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feet, and is characterized by low permeability (0.01 to 0.1 ft/day), limited continuity, low yield and relatively <br />• poor water quality. <br />Above the Wadge Overburden is the first major regional aquifer, known as [he 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 [he <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 Twentymi]e 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 [hick. This is followed by afine-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 />~~Vadae Overburden: Five Wadge Overburden wells (004-79-4, 006-82-74C, 91M006, 008-77-58, and <br />93M001) are currently monitored. water quality sampling is required annually. All monitoring requirements <br />were completed at these sites. Note [hat former Wadge Overburden well 93M003 was abandoned in advance of <br />• mining in August 2000, and former well FBR-11-A was abandoned in December 2001. Data for these <br />abandoned wells can be found in prior AHR's. Tables 1 through 5 provide summaries of the 2004 Water Year <br />data. Tables la through 5a provide some historical statistical summaries of collected data at each site through <br />2003. Tables ]b through 5b provide period of record water level summaries for each site. Figures 3 through 11 <br />provide graphical representations of water levels and selected water quality parameters recorded at each site. <br />The potentiometric surface in [he 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 />2004, the water levels in well 009-79-4 (Figure 3) continue to be comparable to previous years going back [o <br />1984. Well 009-79-4 (Figure 4) has exhibited a genera] decline in total dissolved solids since 1995, although <br />concentrations were up slightly in 2004. Well 006-82-74C (Figures 5 and 6) has retumed 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 />Well 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. In 2004 TDS concentrations (Figure 9) remain at consistent levels seen since ]980s. <br />The potentiometric surface reflected by well 93M001, has generally been observed to slope from southwest to <br />nonheast. Well 93M001 (Figure 10) had indicated declines in water level since the 1999 water year, although <br />• levels went up slightly in 2004. These declines were associated with underground mining of [he Eastern Mining <br />District (EMD). TDS concentrations (Figure 11) in this well remain stable. <br />4 <br />