2003-06-10_REVISION - M1977342 - Laserfiche WebLink

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In a letter dated March 7, 2003, Henderson revised the amendment AM-004 application to provide for the construction of tree and shrub "islands" on the tailing impoundment. Impoundment areas outside of the tree and shrub islands would be reclaimed by placement of 6 to 18 inches of topsoil (average 12 inches) and revegetation with the approved seed mix. No tree and shrub plantings will be required outside of the islands. The islands will consist of discrete areas where thicker cover (minimum 3 feet) will be placed over the mill tailing, and revegetated in accordance with the seeding, tree planting, and shrub planting plan approved in the original Henderson Mill reclamation permit. In the March 7, 2003 letter, Henderson suggests that the ore surge pile could be used to create the thicker cover for the soil islands, but that if the surge pile were not available, the additional cover material would be imported from off-site or borrowed from the Henderson gravel pit. Since the filing of the Mazch 7, 2003 letter, the DMG has communicated to Henderson that use of the surge pile material would not be approved for construction of the soil islands due to potential for acidity in the plant rooting zones. For the purpose of reclamation cost estimation and reclamation bonding, the DMG will assume that the additional cover material for the tree islands will be borrowed from the tailing impoundment perimeter in the same manner as documented in the DMG reclamation cost estimate for task 09e previously provided to Henderson. The volume of additional cover soil to be borrowed will be equal to the volume of the ore surge pile as documented in DMG's cost estimate for task 09a, which is 166,667 loose cubic yards. This is in accordance with Henderson's March 7, 2003 letter. This volume of loose dirt will settle approximately 11 percent upon placement in the islands allowing this additional borrowed cover soil to be sufficient to construct approximately 46 acres of tree islands. The areal extent of the tree islands was calculated as follows: Surge pile volume: 166,667 loose cubic yazds. Earth loam will settle approximately 11 percent following placement. Three foot minimum thickness required for tree islands; reclamation permit already requires and average of one foot soil thickness over the tailing surface. 0.89 x 166,667 loose cubic yards = 148,334 settled cubic yards = 4,005,008 cubic feet 4,005,008 cubic feet / 2 feet thickness = 2,002,504 square feet = 46 acres The DMG evaluated the sufficiency of 46 acres of tree islands as follows. Assuming that 92 islands each one-half acre in size were to be installed on a grid over the entire 1,100 acre pond surface, the spacing between the islands would be approximately 525 feet. This would yield a maximum range from cover for wildlife of just over 250 feet, which is more than adequate according to DMG wildlife experts. Evaluation of a grid of identically sized and spaced tree islands is done only for a check on the sufficiency of the cover fill volume proposed by Henderson. For the execution of the tree island plan, the DMG has stipulated that Henderson will provide a technical revision with a plan and design for the tree islands. The plan should have variations on island size, shape, spacing, and cover thickness to maximize the benefit to wild and domesticated animals. The DMG suggests that the largest islands be in the 2-acre size range and that large islands be incorporated at a rate of at least one for every 40 acres. It is further suggested that a variety of island sizes be constructed from one-quarter acre on up. The location and spacing of the various sized islands can be established considering migration, foraging, and predator pathways that should become established on the reclaimed impoundment over time. Also to be considered are the edge effects where the impoundment reclamation will transition to the surrounding reclaimed and undisturbed areas.