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MWSI Project Phase" Conjunctive Use Summary Report - DRAFT August 22, 1995 CONJUNCTIVE USE Purpose and Scope The purpose of this report is to summarize the MWSI Project's Phase" level investigations related to conjunctive use as a potential water supply for the metro Denver area, The overall intent of Phase" is to develop background information and to initially identify promising water supply options for further study in later Project phases. The Phase" work specific to conjunctive use has included I) a review of background information on surface water and groundwater availability for conjunctive use, 2) identification of a conceptual approach to conjunctive use as a regional water supply source, 3) examination of some of the major operational and facilities aspects ofthis approach to conjunctive use, and 4) a preliminary assessment of yield and economic issues, The information presented in this document is preliminary and wi}1 be refined in Phase III of the Project according to the direction of the Technical Advisory Committee. No specific conjunctive use plans have yet been developed, nor has the new water supply yield potentially available from conjunctive use been definitively quantified, The example approach to conjunctive use and the yield and cost information presented in this document are for illustrative purposes only, Conceptual Description of Conjunctive Use For purposes of this study, conjunctive use is defined as the coordinated use of surface water and groundwater resources to increase water supply availability, By jointly using surface water and groundwater supplies and systems, the opportunity exists to significantly increase the delivery of renewable surface water supplies, In this study it is assumed that in may years there are divertible surface water supplies in excess of the amount needed to meet demands and to fill existing surface reservoirs. Since these unused divertible supplies are unreliable, they are not diverted for use and are lost from the municipal water supply system, However, if groundwater supplies and aquifer storage are made available to "fill in" during periods when the unused surface supplies are unavailable, then a new firm yield can be developed by using both sources. This, then, serves to maximize the use of renewable surface waters. These additional surface water supplies could be used to extend the life of existing non- tributary groundwater-dependent supplies, To the degree that a conjunctive use plan is designed for this purpose, the potential for "new yield" from that conjunctive use plan decreases. At the same time however, the long term operations, maintenance and replacement costs would be lower because of less aquifer decline, 28