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• <br />• <br />C J <br />Hydrologic Data Juterpretation and Jmpacr Auessrnent far dte Permit and Adjacnu Areas <br />ponds are designed to contain the predicted runoff <br />from a statistically determined 10 year, 24 hour <br />precipitation event as well as a calculated sediment <br />storage volume. The ponds are also designed to <br />pass the overflow from a 25 year, 24 hour precipi- <br />tation event safely. <br />The Mt. Gunnison Mine site receives much oC its <br />annual precipitation from snowfall during the <br />winter months. When warmer weather begins in <br />late 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 10 <br />year, 24 hour precipitation event to result From <br />snowmelt or snowmelt coupled with a small precip- <br />itation event. <br />Flowmeters are installed on all discharge points <br />from sediment ponds which discharge lntcrmlttent- <br />ly to continuously record (lows. Information on <br />flows, sampling points, and quality of water dis- <br />charged from the mine site sediment ponds during <br />1990 is included in Appendix J. All water dis- <br />charged was in wmpliancc with NPDES permit <br />effluent limitations. During spring runoff condi- <br />tions, as well as some precipitation events, quality <br />of water in the North Fork of the Gunnison is <br />much worse than the quality of discharges from <br />WECC scdimcnt ponds. It is also well outside <br />NPDES effluent limitations. During winter base- <br />flow and summer conditions water quality in the <br />North Fork is better and is within NPDES effluent <br />limitations. However, this is the time of year when <br />WECC's ponds are rarely discharged. <br />The sediment ponds are treated with cationic <br />Oocculent when necessary to speed clarification <br />and settling of suspended solids. The ponds MB-I, <br />MB-2, MB-5, and MB-b have manually controlled <br />dewatering devices. Ponds MB-3 and MBA have <br />automatic dewatering capability. Ponds MB-], <br />MB-2, MB-5 and MB~.are dewatered intermit- <br />tently with most discharges coming during early <br />spring runoff. Dewatering procedures take usually <br />Icss than one day for all ponds except MB-5. Pond <br />MB-5 is responsive to a high spring groundwater <br />table and sometimes has to be dewatered more <br />often. <br />Ponds MB-3 and MBA did not discharge during <br />1990 and records indicate they have not discharged <br />since they were constructed. MB~I sometimes <br />catches hydraulic oil spills from the Unit Train <br />loadout. When this occurs, the pond is pumped by <br />a licensed contract hauler or the WECC vacuum <br />truck. The pumped oil is deposited in the shop <br />waste oil tank which is in turn pumped by a li- <br />censed waste oil recycling firm. This standard <br />procedure was unnecessary during 1990. <br />Flow volumes from W ECC sediment ponds are low <br />relative to the North Fork of the Gunnison flows. <br />Even during low flow conditions on the North <br />Fork WECC discharges represent an imperceptible <br />difference in stream quality. <br />Based on the analyscs,scdimcnt pond discharges <br />have had no adverse impact on the North Fork of <br />the Gunnison River. Tht: quality of discharges <br />usually exceeds the quality of the North Fork. In <br />addition, pond discharge flows are so small [hat <br />incremental changes in concentrations in the North <br />Fork are imperceptible. <br />Discharges of surface runoff in 1991 are expected <br />to be similar to 1990. <br />IS <br />