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Salim - 2.04.7 <br />Sandstone is at base of Mesa Verde Formation so outcrop is approximate boundary between Kmv <br />• and Km). <br />Any water that maybe contained in fractured zones of the Rollins Sandstone is isolated from both the <br />water table system and underlying units by overlying and underlying low permeability shale and <br />claystone members. These confining units effectively prevent any upward seepage from the Rollins <br />Sandstone to the B coal seam or overlying units or recharge from the overlying perched ground water <br />system. Monitoring indicates relative poor water quality in the Rollins Sandstone, no significant <br />changes over time in water quality, and little or no ground water movement, therefore, it appears that <br />the limited amount of ground water in the Rollins Sandstone may represent connate ground water in <br />place at the time deposition of this unit occurred or minor amounts of ground water occurring in <br />post-depositional fractures. <br />Alluvial/Colluvial Aquifer System - The alluvial/coUuvial aquifer system consists of shallow ground <br />water contained in alluvial/colluvial deposits associated with the North Fork of the Gunnison River <br />and some of its major perennial and intermittent tributaries. The location and extent of known <br />alluvial/coUuvial deposits is shown on the Regional Geology and Hydrology Map, (Map 2.04-M2). <br />Due to the rugged topography and associated high energy stream environment, the alluvial/colluvial <br />deposits in this area contain significant amounts of relatively coarse material resulting from mass <br />wasting of the adjacent steep slopes. As a result of their relatively coarse texture, these deposits <br />typically have relatively high permeabilities and can store and transmit significant quantities of water <br />as evidenced by downstream well yields of up to 100 gpm. <br />In the tributary drainages, the alluviaVcoUuvial deposits are very limited in thickness and extent, <br />frequently forming a very narrow band within and immediately adjacent to the drainage channel. <br />Because most of the tributary drainage channels are relatively steep, alluvial/colluvial deposits of any <br />is significance are frequently confined to the lower drainage reaches where the drainage valleys flatten <br />and widen near their confluence with the North Fork With limited thickness and extent, the <br />alluviaVcoUuvial deposits associated with the tributary drainages store and transmit relatively minor <br />amounts of ground water. <br />Since the only alluviaVcoUuvial deposits of significance are associated with the North Fork of the <br />Gunnison River and these deposits are generally hydrologically downgradient from the surface and <br />underground disturbance associated with the Somerset, Sanbom Creek, and Elk Creek Mmes, detailed <br />characterization of the alluvial/colluvial aquifer has not been undertaken. It is important to note that <br />the alluviaVcoUuvial aquifer system is closely associated with the surface water system of the North <br />Fork of the Gunnison River. When surface flow levels in the river are elevated due to spring <br />snowmelt within the tributary drainage area, the alluviaVcoRuviai aquifer units adjacent to the river are <br />recharged. As flows in the river decline later in the year due to reduced runoff contributions, the <br />alluvial/colluvial aquifer discharges to the river, effectively contributing to a sustained base flow. <br />Figures 2.04.7-F1, Water Level Hydrograph, Alluvial/Colluvial Ground Water (Well TC-2), and <br />2.04.7-F2, Water Level Hydrograph, Alluvial/Colluvial Ground Water (Well WMW-1), illustrate <br />typical seasonal water level fluctuation for shallow alluvial/colluvial ground water. OMI utilizes an <br />infiltration gallery established in the alluvial/colluvial deposits of the North Fork of the Gunnison <br />River south of the Sanborn Creek Mne portal area as the primary mine water supply source. <br />Documentation of water supply withdrawals indicate a maximum sustained withdrawal rate of <br />approximately 50 gpm, which is below the seasonal low flow rate in the alluvial/colluvial aquifer at the <br />point of withdrawal. <br />Perched Ground Water System - The perched ground water system consists of isolated ground water <br />occurrences associated with lithologic units which are hydraulically connected to a recharge source, <br />• have sufficient permeability to store and transmit ground water, and have an underlying low <br />permeability unit which limits downward ground water migration. Most of the lithologic units within <br />the geologic sequence in the mine and adjacent areas are relatively fine-grained and well cemented, <br />with consequent low horizontal and vertical permeabilities. Perched ground water occurrences are, <br />PR04 2.04-21 Revised August 2000