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• 3.1 Bedrock Groundwater Monitorine Hvdrot?ealoav: TCC's undergound mining operations are located on the southwest flank of the Twentymile Pazk Basin. This is an enclosed synclinal Bound water basin, rimmed by outcrops of major lithologic units. Ground water in the Twentymile Park Basin exists primarily under confined conditions within the bedrock units. All Bound water aquifers in the general area, except the alluvial aquifers, are rechazged by infiltration in outcrop areas, located primarily in the southern and western mazgins of the Twentymile Park Basin. Recharge is limited, due to low average annual precipitation and the loss of moisture through evapotranspiration and sublimation in this semi-arid climatic zone. Ground water occurrence and movement within the bedrock aquifer system is controlled by geologic structure and lithology. Ground water movement tends to follow the structural dip of the lithologic units toward the basin axis. The general trend of movement in the permit area is to the north and northwest. This results in confined aquifer conditions down gadient from the outcrop azea. A detailed discussion of the geology/hydrology for the site is available in the TwenJyrrerle Coal Company, Foidel Creek Mine Permit C-82-056, Volume I, Rules 2.04.6 and 2.04.7. The Twentymile Pazk Basin is underlain by a large, high quality coal reserve. This reserve, mined by undergound operations, is known as the Wadge Seam. The Wadge Seam lies within a sequence of sedimentary rocks characterized by shales, claystones, mdstones, siltstones, sandstones, and minor coal stringers referred to as the Wadge Overburden. The Wadge Overburden is hydrologically isolated from the two major bedrock aquifer units in the Twentymile Park Basin, the Twentymile Sandstone aquifer and the Trout Creek Sandstone aquifer, • which lie above and below the Wadge Overburden, respectively. Lying above the Twentymile Sandstone is a minor aquifer unit, known as the Fish Creek Sandstone. All these geologic units are monitored within the Foidel Creek Mine permit area. The Wadge Overburden is directly affected by the mining operation. Dewatering of this zone in the azea of mine operations is currently being conducted. The Wadge Overburden lies approximately 1100 feet below the Bound surface. The Wadge Overburden sequence varies in thickness from 50 to 100 feet, averaging approximately 75 feet, and is characterized by low permeability (0.01 to 0.1 ft/day), limited continuity, low yield and relatively poor water quality. Above.the Wadge Overburden is the first major regional aquifer, known as the Twentymile Sandstone. The Tweniymile Sandstone aquifer is confined above and below by low permeability shale and interbedded siltstone/shale units. The unit varies in thickness from 100 to 200 feet and is isolated hydrologically from the underlying Wadge Overburden sequence by a low permeability marine shale, which has a thickness of up to 700 feet. Approximately 125 feet above the Twenlymile Sandstone is the 35-foot thick Fish Creek Sandstone. The Fish Creek Sandstone is a discontinuous minor unit in the area that was the contact for a small coal seem that had been surface mined. Approximately 200 feet beneath the Wadge Overburden is a sequence of low permeability siltstones, shales and • coals, approximately 250 feet thick, known as the Trout Creek Sandstone. The Trout Creek sandstone is the second major regional aquifer, which exists in the Twentymile Park Basin. There is very little hydrologic connection between the Trout Creek sandstone and the overlying Wadge Overburden. 3