1992-03-12_REVISION - M1977378 (2)

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Tailings Pond #1 File No. M-77-378 Page 2 III . Overburden Covering, Cont'd Areas that are anticipated to carry additional runoff volume over that from uniform sheet flow will be underlain with additional compacted overburden material. Specific examples would be at the base of any benches, if required, and the pond-hillside interface. The most concentrated flow will be at the pond-hillside intersection which at the west end will carry the total surface runoff from the 9.5 acre pond top. This flow is estimated to be 40 cfs with a 100-year thunderstorm event (2.3 inches in 1 hour) . In order to reinforce for this flow, an area ranging in width from 10 feet at the east end (small contributing area) to 20 feet at the west end will be subexcavated 3 feet with 2 1/2:1 side-slopes. The bottom 1 foot will be replaced with one foot of overburden material compacted to 95% of its maximum dry density. An additional 1 foot of loose overburden material will be placed on top and the area seeded. If necessary, erosion control fabric will be placed to minimize erosion. This area will not slope transversely and the 1 foot depth should carry the flow without backing onto the unreinforced pond area (See attached calculation sheets) . The compacted overburden with a permeability in the range of 10"" to 10_Vcm/sec will control downward migration of the runoff water. Upland runoff will be controlled by the existing runoff ditch. Attached is a F.M. Fox generated hydrograph for this interception ditch as originally designed. IV. Seeding and Covering: The regraded area (approximately 27 acres) will be fertilized and seeded either in the fall after completion of earthwork or the following summer during late June or early July (depending on construction progress) . The seed mixture will be the "Mountain Mix" specified in Table E-4 of the Consolidated Permit Application. The seed mixture will be applied at 20 lb/acre (minimum) by hydroseeding. Prior to seeding the overburden will be analyzed to determine the type and application rate of fertilizer required along with any other amendments such as limestone. The borrow material is expected to be similar to that used on Tailings Pond #4. The recommended treatment was application of 0. 9 tons of limestone per acre and fertilization with 200# N, 40# P205 and 60# K20 per acre. A spring tooth harrow will be used to provide a thin soil covering after seeding. Secondary plant communities (forbs and shrubs) as committed to in the Consolidated Permit Application will be planted after establishment of the pioneer communities.