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_21_ All of these coal mines have life-ofanine areas within a 247 square mite subwatershed of the 978 square mile North Fork of the Gunnison River drainage basin (Figure 2), and have been considered as the mines in the general area for this study. All mining operations in the general area are underground operations with the exception of Terror Creek Loadout which is strictly a loading facility. The life-of arsine areas of these mines are plotted on Figure 2. This assessment is divided into the three major subsections: Description of the I{ydrologic Environment, Probable 1{ydrologic Consequences of the Orchard Valley Mine, and Cumulative F{ydrologic Impact Assessment. DESCRIPTION OF THE HYDROLOGIC ENVIRONMENT The first part of this assessment describes the hydrology of the drainage basin of the North Fork of the Gunnison River and the mining activity occurring in the basin. This includes: 1) a description of the regional geology; 2) a description of the ground water regime; 3) a description of the surface water regime; and 4) a discussion of precipitation and runoff in the basin. REGIONAL GEOLOGY The Somerset Coal Field lies on the southeast margin of the Piceance Basin, and just south of Grand Mesa. The sedimentary strata exposed in the Somerset Coal Field dip at 3 to 5 degrees to the north and northeast, and range in age from late Cretaceous to early Tertiary. Coal is produced from the Mesa Verde formation, a 2500-foot-thick sequence of sandstone, shale and coals overlain by the Ohio Creek conglanerate and underlain by the Mancos Shale (Figure 3). The Mesa Verde formation is composed of four members in order of decreasing age--the Rollins Sandstone, the Lower and Upper Coal members and the Barren member. Only minor faulting of limited vertical displacement has been observed in the existing Blue Ribbon, Bear and Hawk's Nest Mines. However, in the Orchard Valley Mine, a fault with a displacement of fifty (50) feet was encountered during mining and drill hole data indicates the presence of other faults in the life-of arsine area with similar displacements. One major fault has been encountered in the Somerset Mine. The faults which have been encountered in existing mines tend to be high angle normal faults. The steep slopes of the stream valleys and the instability of the rock strata in the North Fork drainage basin have contributed to numerous landslides, mud flows and rock falls. These mass wasting features have been mapped by W.R. Junge of the Colorado Geological Survey and published as an open file report, entitled "Geologic Hazards, North Fork Gunnison River Valley, Delta and Gunnison Counties, Colorado". Geologic unit exposed in the North Fork Drainage Basin consist of Late Cretaceous to Early Tertiary Age sedimentary strata, Tertiary Age igneous intrusives, and puaternary Age alluvial and colluvial deposits. The units of