2001-12-11_REPORT - C1980007 - Laserfiche WebLink

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C Hydrologic Data lmaprctanon and Impact Assessma~t for Pamir Area and Adjacau Arcot Because of the small quantity of mine water anticipated, any discharge from ponds would normally be dominated by water quality charac- teristics oC disturbed area runoff as described in the following section. Runoff from Disturbed Area since they were constructed. MB-4 sometimes catches hydraulic oil spills from the Unit Train L.oadout. When this occurs, the pond is pumped by a licensed contract hauler or the WECC vacuum truck. The pumped oil is deposited in the shop waste oil tank which is in turn pumped by a li- censed waste oil recycling firm. This standard procedure was unnecessary during 1989. Overland runoff from snowmelt and precipitation within the disturbed areas is routed to one of six sediment ponds on the mine site. The six sediment ponds are designed to contain the predicted runoff from a statistically determined 10 year, 24 hour precipitation event as well as a calculated sediment storage volume. The ponds are also designed to pass the overflow Crom a 25 year, 24 hour precipi- tation event safely. The Mt. Gunnison Mine site receives much of its annual precipitation from snowfall during the winter months. When warmer weather begins in late February, March or April, most runoff to the sediment ponds is from snowmelt. It is entirely possible Cor a volume of runoff greater than the ]0 year, 24 hour precipitation event to result from snowmelt or snowmelt coupled with a small precip- itation event. The sediment ponds are treated with cationic Bocculent when necessary to speed clarification and settling of suspended solids. The ponds MB-1, MB-2, MB-5, and MB-6 have manually controlled dcwatering devices. Ponds MB3 and MB-4 have automatic dewatcring capability. Ponds MB-1, MB-2, MB-5 and MB-6 are dewatered intermit- tently with most discharges coming during early spring runoff. Dewatering procedures take usually less than one day for all ponds except MB-5. Pond MB-5 is responsive to a high spring groundwater table and sometimes has to be dewatered more often. Ponds MB-3 and MB-4 did not discharge during 1989 and records indicate they have not discharged Flowmeters are installed on all discharge points from sediment ponds which discharge intermittent- ly to continuously record flows. Information on flows, sampling points, and quality of water dis- charged from the mine site sediment ponds during 1989 is included in Appendix J. All water dis- charged was in compliance with NPDES permit effluent limitations. During spring runoff condi- tions, as well as some precipitation events, quality of water in the North Fork of the Gunnison is much worse than the quality of discharges from WECC sediment ponds. it is also well outside NPDES effluent limitations. During winter base- Bow and summer conditions water quality in the North Fork is better and is within NPDES effluent limitations. However, this is the time of year when WECC's ponds are rarely discharged. Flow volumes from WECC sediment ponds are low relative to the North Fork of the Gunnison Bows. Even during low Bow conditions on the North Fork WECC discharges represent an imperceptible difference in stream quality. Based on the analyses, sediment pond discharges have had no adverse impact on the North Fork of the Gunnison River. The quality o[ discharges usually exceeds the quality of the North Fork. In addition, pond discharge Bows are so small that incremental changes in concentrations in the North Fork are imperceptible. Discharges of surface runoff in 1990 are expected to be similar to 1989. 14