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Salim - 2.04.7 Sandstone is at base of Mesa Verde Formation so outcrop u approximate boundary between Kmv • and Km). Any water that maybe contained in fractured zones of the Rollins Sandstone u isolated from both the water table system and underlying units by overlying and underlying low permeabihty shale and claystone members. These confining units effectively prevent any upward seepage from the Rollins Sandstone to the B coal seam or overlying units or recharge from the overlying perched ground water system Monitoring indicates relative poor water quality in the Rollins Sandstone, no significant changes over time in water quality, and little or no ground water movement, therefore, it appears that the limited amount of ground water in the Rollins Sandstone may represent connate ground water in place at the time deposition of this unit occurred or minor amounts of ground water occurring in post-depositional fractures. Alluvial/Colluvial Aquifer System -The alluvial/colluvial aquifer system consists of shallow ground water contained in alluvial/colluvial deposits associated with the North Fork of the Gunnison River and some of its major perennial and intermittent tributaries. The location and extent of known alluviaVcolluvial deposits u shown on the Regional Geology and Hydrology Map, (Map 2.04-M2). Due to the rugged topography and associated high energy stream environment, the alluvial/colluvial deposiu in this area contain significant amounts of relatively coarse material resulting from mass wasting of the adjacent steep slopes. As a resuh of their relatively coarse texture, these deposits typically have relatively high permeabilities and can store and transmit significant quantities of water az evidenced by dowtutream well yields of up to 100 gpm. In the tributary drainages, the alluvial/colluvial deposiu are very limited in thickness and extent, frequently forming a very narrow band within and imrnediately adjacent to the drainage channel. Because most of the tributary drainage channek are relatively steep, alluvial/colluvial deposiu of any • significance are frequently confined to the lower drainage reaches where the drainage valleys flatten and widen near their confluence with the North Fork With limited thickness and extent, the alluviaVcolluvial deposits associated with the tributary drainages store and transmit relatively minor amounu of ground water. Since the only alluvial/colluvial deposits of significance are associated with the North Fork of the Gunnison River and these deposiu ate generally hydrologically downgradient from the surface and underground disturbance azsociated with the Somerset, Sanbom Creek, and Elk Creek Mutes, detailed characterization of the alluvial/colluvial aquifer haz not been undertaken. It is irnportant to note that the alluvial/colluvial aquifer system u closely associated with the surface water system of the North Fork of the Gunnison River. When surface flow levels in the river are elevated due to spring snowmelt within the tributary drainage area, the alluviaVcolluvial aquifer uniu adjacent to the river are recharged. As flows in the river decline later in the y>:ar due to reduced runoff contributions, the alluviaVcolluvial aquifer discharges to the river, effectively contributing to a sustained base flow. Figures 2.04.7-F1, Water Level Hydrograph, AlluviaVColluvial Ground Water (Well TG2), and 2.04.7-F2, Water Level Hydrograph, Alluvial/Colluvial Ground Water (Well WMW-1), illustrate typical seazortal water level fluctuation for shallow alluvial/colluvial ground water. OMI utilizes an infiltration gallery established in the alluvial/colluvial deposiu of the North Fork of the Gunnison River south of the Sanbom Creek Mtne portal area az the primary mine water supply source. Documentation of water supply withdrawak indicate a maximum sustained withdrawal rate of approximately 50 gpm, which is below the seazonal low flow rate in the alluviaVcolluvial aquifer at the point of withdrawal. Perched Ground Water System -The perched ground water system consists of isolated ground water occurrences associated with lithologic uniu which are hydraulically connected to a recharge source, • have sufficient permeability to store and transmit ground water, and have an underlying low permeability unit which limiu downward ground water migration. Most of the lithologic units within the geologic sequence in the mine and adjacent areas ate relatively fine-grained and well cemented, with consequent low horizontal and vertical petmeabilities. Perched ground water occurrences are, PR04 2.04-21 Revised August 2000