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I I I I I I I I I I I I I I I I I I I A weighted ranking system was established to evaluate the key physical properties of the State's 16 highest-potential unconsolidated aquifers and 29 highest-potential consolidated aquifers. Hydrogeologic parameters taken into account in the "aquifer ranking value" include areal extent, depth, saturated thickness, headfreeboard, storage coefficient, and hydraulic conductivity. In addition to calculating a final ranking for the aquifer, the quality ofthe input data was also assessed. The alluvial deposits ofthe South Platte River, its tributary Bijou Creek, and the Arkansas River are the top three ranked unconsolidated aquifers. The High Plains Aquifer, Dakota-Cheyenne Group of southeast Colorado, and the Denver Basin aquifers are the top three ranked consolidated bedrock aquifers. The evaluation of the available storage capacity in Colorado's highest-potential aquifers was guided by the desire to find opportunities to develop large-scale artificial recharge projects, i.e. defined as having storage capacity in excess of 100,000 acre-feet. Thirteen of the 16 primary unconsolidated rock aquifers have sufficient storage capacity to accommodate a large-scale project. In aggregate, the lower South Platte River alluvium and the San Luis Valley alluvium have the capacity to store in excess of one million acre-feet. All but two of the 26 primary consolidated rock aquifers have sufficient storage capacity available to meet the 100,000 acre- feet criterion. Because of their large areal extent and head freeboard, the majority ofthese aquifers can store millions of acre-feet of water. Three types of non-aquifer underground water storage possibilities were assessed statewide: abandoned coal mines, abandoned metal mines, and caves. Storage of water in abandoned underground coal mines is not a new concept, but has only recently been tried in Colorado; most notably by the City of Arvada at the former Leyden coal mine. Overall, the estimated storage capacities of non-aquifer alternatives are much smaller than those of aquifers. An estimated 55,000 acre-feet of underground water storage is available for artificial recharge in inactive coal mines, statewide. Major technical challenges to water storage projects in coal mines include maintaining hydraulic control of stored water, poor water quality (high salinity), and mine subsidence. The potential water storage volumes for abandoned metal mines and natural cave systems are much smaller than for coal mines. Metal mines and natural caves are not a viable option for water storage because of their limited storage capacity, water quality issues, leakage of stored water, and land ownership issues. Artificial recharge projects can increase the total amount of stored ground water in a very specific and calculated fashion. In addition, indirect or passive methods of ground-water recharge such as vegetation control, storm-water retention basins, and leaky ditches are non- specific in application, but can significantly increase overall ground-water storage. Similar to water conservation measures, some changes in legislation and water facility design and engineering, combined with passive recharge structures, would benefit both ground-water and surface-water resources. This study assesses the best aquifers in Colorado for their artificial recharge potential of ground water based primarily on their hydrogeological suitability. Implementation of an AR project must also consider several other factors, including: 1) project objectives, 2) site-specific hydrogeologic conditions, 3) source water availability, 4) water rights, 5) available land surface area and compatible land-use activities, 6) governing water-management districts or entities, 7) facility design criteria, 8) capital costs to construct, 9) operation and maintenance costs, and 10) general storage efficiency and deliverability. 111