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West Elk Mine • 2.04.7 Hydrology Description The purpose of this section is to provide a broad overview of surface water and groundwater hydrology for the permit azea. Important subjects relative to hydrology aze also addressed in other sections including: Geology (2.04.6), Climatology (2.04.8), Soils (2.04.9), and Vegetation (2.04.10). The reader is urged to review these sections. For information pertaining to alternate water supply, refer to Section 2.05.6. To complete specialized reseazch and evaluation, and devote particular attention to hydrologic and subsidence phenomena resulting from mining, including the probable hydrologic consequences of mining, MCC retained Wright Water Engineers, Inc. (WWE) of Denver, and Glenwood Springs, Colorado. WWE has 40 years of experience on diverse water resource assignments. For this assignment, W WE has utilized the following experts or advisors: Mr. C. Richard Dunnrd, P.E., subsidence and hydrologic impacts from subsidence Mr. John Rold, P.G., geology, hydrogeology, and natural hazards Mr. Emest Pemberton, P.E., sediment transport and fluvial morphology 2.04.7(1) Groundwater Information West Elk Mine has historically been considered a dry mine. Previous studies by MCC and its • consultants have shown that groundwater inflows encountered within the mine workings were associated with perched conditions within the Upper and Lower Coal Members of the Mesaverde Formation. These studies concluded that there was no stratigraphic unit above the Rollins Sandstone that had the stratigraphic continuity or water-yielding capacity to be considered a potential regional aquifer (see Figures SA, SB, SC, SD, SE, and SF in map pockets). See Section 2.04.6 (Geology Description) for a detailed discussion of the geologic units associated with, and in close proximity to, the West Elk Mine workings. See also Exhibits 17A, 18, and 18B for additional discussions relative to groundwater conditions and relationships. Within West Elk Mine, groundwater inflows have manifest themselves as roof inflows from sandstone channels located in the lower portion of the Mesaverde Formation, floor inflows associated with the underlying sandstone unit, rib/roof inflows associated with fractures storing finite volumes of groundwater, and, as manifest in early 1996, damage zones associated with fault systems. Conceptual groundwater flow is shown on Figure 7F. In early March 1996, B Seam development mining of the B East Mains near the Northeast Panels intersected a SE-NE trending fault system (B East Mains (BEM) Fault) which initially produced about 500 gpm. This same fault was subsequently crossed by mining several times with observed groundwater inflows reaching as much as 2,500 gpm. Each progressive mining intersect of the BEM Fault was generally down-dip from the previous one resulting in a relatively large initial inflow that diminished over time. Each new mining intersect generally resulted in the loss of inflow from the previous mine crossing of • the BEM Fault. 2.04 -5! Revised November 1004 PRIO