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APPENDIX 1 <br /> Managed Groundwater Recharge. Colorado's water-related contribution to the Proposed <br /> Program will come from the development of new managed groundwater recharge facilities and <br /> the utilization of existing groundwater recharge facilities that will recharge and re-regulate <br /> excess water supplies that are physically and legally in excess of existing and future water <br /> demands in Colorado within the lower reaches of the South Platte River pursuant to Colorado <br /> water law and administrative constraints. <br /> Because of the underlying physical characteristics of the South Platte River alluvial aquifer, <br /> water infiltrating into the soil surface and not consumed by evaporation or transpiration by plants <br /> eventually makes its way into the underlying alluvium. The alluvium underlying the majority of <br /> the South Platte River is hydraulically connected to the river channel. The river channel acts as a <br /> drain for the aquifer once the alluvium is `filled' to an elevation greater than the water surface of <br /> the South Platte River. With recharge, a gradient develops and water within the alluvium begins <br /> to migrate toward, and eventually discharges to or augments,the river flows. The time required <br /> for water to travel from the point of recharge to the river channel is largely a function of the <br /> distance between the recharge facility to the river channel,the ease of which water flows <br /> horizontally through the alluvium(aquifer transmissivity), and the thickness of the saturated <br /> alluvium. Modeling the rate at which recharged water returns to the stream of origin can be <br /> performed by several groundwater models and is outlined in Tab 3A. To maximize the yield <br /> from a recharge facility, it should be positioned such that the water recharged by the facility <br /> during periods of excess appears as return flow or accretion to the river during periods of <br /> shortage. <br /> Employing managed groundwater recharge as a means to develop additional supplies of water <br /> within the South Platte River Basin in Colorado is not a new concept. Since the early 1970's <br /> South Platte tributary well owners have developed and relied heavily upon such water sources to <br /> offset the depletions to the stream system that result from the pumping of their junior water <br /> rights. By using existing diversion structures and canal systems, groundwater recharge is <br /> considered by well owners to be the most cost-effective method to develop additional water <br /> supplies for meeting augmentation and other water supply needs. Typically, an irrigation or <br /> canal company will utilize its canal and nearby natural depressions as recharge facilities. Ideally <br /> the topsoil (material overlying the alluvium) in such areas should have a high infiltration rate. <br /> Companies pursuing recharge projects typically divert water from the South Platte River for <br /> recharge purposes from the late fall through early December and from early March through mid- <br /> June. Divertable river flows throughout these periods are often unconstrained by senior water <br /> right holders or inclement weather(ice). <br /> In addition to utilizing existing canal infrastructure to divert recharge water, some water users <br /> not having access to a canal system have installed pumping plants near the river and pumped the <br /> water via underground pipelines to recharge basins located up to 1.5 miles from the river <br /> 10 <br />