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<br />Artificial Recharge of Ground Water in Colorado <br />A Statewide Assessment <br /> <br />Artificial recharge technologies are broadly grouped according to whether water is recharged at <br />the surface or underground, and then by whether water is recharged into the unsaturated zone or <br />directly into the saturated zone of the aquifer. <br />. Surface infiltration is the impoundment of water at the ground surface for the purpose of <br />infiltration to the underlying near-surface, unconfined aquifer. <br />. Subsurface infiltration is the application of water below the ground surface for <br />infiltration to the underlying unconfined aquifer. <br />. Direct injection differs from infiltration systems by recharging water directly into the <br />saturated zone of the aquifer. <br />. Aquifer storage and recovery (ASR) wells are wells through which water is injected into <br />aquifer storage during times oflow demand and high surface-water supply and <br />subsequently recovered by pumping at a later date when demand exceeds surface supply. <br />. Modification of natural recharge involves man-made changes to the land surface or <br />hydrogeologic conditions to increase the amount of recharge from natural and local <br />sources. <br />. Underground (non-aquifer) water storage technologies apply to storage and retrieval of <br />water in natural or manmade voids in the subsurface, such as abandoned mines or natural <br />caverns. <br />The selection of a particular technology requires detailed site investigation and depends on the <br />hydrogeologic setting of the target aquifer, land availability and uses, and the project objectives. <br /> <br />Artificial recharge is being used in at least 32 states in the U.S. and at least 26 countries <br />worldwide. The methods used span the entire spectrum of known technologies, but the dominant <br />methods are injection wells and infiltration basins. The larger scale projects are generally located <br />in drier areas of the U.S. (i.e., the west and southwest), or areas in which the growing population <br />has overtaxed the available water supply (e.g., California, Florida, New Jersey, New York). <br /> <br />An inventory of artificial recharge projects within Colorado identified 19 active operations <br />including <br />. augmentation in the lower South Platte River basin, <br />. seasonal storage as part of conjunctive use of ground water and surface water in the San <br />Luis Valley, <br />. direct injection by two water districts in the Denver Basin, and <br />. regulation of water supply and water quality at several smaller municipal water systems. <br /> <br />The occurrence and distribution of Colorado's water resources are inherently linked to the state's <br />geography and underlying geology. As a result of Colorado's complex geology, a multitude of <br />aquifers in various areas of the state are suitable for artificial recharge projects. The geologic <br />units containing these aquifers can be broadly classified as unconsolidated sediments, poorly <br />consolidated sediments, or consolidated rock. The amount of storage available in an aquifer is <br />dependent upon the aquifer's (I) storage coefficient (storage ability), (2) areal extent, and (3) <br />freeboard (amount the water level could rise above present water level). In general, unconfined <br />aquifers have smaller areal extent, tens of feet of freeboard, and a high storage coefficient. <br />Confined aquifers, on the other hand, often have a large areal extent and hundreds of feet of <br />available freeboard, but a very low storage coefficient. <br /> <br />11 <br />