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1 <br />' F~cecutive Summ3r <br />This annual hydrologic report is the fifth report submitted for the Mt. Gunnison <br />' No. 1 Mine and covers the period from October 1, 1985 through September 30, <br />1986. The report provides a summary and interpretation of hydrologic monitoring <br />data at the Mt. Gunnison No. 1 Mine through Water Year 1986 and an assessment of <br />' hydrologic impacts. Detailed listings of monitoring data for the year aze <br />contained in the quarterly reports, "Hydrologic Studies". <br />This report is divided into two sections. The first section describes <br />' ironitoring activities through Water Yeaz 1986. The second section interprets <br />the hydrologic data collected during the yeaz and relates this information to <br />mining and reclamation activities at Mt. Gunnison to assess the impacts, if any, <br />on the hydrologic balance. This section also addresses hydrologic impacts that <br />might be expected to occur during Water Yeaz 1987. <br />An extensive baseline monitoring data base has been developed for the Mt. <br />Gunnison No. 1 Mine. Operational monitoring started in August 1981 with <br />commencement of construction activities at Mt. Gunnison. <br />t Monitoring data for the period October 1, 1985, to. September 30, 1986, shows no <br />significant adverse impacts to the hydrologic balance from mining activities at <br />the Mt. Gunnison No. 1 Mine. <br />' Mine inflows during water year 1986 were much lower than water year 1985. The <br />volume of water discharged fran the underground workings in 1986 was 2.4 acre <br />feet versus an estimated 30.5 acre feet in 1985. This may be attributed to a <br />' reduced amount of inflow once initial dewatering of the area immediately near <br />the mine workings took place. Most, if not all, significant inflow in 1986 was <br />near Sylvester Gulch where low overburden and natural fractures wntribute to <br />' mine inflows. Although the Sylvester Gulch inflows aze responsive to <br />precipitation and snowmelt, peak inflow generally corresponds to the peak spring <br />runoff (with sane lag time). There was above average precipitation and spring <br />' runoff in 1986 that did not sustain mine inflaa rates of the previous year. In <br />fact mine inflow was much lower than previous years. <br />The quality of mine water sampled in 1986 was good (see tables 1.4-1 and 1.4-2), <br />' and all dischazges were well within NPDES effluent limitations without <br />treatment. <br />' Mine water discharge in 1986 was entirely fran the Sylvester Gulch underground <br />sump. This dischazge had no measurable effect on the quality of the North Fork <br />of the Gunnison River. <br />' Activities at the Lower Refuse Pile had no measurable effects on surface or <br />groundwater Lal.ity during water yeaz 1986. <br />' Underground mining activities have not produced a visible or measurable effect <br />~ any springs, groundwater wells, or surface water monitoring stations during <br />water year 1986. <br />1 <br /> <br />-ii- <br /> <br />