2000-03-10_GENERAL DOCUMENTS - C1980007

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i. Refuse Material Disposal Impacts on Surface Water - Existing Refuse Piles -Runoff from both the Lower Refuse Pile (LRP) and the Refuse Pile Expansion (RPE) sites will be routed through the sediment control system and must meet effluent limitations. Final designs for the waste rock pile, which include surface and subsurface drainage systems, allow for the collection of water leachate from the pile. Analysis of sample rock provided by MCC indicates that potential toxicity levels are below the Environmental Protection Agency (EPA) standards. Consequently, it appears that if runoff were to develop from the waste piles, minimal impact will occur on the North Fork of the Gunnison River. MCC will be monitoring for both groundwater quality and levels under the piles, leachate quality and levels from the subdrains, and spring quality from the base of the pile site. MCC has had leach tests performed on representative samples of waste rock. Of analyses conducted at three labs, the leachate exceeded drinking water standards in four parameters. One lab reported dissolved iron concentrations of 6.62 mg/I (secondary drinking water standards = .3 mg/l); the other labs reported iron at .OS mg/I and .06 mg/I. Two labs reported pH's of 8.9 and 9.1 (secondary drinking water standards = 6.5 - 8.5 pH). Selenium exceeded standards at two labs which were .04 mg/1 and .025 mg/1 (primary drinking water standards = .Ol mg/1). One lab analysis exceeded nitrate levels with 30 mg/I (primary drinking water standards = 10 mg/I). TDS levels were 782 mg/1 and 720 mg/I at two labs. The amount of leachate flow in the underdrains is expected to be low and will be diluted in the sediment ponds with surface runoff. The existing refuse piles are expected to have no significant effects to the surface water system even under worst-case low flow conditions. The amount of leachate generated from the pile is small in relation to the adjacent drainage areas. The quality of the leachate is predicted to be comparable to similar waste piles in the North Fork valley with moderately high TDS, elevated sodium and sulfates, and low concentrations of metals. A worst-case discharge projection from the Lower Refuse Pile has been estimated. Based on an estimated annual infiltration of 5.7 inches, an annual leachate production of about 19 acre-feet is predicted from the 40-acre upper refuse site. A seasonal maximum leaching rate was estimated at twice the average annual leaching discharging rate of 11.8 gpm. When added to the leaching rates estimated for the Lower Refuse Pile, a worst case total seasonal discharge of nearly 50 gpm could result. 44