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1 Assessment of impacts typically involves oanparison of data with baseline conditions. Since no mining activity will occur in the Minnesota Creek basin during this term of permit, monitoring data for Water Year. 1986 from the Minnesota Creek basin is considered an extension of baseline monitoring. Surface water, springs, and groundwater are considered separately even though they are interrelated. The report attempts to show the interrelationships that are significant or could be expected. Since mine water and groundwater aze so interrelated, they are discussed together. ' 2.1 Assessment of Groundwater There aze essentially two ways in which mining activities az Mt. Gunnison could affect groundwater. First, extension of mine entries and ' mining of the opal , could cause water to move from formations above the opal seam into the mine. Of particular concern is the role of possible roof collapse and mine subsidence on water bearing formations above the coal. Second is the possible impact of ' refuse material disposal on groundwater quality. Although the permeability of the F seam and overlying strata is very low and the ' quantity of water stored in these units is also low, water from these strata will occasionally seep into the mine. The entries located neaz the outcrop, near the portals, and in Sylvester Gulch, flowed during snowmelt runoff in the Spring and Summer of 1986. Inflows occurred primarily from the roof in areas of low overburden cover and near the F Seam outcrop. Sustained inflows were encountered only at the three locations described previously. The estimate of sustained inflows is provided in table 1.4-3, The three locations where inflows were encountered are shown on E7~hibit III. Sane mine dischagge water is lost to evaporation prior to discharge. ' Imported water is to a large extent transported out of the mine with the coal, or lost in ventilating air. Sane seepage may account for a minimal amount of water loss. ' The seasonal pattern of mine inflows observed near the Sylvester Gulch Return and the mine entries indicate that the colluvium, coal and bedrock are rechazges locally. The area near the Sylvester Gulch Fan location is also an area with many naturally occurring fractures. Water moves down slope and discharges as springs and seeps. Water inflows appear to be controlled by precipitation, depth of overburden and location of fracture zones, and not by the extent of underground workings. The lower mine inflows during water year 1986 may be attributed to a localized dewatering effect in the Sylvester Gulch area. Once mining was completed the flows encountered during mining decreased since the area immediately near the iorkings had been dewatered to a certain extent. Transmissivity to the surface may not be as great as transmissivity in the immediate vicinity of the vx~rkings. Precipitation was above average during the 1986 water yeaz but was less than in 1984 and 1985. This may also have contributed somewhat to lower inflows. Precipitation was likely a small contributor to decreased mine inflows, however, since base levels of the groundwater wells responsive to precipitation and runoff remained as high as the 1984 and 1985 levels. Mine water inflows for the 1987 water year aze expected to be similar to the 1986 water year. Sane minor inflows of the roof dripper, floor seep, rib seep ' (11)