municipal purposes. To some extent, this stigma is beginning to disappear as we learn more about the nature and extent of the resource and the possibilities for its utilization. It is currently estimated that the Denver Basin, covering an area of approximately 6,600 square miles stretching from Colorado Springs to Greeley, contains between 175 and 250 million acre-feet of theoretically recoverable groundwater. These figures indicate that, even without recharge, non-tributary groundwater could meet much of our municipal water supply needs for hundreds of years longer than the useful life of any surface water supply facility such as Two Forks. In general, the quality of the water is good; it seldom requir~s treatment beyond minimal disinfection to meet state standards. Where problems do exist (e.g., the Laramie Fox Hills aquifer), they can usually be alleviated by blending the well water with water from other sources. The Metropolitan Denver Water Supply EIS estimated that development of unappropriated non-tributary groundwater under municipal boundaries has the potential for providing a firm annual yield of up to 145,000 acre-feet per year. (Army Corps of Engineers, 1988) This estimate did not include consideration of natural recharge rates or artificial recharge potential. Nor did the EIS consider the advantages associated with conjunctive use of surface and ground water systems. Conjunctive use would involve the integration of both existing (estimated at about 23,100 acre-feet) and future groundwater development into surface water supply systems. In years when surface water is available, groundwater would not be used and wells would only be pumped to the minimum extent necessary for maintenance purposes. In dry years, the groundwater supplies would be pumped into the surface water system. This would allow greater use of the surface water system, because in most years more water is available than the firm annual yield of the surface water system. (See discussion above under agricultural acquisitions.)