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infiltration in outcrop areas, located primarily in the southern and western margins of the Twentymile Park Basin. <br />Recharge is limited, due to low average annual precipitation and the loss of moisture through evapotranspiration <br />and sublimation in this semi -arid climatic zone. Ground water occurrence and movement within the bedrock <br />aquifer system is controlled by geologic structure and lithology. Ground water movement tends to follow the <br />structural dip of the lithologic units toward the basin axis. The general trend of movement in the permit area is to <br />the north and northwest. This results in confined aquifer conditions down gradient from the outcrop area. A <br />detailed discussion of the geology/hydrology for the site is available in the Twentymile Coal Company, Foidel <br />Creek Mine Permit C-82-056, Volume I, 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 />underground 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 Basin, the Twentymile Sandstone aquifer and the Trout Creek Sandstone aquifer, <br />which lie above and below the Wadge Overburden, respectively. Lying above the Twentymile Sandstone is a <br />minor aquifer unit, known as the Fish Creek Sandstone. All 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 the area of mine <br />operations is currently being conducted. The Wadge Overburden sequence varies in thickness from 50 to 100 <br />feet, averaging approximately 75 feet, and is characterized by low permeability (0.01 to 0.1 friday), limited <br />continuity, low yield and relatively 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 from the <br />underlying Wadge Overburden sequence by a low 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 />Beneath the Wadge Overburden is a sequence of low permeability siltstones, shales, and coals, approximately <br />200 feet thick. This is followed by a fine-grained massive sandstone, approximately 250 feet thick, known as the <br />Trout Creek Sandstone. The Trout Creek sandstone is the second major regional aquifer, which exists in the <br />Twentymile Park Basin. There is very little hydrologic connection between the Trout Creek sandstone and the <br />overlying Wadge Overburden. <br />Table 1 presents a summary of bedrock well monitoring frequencies, which was last updated for Permit Revision <br />6 (PR 03-06) in March of 2004. <br />4 <br />