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~ Hydrologic Data Interpretation and Impact Assessment for the Permit and Adjacent Areas L J This section focuses on three aeeas. First, it analyz- es the hydrologic data gathered during water year 1991. Second, it assesses impacts of mining on the quality and quantity of water is surface streams, springs, groundwater aquifers, mine inflows, and mine discharges during water yeaz 1991. Lastly, it estimates potential impacts of mining on these waters during water year 1992 Surtacewater, springs, and groundwater are consid- ered separately even though they are interrelated. The report attempts to show the interrelationships that are significant or could be expected. Beptise mine water and groundwater are so interrelated, they are discussed together. Assessment of Mine Water and Groundwater Mine Wafer The underground mining methods in use at the West Elk Mine can affect groundwater in essential- ly two ways. First, development mining in areas where groundwater is present in or near the coal seam can pose localized movement of this water into the mine workings. To date, groundwater movements of this type have occurred in areas of low cover which are highly fraaured (near out- crops and near stream channels) and where sandstone channels bearing water immediately overlie the coal rpm. Secondly, retreat mining pn pose subsidence and the fracturing of overlying groundwater formations. Again, water usually moves into the mine workings. At the West Elk Mine, the permpbiliry of the "F" seam and overlying strata is very low and the areal extent and quantity of water stored in these units are small. During 1991, the mine workings located near the outcrop, the main portal area, and along Sylvester Gulch behaved similarly to previous years, relative to inflows. Inflows occurred primarily from the roof in areas of low overburden and near the "F" seam outcrop. Inflow locations were essentially the same as previous years and are shown on Exhib- it IL The estimated flows at these locations are also shown on Exhibit II. Separate estimates are provided for low flow conditions and spring runoff conditions. Table 6 shows total estimated sustained mine inflows for the 1991 water year. The seasonal pattern of mine inflows observed at the main portal area and along Sylvester Gulch indicates the colluvium, coal, and bedrock recharge loplly. The area along Sylvester Gulch contains many naturally occurring fractures. Water moves down slope through the colluvium and fracture zones and discharges as springs and seeps. Water inflows appear to be controlled by the amount of precipitation (mostly snowmelt), depth of overbur- den, and location and extent of fracture zones and not by the extent of the underground workings. Mine water inflows for the 1992 water year in the Sylvester Gulch and the main portal areas are expected to be similar to those of the 1990 and 1991 water years. However, precipitation could have an impatx on inflow rates if there is a dramat- ic change in 1992. Data from NOAA's Somerset lE Station and Colorado Climate Report indipte precipitation during 1991 was similar to that in 1989 and 1990 (averaging 21 inches/month). Mine water inflows for other areas of the mine are related to particular geologic and topographic features as well as water-bearing formations. In the "F" seam, these areas include the 1st West Submain, Panels 1W1S, 1W2S, 1W3S, 1W4S, 1 WSS, 1 W6S,1 W 1N, 2W 1 N, the Ist East Submain, the 1 Northwest and 2 Northwest Submains, and Panels NW1S and NW2S. In the "B" seam, the areas include the West Mains. Bepuse of these lopl features, each area is discussed separately. •