2012-02-16_REVISION - M2002020 (9)

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Nature of Deposit to be Mined Materials along the Poudre River near the City of Greeley are composed of Pleistocene and recent age unconsolidated, valley -fill materials that make up the alluvial floodplain of the Cache la Poudre River. The alluvial deposit extends from the surface or near - surface to a variable depth of 50 to 60 feet below the ground surface. The alluvial material is generally underlain by Upper cretaceous age bedrock of the Laramie Formation, Fox Hills Formation, and Pierre Shale. Although the Laramie Formation may be absent in some areas, where it is present it consists primarily of silty to sandy blue to gray shale. The Fox Hills Sandstone is a silty massive sandstone with black shale lenses. Beyond the weathered transition zone with the Fox Hills Sandstone, the Pierre Shale is predominantly dark gray, un- weathered shale. The relatively impermeable characteristics of the shale create a bottom seal for reservoir construction and water storage development. Topsoil and overburden on most of the site is thin or non - existent. Although past mining activities may have removed previously existing topsoil and overburden material, the site is within the Cache la Poudre floodplain where natural stream and wind -blown surface deposits would be expected. Native soils and overburden for the site were likely thin and developed from alluvium and eolian deposits of clay, silts, and sands. Mine Phasing The City of Greeley anticipates mining and reclaiming the amended 25 Avenue Site in approximately 15 years. However, the rate of mining and overall life of the mine is dependent on several factors including product demand. The mining plan currently includes mining the two amendment area SW ponds to bedrock, a mining cell of approximately 42 acres (see Exhibit C -5). Although previous mining established the ponds that are estimated to be 20 feet deep, borehole drilling and testing has verified that commercial deposits of sand and gravel exist up to 60 feet below the surface of the ponds. In addition to the commercial sand and gravel materials, clay, silt, and other non - marketable materials excavated from the beneath the ponds will be used on -site to backfill previously excavated pits. Due to the small size of the mining area, mining will not be subdivided into phases or areas. If additional mining locations are added within the affected area in the future, a revised mining plan with phasing information will be submitted to the Division of Reclamation, Mining and Safety (the Division). In general, there will not be a distinctive direction of mining for the mining cell. Mining will consist of dewatering the ponds, expanding the pond cavities to create one larger pit, and excavating material from the pit bottom. Since good marketable material is expected to occur intermittently with non - marketable clay and silt lenses, excavation may move from one area to another in the bottom of the pit based on material needs. Even though the proposed mining cell is relatively small, integration of mining and reclamation is still applicable. Previous mining on the site has created a lack of suitable overburden material for reservoir slope construction. Therefore, mining will excavate at 3H:1 V slopes along the pit edges to facilitate final reclamation of reservoir slopes. Installation of the slurry wall will likely Greeley 25` Avenue Site DRMS 112 Permit Amendment Application Page 6