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Meeting Water Demand Without Gunnison Water Potable reuse refers to the physical treatment of wastewater to drinking water standards and re-circulating it through the regular distribution system. This can be accomplished by treating the effluent and directly releasing it into the distribution system or by blending treated effluent with raw water, then treating this mixture to meet drinking water standards. Substitution arrangements-taking the form of exchanges, plans of augmenta- tion, first-use agreements, and water trades-allow for the delivery of effluent to downstream water users in exchange for Denver's use of upstream water. For exam- ple, a downstream farmer may not mind that the water he diverts is below drinking water standards. He might be willing to exchange a more permanent supply of return flows from a city's water system for rights he has to water stored in a pristine head- waters area (water that would require less treatment by the city). Both parties must agree to the quantity and quality of the exchanged water and must, in order to com- ply with Colorado law, keep stream flows between the points of exchange at sufficient levels to protect intervening water users. Technical and political feasibility of large-scale potable reuse have been demonstrated or are planned in other parts of the country. A plant in Fairfax County, Virginia, has re-purified water for human consumption since 1978, without health problems. The San Diego County Water Authority proposes to construct a 20 million gallon/day water re-purification facility.""'. The town of Cary, North Carolina has invested $10 million in a reclaimed water system to supply irrigation water to its resi- dential and commercial customers."' Notably, conventionally treated wastewater from upstream cities makes up a good portion of the stream flow from which most Denver providers currently draw supplies.1' Large-scale effluent re-use as a Front Range water supply strategy is not with- out its own challenges, including public acceptability and potential impacts on down- stream water users. However, the enormous potential of re-use to help meet demand at costs and with impacts that are less than those of a Gunnison Basin import suggest that it must be part of a any sound Front Range water supply strategy. b. Conjunctive use Conjunctive use means the coordinated use of ground and surface water resources. In a nutshell, in "wet" and average years, surface supplies would be tapped to the fullest extent possible, even storing some extra water in groundwater aquifers that would then be available for drought years. Denver's southern suburbs, some of which already rely heavily on groundwater,1' are some of the best candidates for conjunctive use. The Denver Basin aquifer system, although not a source of supply that will last forever, is likely to provide a significant portion of southern suburbs' water sup- ply for many centuries. The Denver Basin Aquifers contain approximately 300 mil- lion AF of drainable water storage."' This volume of water amounts to a reservoir 15 times greater than the active storage of Lake Powell (20 million AF), and 400 times greater than the active storage of Colorado's Blue Mesa Reservoir (750,000 AF).. Gunnison Basin Water • 43