2013-01-29_PERMIT FILE - C1981022A (3)

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SetYiae - 2.05.6 water quality because the flow path between the recharge source and the discharge point is typically • relatively short, resulting in limited water quality degradation. Perched ground water and stored mine water which haz been in prolonged contact with shales and mudstones may exhibit elevated TDS levek due to dissolution of soluble ttvnerals and salts. Water quality f or the limited water which exisu in the Rollins sandstone is generally poor, with high levek of dissolved solids and hardness, reflecting very low transmissivity, long residence times, and dissolution of soluble minerals in a reducing environment. Ground water occurrence and movement is generally controlled by the semi-arid climate, rugged topography, stratigraphy, and surface drainage patterns. AlluviaUcolluvial deposits in the Nonh Fork Valley and major tributaries are recharged by and discharge to the azsociated flowing drainages with seasonal fluctuations in surface water flow levek. The semi-arid climate limits precipitation and results in relatively high evaporation and transpiration rates, both of which limit infiltration and ground water recharge. The limited infiltration, which does occur, percolates readily through unconsolidated surficial materials until it reaches the underlying bedrock Most of the bedrock units are relatively fine-grained, with limited permeability, so infiltration from the uncorsolidated 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 pemteability (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 amts and the numerous deeply incited drainage valleys intercepting the upper bedrock amts and resulting in outcrop exposures, the potential Eor 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 is distinct from either the surficial alluviaVcolluvial 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 Rollias Sandstone, which because of iu position at the base of the coal-bearing sequence, unique regional continuity, thickness, and exposure is considered a regional marker bed. The D coal seam occurs relatively high in the stratigraphic sequence and outcrops or sub-crops in most of the small tributary drainages on the north side of the North Fork Valley. Because it u high in the sequence and has somewhat higher permeability when compared with the other azsocia[ed units in the sequence, i[ receives recharge from both overlying unconsolidated materials and along outcrop/sub-crop exposures in the small drainages when they are flowing. The Rollins sandstone is the only regionally extensive and laterally continuous bedrock unit in the general area. The Rollins sandstone outcrops in the channel of the North Fork of the Gunnison River west of the Town of Somerset, but this unit contaitu only limited amounts of ground water, which typically is of very poor quality. Bazed on iu relative position in the stratigraphic sequence, its depth, the existence of a sequence of relatively impermeable overlying shales, the limited amount of contained ground water, the lack of significant ground water movement, and poor ground water quality, it appears that ground water occurrence in the Rollins sandstone is isolated from the water table system and may represent comate ground water in place at the time deposition of this unit occurred or motor amounts of ground water occurring in post-depositional fractures. PR04 2.05-76 Revised August 2000