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West Elk Mine <br />0 Regional Groundwater Conditions <br />For the most part, the Mesaverde Formation is dry or the rock units are of such low permeability <br />that they yield insufficient water for sustained use. Even the extensive Rollins Sandstone, a thick <br />basal sandstone immediately above the Mancos Shale, has been found to be highly cemented and a <br />low water producer. Due to its areal extent, the Rollins Sandstone was thought to be a potential <br />aquifer of regional importance, but the Rollins Sandstone has been found to have insufficient yield <br />to sustain groundwater supply wells (Bowie No.l NW). In addition, petrographic analysis <br />revealed that the sandstone units within the Rollins Sandstone are depositionally immature and <br />have been designated as a "tight gas sand" by the Colorado Oil and Gas Conservation Connnission <br />(Mayo and Associates, 1998 and 1999). <br />Furtherniore, groundwater monitoring wells completed in the Rollins Sandstone do not provide <br />predictable information regarding Rollins Sandstone groundwater conditions, as determined by <br />Mayo and Associates during their investigation of fault-related groundwater inflows from the <br />Rollins Sandstone at West Elk Mine (1998). In that study, and their hydrogeologic study completed <br />in 1999, it was determined that the groundwater systems associated with the Rollins Sandstone are <br />not areally extensive and not in hydrodynamic communication with each other. For these reasons, <br />MCC does not plan to continue monitoring or complete any additional monitoring wells in the <br />Rollins Sandstone. <br />• Most of the stratigraphic members of the Mesaverde Formation above the Rollins Sandstone <br />outcrop within close proximity to West Elk Mine. Up-dip B Seam outcrops occur 0.6 to 4 miles <br />from underground workings. Most workings intercept the B Seam within 2 miles of the up-dip <br />outcrop. Existing and potential mining areas in the overlying Upper Coal Member have similar <br />distances to the up-dip outcrop. The Lower Coal Member (i.e., B Seam) extends down dip beneath <br />the Gunnison River and does not crop out down-dip in the permit area. Down-dip outcroppings of <br />rock units from the Bowie Sandstone and higher occur along the Gunnison River and in Sylvester <br />Gulch in the mine permit area. Up-dip outcrops of rocks units associated with mineable coal tend to <br />occur on steep slopes and thus have limited potential for groundwater recharge from the active zone. <br />The stratigraplnic sequence, observed spring locations and topography suggests groundwater <br />recharge is from snowmelt and precipitation and discharges mainly from bedrock and colluvial <br />springs on steeper slopes above the major drainages and is restricted to active zone groundwater <br />systems. A small amount of water infiltrates downward through the bedrock units, while most <br />moves laterally on top of relatively impermeable rock units, discharging where more permeable <br />units outcrop/subcrop. <br />Sandstone units within the Mesaverde Formation are generally lenticular, regionally discontinuous <br />and are usually highly cemented. In general, the only extensive units are the persistent coal seams, <br />especially the B Seam, and the Rollins Sandstone. The dip of these fonmations is to the northeast at <br />generally less than 5 degrees. <br />0 <br />2 04 -54 Remised June 2005 PRIO, March 2006; Rev. April 2006 PRIO, Sep. 2007PR12