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silts[one(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 ]ow permeability mazine 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 [hat 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 afine-grained massive sandstone, approximately 250 feet thick, known as [he <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 [he Trout Creek sandstone and the <br />overlying Wadge Overburden. <br />Wade Overburden: Five Wadge Overburden wells (009-79-4, 006-82-74C, 91M006, 008-77-58, and <br />93M001) are currently monitored. In 2003, as a result of changes from TR03-0.2, monitoring for water levels, <br />field parameters continued on asemi-annual basis, while water quality sampling changed from semi-annual to <br />annual. All monitoring requirements were completed at these sites. Note that former Wadge Overburden well <br />93M003 was abandoned in advance of mining in August 2000, and former well FBR-11-A was abandoned in <br />December 2001. Data far these abandoned wells can be found in prior AHR's. Tables 1 through 5 provide <br />summaries of the 2003 Water Year data. Tables la through 5a provide historical POR statistical summaries of <br />collected data at each site. Tables lb through 5b provide period of record water level summaries for each site. <br />• Figures 3 through 11 provide graphical representations of water levels and selected water quality pazameters <br />recorded at each site. <br />The potentiometric surface in the Wadge Overburden generally slopes from south to north in monitored wells in <br />the vicinity of the TCC facilities, as shown by water levels in wells 009-79-4, 006-82-74C, and 91M006. During <br />2003, the water levels in well 009-79-4 (Figure 3) continue to be comparable to previous yeazs going back [o <br />1984. Well 009-79-4 (Figure 4) has exhibited a general decline in total dissolved solids since 1995, although <br />concentrations were up slightly in 2003. Well 006-82-74C (Figures 5 and 6) has returned to water levels and <br />TDC concentrations typical of the early 1990s. Well 91M006 (Figure 7) continues to show a steady decline in <br />water level. <br />We11 008-77-58 (Figure 8) has been unaffected by mining and exhibits seasonal water level fluctuations. During <br />1998, this well experienced an unusually long seasonal decline, and subsequently an established recovering water <br />level trend resumed. 2003 also saw a rise in TDS concentrations (Figure 9) to levels last seen in the early 1980s. <br />The potentiomeuic surface reflected by well 93M001, has generally been observed to slope from southwest to <br />northeast. Well 43M001 (Figure 10) had indicated declines in water level since the 1499 water year, although <br />levels went up slightly for the first time in 2003. These declines were associated with underground mining of the <br />Eastern Mining District (EMD). TDS concentrations (Figure 11) in this well remain stable. <br />• <br />4 <br />