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SECTIONONE Purpose and Scope of Study <br />The Henderson Mill is located along the Williams Fork River approximately 20 miles south of <br />Parshall, Colorado in Grand County, Colorado. The mill and tailing dam are part of the <br />Henderson Mine complex operated by Climax Molybdenum Company (Climax). Tailing <br />deposition began at the site in July 1976. <br />1.1 PROJECT HISTORY <br />Tailing from the mill is slurried and flows by gravity through a tailing delivery line to the tailing <br />storage impoundment at the eastern side of the the Park basin. The dam consists of three smaller <br />dams: Nos. 1, 2, and 3. Dam No. 1 is the main dam, and Dams No. 2 and No. 3 are auxiliary to <br />Dam No. 1. Dam No. 1 will ultimately incorporate Dam No. 3 and tailing will cover Dam No. 2. <br />The dams are being constructed of tailing, using the upstream method of construction and header <br />and spigot deposition. Spigot deposition is used to build the dam and raise the crest to create <br />storage during the summer months from May through October. Open -end lead -offs (single point <br />discharge) is used for winter deposition from November through April. <br />Tailing Dam No. 1 <br />Dam No. 1 is approximately 200 feet high' (crest elevation 8800) and has a crest length of over <br />7,000 feet. The downstream slope is about 4H:1V (horizontal:vertical). The ultimate planned <br />height for the dam is about 300 feet (crest elevation 8900). At the ultimate height, the crest <br />length will be about 11,000 feet and the dam will impound about 300 million cubic yards of <br />tailings. <br />Dam No. 1 started with the construction of a 40 -foot high starter dam composed of native soils. <br />Tailing deposition commenced behind this dam. The dam is raised by controlled tailing <br />deposition using groups of spigots to create a course sand beach. Continued deposition raises the <br />dam crest in the upstream direction. This deposition method results in an engineered shell of <br />free - draining tailing. Fine tailing and water are contained by the engineered shell, creating the <br />impoundment. A drainage system was constructed beneath and upstream of the starter dam. The <br />drain system dewaters the engineered dam shell and controls the phreatic surface and seepage in <br />the dam shell. The drains consist of a network of corrugated metal drainpipes surrounded by <br />gravel. They discharge to a seepage collection trench located downstream of the starter dam. <br />Decant Structure <br />Water collected in the decant pond is the result of either direct precipitation and run -on or tailing <br />deposition. Water is removed from the pond by a decant structure. The decant structure consists <br />of two separate and independent gravity pipelines: a decant line and a spillway line. The decant <br />line recovers water for reuse in the milling process and the adjacent spillway line serves as an <br />emergency overflow spillway for the impoundment during extreme storm events. The spillway <br />and decant lines are two parallel steel pipes encased in reinforced concrete. These pipes extend <br />through and beneath the starter dam and into the decant pond. <br />I Based on May 2000 Survey <br />N: PROJECTS16844618 HENDERSON MILL BARG\SUB ODIBA PROJ -MILL R- 3.DOC124JUL -01% 1 -1 <br />