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MWSI Project Phase II Conjunctive Use Summary Report - DRAFT August 22, 1995 require higher pumping costs due to well interference effects, If a conjunctive use plan if pursued, creative planning efforts will be needed to minimize these concerns through well field locations, service area selection and land use planning techniques, Surface water supplies for a conjunctive use plan could be obtained from existing municipal water supply systems or from new surface water development. For the purpose of developing a conceptual example of conjunctive use, this study has focused on supplies available from existing systems and on increasing the use of those supplies, Conjunctive Use Plan Formulation Options Within the concept of conjunctive use, there are certain basic options for formulating a conjunctive use plan which relate to I) diversion of surface supplies and use of surface reservoirs, 2) recharge of aquifers, and 3) ultimate use of the additional water supplies developed, These options are presented below, Diversion of Surface Supplies and Use of Surface Reservoirs Conjunctive use plans can vary in the manner in which they divert surface supplies, In the most conservative approach, surface supplies for conjunctive use would be diverted only during periods of high river flows, These surface supplies would be used directly or would be stored locally to meet ongoing municipal demands and/or for recharge, as shown in Figure I. As a more aggressive option, the conjunctive use plan could utilize divertible surface supplies whenever available and could "borrow" additional water supplies from surface water storage, within certain limitations, While this would result in increased drawdowns of surface reservoirs, these drawdowns would be compensated for either through increased capture of storable flows during subsequent runoff seasons or through groundwater pumping in times of drought, thereby ensuring "payback" as depicted in Figure 2. This can be a very significant aspect of a conjunctive use plan since allowing this drawdown can greatly increase the efficiency of storage in capturing storable surface water supplies, Recharge of Aquifers In either case, a choice exists with regard to the recharge of the groundwater aquifers. Without recharge, conjunctive use can help to extend the life of the aquifer system and possibly increase system yields, however long term depletion of aquifers would be unavoidable and any new yield generated would have a finite life. In a conjunctive use plan which includes recharge, the groundwater system can act as an underground storage reservoir for divertible surface supplies, These stored divertible surface supplies could then be used to I) generate new firm yield or to 2) reduce or even eliminate depletion of the aquifers caused by existing groundwater use, or 3) a combination of both. This scheme is depicted in Figure 3. 30