2009-06-15_GENERAL DOCUMENTS - C1982056 (2)

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t • movement within the bedrock aquifers. The doubly plunging syncline of Twentymile Park forms an enclosed basin that is rimmed by the Twentymile Sandstone. Groundwater movement follows the structural dip toward the axis of the basin where it is confined by the overlying shale. Artesian pressures are encountered in the central part of the basin. The southern portion of Twentymile Park basin is heavily faulted. It is thought that many of these faults form conduits for vertical groundwater movement as well as barriers to normal down-dip movement. The Williams Fork Formation consists of massive sandstone beds. thick shale sequences. and interbedded shale, sandstone. siltstone, and coal sequences. There are four primary aquifer systems in Twentymile Park. These are. in ascending order. the Trout Creek Sandstone, the Wadge overburden, and the Twentymile Sandstone bedrock aquifers, and the alluvial aquifers underlying major surface drainages and in reclaimed spoils of adjacent surface-mined areas. The fine-grained, massive Trout Creek Sandstone is a major aquifer with good lateral continuity and forms striking white cliffs on outcrop. This sandstone averages about 100 feet in thickness (Bass, et. al. 1955) and has been known to vary from 50 to 400 feet thick in the area. according to Cyprus Western. Recharge occurs during spring snowmelt at the outcrop area along the southern rim of the basin. There is approximately 250 feet of interbedded shales, siltstones, • thin sandstones, and coal beds between the Trout Creek Sandstone and the overlying Wadge overburden aquifers. These low permeability sediments create confined aquifer conditions in the Trout Creek toward the center of the basin and artesian pressures are commonly encountered. Although the 250 feet of sediment allows very limited hydrologic connection between the Trout Creek Sandstone and the Wadge overburden aquifer above, more significant communication may be occurring along fault zones. Water in the Trout Creek aquifer is a sodium bicarbonate type with moderate concentrations of sulfate and very low calcium and magnesium concentrations. Total dissolved solids (TDS) values are typically about 900 mg/1. The Wadge overburden aquifer will be most directly affected by mining in the Foidel Creek mine. It averages 75 feet thick and is made up of the Upper and Lower Wadge seams as well as the interbedded siltstones. shales. fine-grained sandstones, and coal stringers that lie above and below the Wadge seam. Owing to the interbedded nature of the sediments and the presence of coal. this aquifer demonstrates low permeability, low well yields. and relatively poor water quality. Some of the poor water quality of the Wadge overburden aquifer can be attributed to the adjacent spoil piles from the surface mining activities. Recharge to this aquifer occurs from rainfall and snowmelt at the outcrop along the rim of Twentymile Park basin and the spoil piles of • the surface mines. Thus. this aquifer exhibits different water quality characteristics depending upon the proximity to the existing spoil piles. Water passing through the spoil piles tend to be of a