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the following discussions. Statistical summaries of the historical data base are provided after presentation of the <br />2005 data. <br />3.1 Bedrock Groundwater Monitoring <br />Hydro¢ealoay: TCC's underground mining operations are located on the southwest flank of the Twentymile <br />Park Basin. This is an enclosed synclinal ground water basin, rimmed by outcrops of major lithologic units. <br />Ground water in the Twentymile Park Basin exists primarily under confined conditions within the bedrock units. <br />All ground water aquifers in the genera] area, except the alluvial aquifers, are recharged by infiltration in outcrop <br />areas, located primarily in the southern and western margins of the Twentymile Pazk Basin. Recharge is limited, <br />due to low average annual precipitation and the loss of moisture through evapotranspiration and sublimation in <br />this semi-arid climatic zone. Ground water occurrence and movement within the bedrock aquifer system is <br />controlled by geologic structure and lithology. Ground water movement tends to follow the swctural dip of the <br />lithologic units toward the basin axis. The general trend of movement in the pernvt area is to the north and <br />northwest. This results in confined aquifer conditions down gradient from the outcrop area. A detailed <br />discussion of the geology/hydrology for the site is available in the Twentymile Coal Company, Foidel Creek Mine <br />Permit C-82-056, Volume 1, Rules 2.04.6 and 2.04.7. <br />The Twentymile Park Basin is underlain by a large, high quality coal reserve. This reserve, mined by <br />uhderground operations, is known as the Wadge Seam. The Wadge Seam lies within a sequence of sedimentary <br />• rocks characterized by shales, claystones, mudstones, siltstones, sandstones, and minor coal stringers referred to <br />as the Wadge Overburden. The Wadge Overburden is hydrologically isolated from the two major bedrock aquifer <br />units in the Twentymile Park $asin, the Twentymile Sandstone aquifer and the Trout Creek Sandstone aquifer, <br />which lie above and below the Wadge Overburden, respectively. Lying above [he Twentymile Sandstone is a <br />minor aquifer unit, known as the Fish Creek Sandstone. Ail these geologic units are monitored within the Foidel <br />Creek Mine permit area. <br />The Wadge Overburden is directly affected by the mining operation. Dewatering of this zone in [he area of mine <br />operations is currently being conducted. The Wadge Overburden lies approximately 1100 feet below the ground <br />surface. The Wadge Overburden sequence varies in thickness from 50 to 100 feet, averaging approximately 75 <br />feet, and is characterized by low permeability (0.01 to 0.1 fdday), limited continuity, low yield and relatively <br />poor water quality. <br />Above the Wadge Overburden is the first major regional aquifer, known as the Twentymile Sandstone. The <br />Twentymile Sandstone aquifer is confined above and below by low permeability shale and interbedded <br />siltstone/shale units. The unit varies in thickness from 100 to 200 feet and is hydrologically isolated fmm the <br />underlying Wadge Overburden sequence by a ]ow permeability marine shale, which has a thickness of up to 700 <br />Feet. Approximately 125 feet above the Twentymile Sandstone is the 35-foot thick Fish Creek Sandstone. The <br />Fish Creek Sandstone is a discontinuous minor unit in the area that was the contact for a small coal seam that had <br />• been surface mined. <br />3 <br />