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Sa7rcvr - 2.04.7 <br />Stored Mme Water System -Historical and ongoing coal mining in the area has resulted in extensive <br />• underground mine workings. In those areas where the mine workings are associated with perched <br />ground water units or where mine subsidence results in fractures connecting to perched ground water <br />occurrences, ground water inflow has occurred or is occurring, resulting in some accumulation of <br />stored ground water in the mine workings. Both historic and potential future contributions to mine <br />inflow from either surface water flows in the North Fork of the Gunnison River or the associated <br />alluviaUcolluvial system are considered neghgible. The historic Hawk's Nest Mute and the Elk Creek <br />Mme workings are or will be well above the present level of the river. The Somerset Mine and <br />Sanborn Creek Mine workings, while partially below river level, are in coal seams which are <br />stratigraphically above river level and laterally separated from the margins of both the river and the <br />associated alluviaVcolluvial deposits. <br />Historic mine inflow rates have been low to moderate, as discussed in the previous section, ranging <br />from 0 to 400 gpm Based on monitoring of flooding of the abandoned B-Seam workings in the <br />Somerset Mine (Wells B-6 and H-10), the rate of flooding is relatively slow, occurring over a period of <br />5 to 10 years as illustrated by F~s 2.04.7-F5; Water Ixvel Hydrograph ,Somerset Mute - B Coal <br />Seam (Well B-6), and 2.04.7-F6, Water Level Hydrograp6 ,Somerset Mine - B Coal Seam (Well H- <br />10). With the constraints of existing hydrologic connections between the mine workings and <br />variations in site geology, flooding generally progresses until a state of dynamic equilibrium is reached <br />and a stable water table u restored. Flooding of the abandoned workings h neither complete nor <br />uniform, as evidenced by differences in equilibrium water level elevations between the referenced <br />monitoring wells (Well B-6 at elevation 6,050' and Well H 10 at elevation 6,068). <br />Water levek in the abandoned Somerset Mme B-Seam workings have stabilized at an elevation well <br />below any of the mine openings and there has been no mine water discharge from the Somerset Mtne <br />nor is there expected to be any in the future. The only mine water discharge to date has been minor <br />• flow from Spring SP-8 (Oliver Mme). Historic flows from this spring averaged 12gpm and likely <br />reflect discharge from the abandoned D- and E-Seam mine workings. <br />Based on an estimated average ground water infihration and recharge rate of 0.6 inches per yt az and <br />an area of approximately 9.9 square miles of existing and proposed mine workings, the m=*+mtun <br />potential amount of ground water which could be intercepted by the mine workings (coriservatively <br />assumes that all infiltration u captured by mine) annually a estimated at 315 acre-feet. This quantity <br />of water a relatively small when compared to both surface runoff/evapotranspirauon of <br />approximately 8,600 acre-feet annually for the corresponding surface watershed and m~rm„m <br />infiltration of approximately 40,000 acre-feet (0.6 inches and 1,150 square miles) for the <br />encompassing ground water basin. Some of the mine water which may potentially be stored in the <br />upper mine workings (Hawk's Nest and Oliver Mmes - D and E seams) may be drained by mining <br />and related subsidence in the B and C seams by the Sanborn Creek Mme. Potential mine water <br />inflows and drainage and the proposed related control and mitigation measures, are addressed by a <br />detailed repon titled, "Water Communication Potential From Overlying Workings at the Sanborn <br />Creek Mme' (Abel, 1992), included in Exhibit 2.04-E4, Hydrologic Information. <br />During attive muting operations, mine water inflows will generally be collected and routed to <br />underground sumps. To the extent possible, water which collecu in the sumps will be utilized to <br />meet operational mine water needs for dust control and other purposes. Any excess mine water will <br />be treated, if necessary, to meet applicable effluent standards and discharged to surface drainages. <br />Generally, following completion of mining, it u anticipated that ground water inflows to the mine <br />workings will continue until equilibrium is established between inflows, any discharge or ground water <br />recharge, and the associated perched water table(s). <br />. Ground Water Quality - Ground water quality for the mine and adjacent areas u highly variable <br />dependent on the recharge source, flow path, strata with which the water comes in contact, and <br />discharge mechanism Generally, water quality for alluvia]/colluvial ground water is very sirrtilar to <br />the surface water quality of the associated surface water source due to close surface and ground water <br />interaction and limited residence time. Water quality for most shallow perched ground water sources <br />PR04 2.04-37 Revised August 2000 <br />