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~J HYDROLOGIC DATA INTERPRETATION AND IMPACT ASSESSMENT FOR THE PERMIT AREA AND ADJACENT AREAS • I~ L Y:A:X'R:'AC~.J.CV:T:'.G:`.y.'~'."~x YY..'..r..;o,..`^~::'.. X".S.w.~':5::`Sf:1:A:Y6n.M.I.:SY~Y'.C..^...:rt':4.:`Y~:::i'.'S`.y":C,^.'C.3.':2:acn:/M93?R:`).~C5:6~yvmx;:.:m:;;~.;C"S:.'^`~f.^.~)W.9.C:W:C~b".A:.p'.'.MY This section contains an analysis of the hydrologic data gathered during water year 1988; an assessment of impacts of mining on the quality and quantity of water in surface streams, springs, groundwater aquifers, mine inflows, and discharges during water year 1988; and an estimate of poten- tial impacts of mining on these waters during water year 1989. Surface water, springs, and groundwater are considered separately even though they are inter- related. The report attempts to show the inter- relationships 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 Generally, underground room and pillar mining methods, which are currently in use at the Mt. Gunnison No. 1 Mine, can affect groundwater in essentially two ways. First, development mining in areas where groundwater is present in or im- mediately adjacent to the coal seam can cause 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 fractured (near outcrops and near stream channels) and where sandstone channels bearing water immediately overlie the coal seam. Secondly, retreat mining, which causes subsidence, can fracture overlying groundwater formations causing movement o[ the water, usually into the mine workings. At the Mt. Gunnison No. 1 Mine, the permeability of the "F' seam and overlying strata is very low and the areal extent and quan- tity of water stored in these units are small. The mine workings located near the outcrop, the main portal area, and along Sylvester Gulch behaved similarly to previous years with respect to inflows. Inflows occurred primarily from the roof in areas of low overburden and near the "F' seam outcrop. Inflow locations are essentially the same as previous years and are shown on Exhibit If. The estimated flows at these locations are also shown on Exhibit II for low flow conditions as well as spring runoff conditions. Table 5 shows total estimated sustained mine inflows for the 1988 water year. The seasonal pattern of mine inflows observed in the mine workings at the main portal area and along Sylvester Gulch indicate that the 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 overburden, and location and extent of fracture zones and not by the extent of the underground workings. 9 Mine water inflows Cor the 1989 water year in the Sylvester Gulch area and the main portal area are expected to be similar to those of the 1987 and 1988 water years; however, precipitation could have an impact on inflow rates i[ there is a dramatic change in 1989. Mine water inflows for other areas of the mine, namely the ls[ West Submain, Panels 1W2S, 1W3S, 1W4S, 1WSS, 1W6S, and the 1st East Submain are related to particular geo- logic and topographic features as well as water bearing formations in these areas. Consequently, each of these areas is discussed separately.