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~ Hydrologic Data Interpretation and Impact Assessment for the Permit and Adjacent Areas ce::.asx~x .. .:x m~~.,ar~::wcr~^,zxxxe~~mx.er~^nix::azs;~e.:.xc::sx.~:c~z.a.-s:;racx:ararG~?ma:.,msxsxae • This section focuses on three areas. First, it analyz- es the hydrologic data gathered during water year 1990. Second, it assesses impacts of mining on the quality and quantity of water in surface streams, springs, groundwater aquifers, mine inflows, and mine discharges during water year 1990. Lastly, it estimates potential impacts of mining on these waters during water year 1991. Surfacewater, 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. Because mine water and groundwater are so interrelated, they are discussed together. Assessment of Mine Water and Groundwater Mine Water The underground room and pillar mining methods in use at the Mt. Gunnison No. 1 Mine can affect groundwater in essentially two ways. First, dcvclop- mcm mining in areas where groundwater is present in or near the coal seam can cause localized movement of this water into the mine workings. To dart, groundwater movements of this type have occurred in areas of low cover which are highly fractured (near outcrops and near stream channels) and where sandstone channels bearing water immediately overlie the coal seam. Secondly, retreat mining can cause subsidence and the Fracturing of overlying groundwater formations. Again, water usually moves into the mine work- ings. At the Mt. Gunnison No. 1 Mine, the perme- ability 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 1990, 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 root in areas of low overburden and near the "F' seam outcrop. 1n0ow lotations were essentially the same as previous years and arc shown on Exhib- it 11. The estimated Oows at these locations arc 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 1990 water year. The seasonal pattern of mine inflows observed at the main portal area and along Sylvester Gulch indicatesthe colluvium,coal,and bedrock recharge locally. 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 Cracture zones and not by the extent of the underground workings. Mine water inflows for the 1991 water year in the Sylvester Gulch and the main portal areas arc expected to be similar to those of the 1989 and 1990 water years. Ijowcvcr, precipitation could have an impact on inflow rotes if there is a dramat- ic change in 1991. Although specific data are not available, precipitation during 1990 is assumed to have been similar to that in 1988 and 1989. Mine water inflows for other areas oC the mine are related to particular geologic and topographic features as well as water-bearing (ormations. These areas include the 1st West Submain, Panels 1W1S, 1W2S, 1W3S, 1W4S, IW~S, 1WGS, 1W1N,2WIN, the 1st East Submain, and the Northwest Sub- mains. Because of these local features, each area is discussed separately. 1st West Submain The 1st West Submain was advanced one crosscut during 1990. Then it was retreat mined to the last entry of the 1WSS panel. The new area was rela- •