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f~ecutive Summary This annual hydrologic report is the fifth report submitted for the Mt. Gunnison No. 1 Mine and wvers the period from October 1, 1985 through September 30, 1986. The report provides a summary and interpretation of hydrologic monitoring data at the Mt. Gunnison No. 1 Mine through Water Yeaz 1986 and an assessment of hydrologic i~acts. Detailed listings of monitoring data for the year are contained in the quarterly reports, "hydrologic Studies". This report is divided into two sections. The first section describes monitoring activities through Water Yeaz 1986. The secorui section interprets the hydrologic data ~llected during the year and relates this information to mining and reclamation activities at Mt. Gunnison to assess the impacts, if any, on the hydrologic balance. This section also addresses hydrologic impacts that might be expected to occur during Water Yeaz 1987. _ An extensive baseline monitoring data base has been developed for the Mt. Gunnison No. 1 Mine. Operational monitoring started in August 1981 with commencement of construction activities at Mt. Gunnison. ' Monitoring data for the period October 1, 1985, to.September 30, 1986, shows no significant adverse impacts to the hydrologic balance from mining activities at the Mt. Gunnison No. 1 Mine. Mine inflows during water year 1986 were much lower than water year 1985. The volume of water discharged from the underground workings in 1986 was 2.4 acre feet versus an estimated 30.5 acre feet in 1985. This may be attributed to a ' reduced amount of inflaa once initial dewatering of the area immediately near the mine workings took place. Mast, if mt all, significant inflow in 1986 was neaz Sylvester Gulch where low overburden and natural fractures contribute to ' mine inflows. Although the Sylvester Gulch inflows aze responsive to precipitation and snowmelt, peak inflow generally corresponds to the peak spring runoff (with scm~ lag time). There was above average precipitation and spring ' runoff in 1986 that did not sustain mine inflow rates of the previous year. In fact mine inflow was much lower than previous years. The quality of mine water sampled in 1986 was good (see tables 1.4-1 and 1.4-2), ' and all discharges were well within NPDES effluent limitations without tceatment. I Mine water discharge in 1986 was entirely from the Sylvester Gulch underground sump. This discharge had no measurable effect on the quality of the North Fork of the Gunnison River. Activities at the Lower Refuse Pile had no measurable effects on surface or groundwater quality during water year 1986. I Underground mining activities have not produced a visible or measurable effect on any springs, groundwater wells, or surface water monitoring stations during water year 1986. I I -ii-