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• N~rologle Qom rnterpnmdon m~C hnpaet Asreemiw Jor Pemdr Area and Ad/atenr Arcar <br />• <br />u <br />year, 24 hour precipitation even[ as well as a <br />calculated sediment storage volume. The ponds <br />are also designed to safely pass the overflow from <br />a 25 year, 24 hour precipitation event. The Mt. <br />Gunnison Mine site receives much of its annual <br />precipitation from snowfall during the winter <br />months. When warmer weather begins in late <br />February, March or April, most runoff to the <br />sediment ponds is from snowmelt. It is entirely <br />possible for a volume of runoff greater than the <br />10 year, 24 hour precipitation event to result from <br />snowmelt or snowmelt coupled with a small <br />precipitation event. <br />The sediment ponds are treated with cationic <br />flocculent when necessary to speed clarification <br />and settling of suspended solids. The ponds MB-1, <br />MB-2, MB-5, and MB-6 have manually controlled <br />dewatering devices. Ponds MB-3 and MB-4 have <br />automatic dewatering capability. Ponds MB-1, <br />MB-2, MB-5 and MB-6 are dewatered intermit- <br />tently with most discharges coming during early <br />spring runoff. Dewatering procedures take usually <br />less than one day with the exception of MB-5 <br />which is responsive to a high spring groundwater <br />table and sometimes has to be dewatered mare <br />often. <br />Ponds MB3 and MB-4 did not discharge <br />during 1988 and records indicate they have not <br />discharged since they were constructed. MB-4 <br />sometimes catches hydraulic oil spills from the <br />Unit Train L.oadout and is pumped by a licensed <br />contract hauler or the WECC vacuum truck which <br />deposits [he oil in the shop waste oil tank which <br />is in turn pumped by a licensed waste oil rery- <br />cling firm. This is a standard procedure. <br />Flowmeters have been installed on all dis- <br />charge points from sediment ponds which dis- <br />charge intermittently to continuously record flows. <br />Information on flows, sampling points, and <br />quality of water discharged from the mine site <br />sediment ponds in 1988 is included in Appendix I. <br />All water discharged was in compliance with <br />NPDES permit effluent limitations. <br />During spring runoff conditions as well as <br />some precipitation events quality of water in the <br />North Fork of the Gunnison is much worse than <br />the quality of discharges from WECC sediment <br />ponds and well outside NPDES effluent limita- <br />tions. <br />During winter baseflow and summer condi- <br />tions water quality in the North Fork is better <br />and is within NPDES effluent limitations, how- <br />ever, this is the time of year when WECC's ponds <br />are rarely discharged. <br />Flow amounts from WECC sediment ponds <br />are low in comparison to the North Fork of the <br />Gunnison flows. Even during low flow conditions <br />on the North Fork WECC discharges represent an <br />imperceptible difference in stream quality. <br />It is apparent from the analyses that sediment <br />pond discharges have had no adverse impact on <br />the North Fork of the Gunnison River. The <br />quality of discharges usually exceeds the quality of <br />the North Fork; and discharge flows are so small <br />that incremental changes in concentrations in the <br />North Fork are imperceptible. <br />Discharges of surface runoff in 1989 are <br />expected to be similar to 1988. <br />15 <br />