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<br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br /> <br /> <br />Artificial Recharge of Ground Water in Colorado <br />A Statewide Assessment <br /> <br />Non-Aquifer Geo101!ic Stora1!e Options <br /> <br />There are several types of unconventional, non-aquifer geologic storage options. This study <br />considers these storage options as a specific type of AR, called underground water storage. <br />Underground water storage is the storage of water beneath the ground surface in natural or <br />human excavated voids such as mines or caves. Strictly speaking, caves are generally part of <br />larger carbonate rock (limestone, dolomite) aquifers, but are treated separately here because of <br />the potential for open void space that could be artificially recharged. <br /> <br />Abandoned Coal Mines <br />A literature review was conducted of existing water storage and recovery projects in abandoned <br />coal mines to evaluate the storage potential ofthis media in Colorado. Nationally, coal mines are <br />currently being utilized for water storage with most of the active projects located in the Central <br />Appalachia Coal Basin. In Central Appalachia, abandoned coal mines are being used to supply <br />selected municipalities in West Virginia, Kentucky, and Ohio. All of these water-supply projects <br />use mines that are naturally recharged rather than artificially injected. <br /> <br />Successful water storage deployment in coal mines is very dependent upon the hydrogeologic <br />characteristics of the mine environment and surrounding host rock, and geochemical interactions <br />that influence water quality. Roof collapse and associated surface subsidence represent <br />additional safety and logistical concerns. The quantity of water available for extraction from <br />abandoned underground coal mines is dependent on the mined-out void space and rate of natural <br />ground water recharge into the mine. In the Appalachia region, many public water suppliers <br />have experienced difficulties in obtaining dependable water supplies from coal mines because of <br />erratic fluctuations in the quantity and chemical quality of the water (Ferrell, 1992). Coal mines <br />respond differently to pumping and injection than do natural ground-water systems. Recharge <br />rates vary according to local climatic conditions, proximity and elevation of adjacent stream <br />courses, lithologic variations in the overlying rocks, the amount and depth of fracturing in those <br />rocks, and the watershed area (thus, infiltration potential) draining into the mine. The <br />Appalachia experience indicates that the total volume of stored water cannot be recovered. Mine <br />configuration, roof collapse, and leakage from the mine limits the amount of stored water. <br /> <br />Colorado contains eight major coal regions (Figure VII-4). In terms of abandoned coal mines, <br />Colorado has over 1,700 locations (Carroll and Bauer, 2002); 1,430 of these are underground <br />mines. They vary in size, but only 141 are considered large mines; that is, having produced more <br />than I million tons of coal. These large mines, as well as hydraulically connected smaller mines, <br />are considered the best candidates for significant underground water storage projects in <br />Colorado. <br /> <br />Based on the distribution of the large mines, a total of eleven potential storage sites were <br />identified for Colorado (Figure VIJ-4). More than 650 million tons of coal was produced from <br />these sites (Table VII-2). To convert tons of coal produced to an equivalent mine volume in <br />acre-feet, the following relationship was used: <br /> <br />Coal density (in tonslcu ft) x tons of coal produced I 43,560 cu ft I ac-ft <br /> <br />61 <br />