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MWSI Project Phase" Conjunctive Use Summary Report - DRAFT August 22, 1995 controlled by a maximum volume limit or by minimum storage requirements in the surface water storage system, Whenever borrowing reaches the limit or the surface storage system reaches its minimum, borrowing of surface water would be curtailed and groundwater would be relied upon. Requirements for payback of borrowed water in a timely manner with respect to droughtevents would also be developed, along with rules regarding canceling of borrowing debts if the surface system subsequently fills, The operational plan for existing surface supply reservoirs is a key component of a conjunctive use system that allows borrowing. The operational plan should be developed by detailed modeling of the surface supply system with borrowing limits, minimum reservoir storage and groundwater payback rates. The ability to borrow water and to modify operation plans would be dependent on many factors such as water rights concerns, agreements on operations, and environmental issues including stream depletions and recreational reservoir levels. Groundwater Wel!field , An extensive deep groundwater well field into the Denver Basin would be a major infrastructure component of any conjunctive use plan, Such a well field, together with collection pipelines and water treatment, would be required to deliver groundwater in the event of a shortage of supply from surface water sources. The well field, including the number of wells and spacing, would be developed to allow discharge of the needed volume at a constant rate throughout the year, Well field production would need to be sufficient to meet annual demands under a conjunctive use plan and to payback, when needed, any water supplies borrowed from the surfaee supply system, The well field required for a conjunctive use plan may be a series of new wells or may utilize existing well field infrastructure. Under a conjunctive use plan the well field may not be significantly used for several years but then may be pumped heavily for a period of 2 to 4 years, Local Peaking Reservoir In addition to the major surface supply reservoirs, there would be a need for a local peaking reservoir or reservoirs as a component of the system to meet conjunctive use demands. The primary purpose of the peaking reservoir would be to maximize the efficiency of well use in the groundwater system, Wells would be most efficient pumping at a constant rate year round with winter discharges to the reservoir and summer deliveries from the reservoir to meet seasonal peak demands. This reservoir would be used only as a well peaking reservoir, filling in the winter and being drawn down in the summer without any planned carryover storage, In addition, during times of delivery from the surface water system, the local reservoir would be used to store surface water deliveries, This provides for additional capture of surface supplies while at the same time providing storage for summer peaking and groundwater recharge. 34