2013-03-14_REPORT - C1982056 (3)

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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 gradient from the outcrop area. A detailed discussion of the geology/hydrology for the site is available in the Twentymile Coal Company, Foidel Creek Mine Permit C -82 -056, Volume !, Rules 2.04.6 and 2.04.7. The Twentymile Park Basin is underlain by a large, high quality coal reserve. This reserve, mined by underground operations, is known as the Wadge Seam. The Wadge Seam lies within a sequence of sedimentary rocks characterized by shales, claystones, mudstones, 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 area of mine operations is currently being conducted. 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 Twentymile 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 hydrologically isolated 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 Twentymile 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 seam that had been surface mined. Beneath the Wadge Overburden is a sequence of low permeability siltstones, shales, and coals, approximately 200 feet thick. This is followed by a fine - grained massive sandstone, 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. Table 1 presents a summary of bedrock well monitoring frequencies, which was last updated for Permit Revision 6 (PR 03 -06) in March of 2004. Wadge Overburden: Five Wadge Overburden wells (009 -79 -4, 006- 82 -74C, 91M006, 008- 77 -58, and 93M001) are currently monitored. Water quality sampling is required annually. All monitoring requirements were completed at these sites. Tables 2 through 6 provide summaries of the 2012 Water Year data. Tables 2a through 6a provide some historical statistical summaries of collected data at each site through 2012. Figures 3 4