2009-02-10_PERMIT FILE - C1981022 (5)

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Ste- 2.04.7 Most of the bedrock units (including the Bowie and Rollins Sandstones) are relatively fine-grained, is with limited permeability, so infiltration from the unconsolidated surface materials is intercepted by the first relatively impermeable unit, creating a perched water table. Any ground water, which accumulates above the relatively impermeable unit, flows downgradient and laterally along the contact until it either encounters a zone of increased secondary permeability allowing downward percolation to another confining layer, or discharges where the confining layer outcrops at the ground surface. The rate and extent of downgradient and lateral ground water movement is dependent on the relative permeability (primary or secondary) of the unit(s) above the confining layer. In most cases, the volume and rate of any discharge at stratigraphic outcrops is limited and is lost to evaporation and transpiration before it can reach surface drainages. With the limited lateral continuity of most of the bedrock units and the numerous deeply incised drainage valleys intercepting the upper bedrock units and resulting in outcrop exposures, the potential for significant accumulation or flows of perched ground water is limited. These conditions of ground water occurrence and movement are commonly referred to as an unconfined water table aquifer. There are two cases in the immediate mine area where ground water occurrence may be distinct from either the surficial alluvial/colluvial or water table aquifer systems. The first is the D Coal Seam, which is a component of the general water table aquifer system, but which has a distinct recharge mechanism. The second is the Rollins Sandstone which, because of its unique regional continuity and position at the base of the coal-bearing sequence, is considered a key geologic marker bed. Based on its depth, the existence of a sequence of relatively impermeable overlying shales and underlying claystones, the limited amount of contained ground water, the lack of significant ground water movement, and poor ground water quality, the Rollins Sandstone is not considered a ground water aquifer. Existing underground mining disturbance consists of the Somerset and Sanborn Creek Mutes in the B • and C coal sears, and the Hawk's Nest Mine in the D and E coal seams, as shown on the Coal Seam Isopachs, (Maps 2.04-144A, 2.04-M4B, 2.04-M4C, and 2.04-M4D) and Mme Plan Maps, (Map 2.05- M2). The abandoned Somerset Mme workings extend from the east side of Elk Creek to the north and west to Hubbard Creek The active Sanborn Creek Mme workings extend from east of Sanborn Creek to the north and east through Hawk's Nest Creek In the Elk Creek Mme, the D Coal Seam will be mined from the east side of Elk Creek to the north and west to Hubbard Creek Hstorical, existing, and proposed mining operations either have or have the potential to directly affect those stratigrapbic units above the B coal seam in areas where mining extraction or mining-related subsidence occur. No known aquifers exist below the mineable coal sequence. Potentially Affected Ground Water Units and Ground Water Aquifer Characteristics - The hydrogeologic conditions limit ground water occurrence and movement to the following primary ground water aquifer systems: • Alluvial/Colluvial Aquifer System • Perched Ground Water System • Stored Mme Water System These aquifer systems are individually described in the following sections. It should be noted that the Rollins Sandstone had previously been referenced as an aquifer in this area, however, a number of factors including ongoing monitoring by several of the mines in the area have resulted in a determination that while the Rollins Sandstone does contain connate ground water and limited ground water in post-depositional fractures, it does not function as an aquifer. The Rollins Sandstone is the only regionally extensive and laterally continuous sedimentary bedrock unit in the general area. • It lies 40 to 50 feet below the B coal seam, is approximately 120 to 200 feet thick, and is composed of massive cliff-forming, locally cross-bedded, medium- to coarse-grained buff to white Sandstone. The Rollins Sandstone outcrops in the channel of the North Fork of the Gunnison River west of the Town of Somerset, as shown on the Regional Geology/Geology Map, (Map 2.04-M2, Rollins PR04 2.04-20 Revised May 2001