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surface at greater than 100 gpm when first drilled (Circa 1973). In contrast, the No. SA mine supplemental well, drilled within 16 feet of the No. 5 mine well, yielded less than 10 gpm and has a permeability of approximately 0.01 H/day as • shown in Figure 11 a, No. 5 Mine Replacement Well Draw Down. The highly variable yields of the Trout Creek Sandstone wells, indicated that the aquifer may have localized fracture-induced high permeability zones. Middle Sandstone The Middle Sandstone lies approximately 160 to 360 feet above the "F" coal seam and lies approximately 165 to 266 feet below the overlying Twentymile Sandstone. The Middle Sandstone averages 105 feet thick within the permit boundaries. The Middle Sandstone dips gently to the north-northwest at approximately 8 to 10 degrees. For information regarding the structure of the Middle Sandstone, refer to the Structure and Isopach Map of Middle Sandstone (Map 12). The Middle Sandstone is composed of fine to very fine grains of sand cemented with calcium carbonate. The sandstone is exposed in the valley walls of the Yampa and Williams Fork Rivers in the southern portion of the permit area, and may lie up to 1,700 feet below ground surface near the northern boundary. Six (6) monitoring wells have been completed in the Middle Sandstone. These aze: 1) TR-4, 2) TR-7a, 3) 81-O1, 4) 83-O1, 5) 83-02, and 6) 83-03. Water well locations are shown on the Hydrologic Monitoring Program Location Map (Map 11). Water levels on all six (6) wells indicate that the aquifer is artesian. The aquifer is probably artesian over most of the permit area except in the immediate vicinity of the outcrop. Hydrologic properties of the Middle Sandstone were determined from aquifer testing performed on many of the wells. Results of this testing yielded an average permeability of 0.5 gpd/ft and a storage coefficient of 0.00033 to 0.0015. Historical water levels for the six (6) wells are plotted in Figure 12, Plot of Water Level Data - Well TR-4, Figure 13, Plot of Water Level Data -Well TR-7A, Figure 14, Plot of Water Level Data -Well 81- 01, Figure I5, Plot of Water Level Data -Well 83-O1, Figure 16, Plot of Water Level Data -Well 83-02, and Figure 17, Plot of Water Level Data- Well 83-03. • Declines in piezometric levels, shown for observation wells in these figures for the Middle Sandstone and the Trout Creek Sandstone wells are due primarily to mining activities. No declines in piezometric levels have been observed in the Twentymile Sandstone. Further discussion of these observation well responses is provided in Section 2.05.6 under the Probable Hydrologic Consequences section. The piezometric declines in the Trout Creek Sandstone are thought [o be related primarily to pumping of the mine water supply wells completed in the unit and not from mining the overlying E and F coal seams. The piezometric levels in the Middle Sandstone, show fluctuations which are apparently related to dewatering and subsidence associated with Mines 5 and 6. A piezometric surface map has been prepared for the middle Sandstone for water levels measured in 1992. The water levels are presented on the Piezometric Surface -Middle Sandstone 1992 Map (Map 13). Based on these water levels, ground water gradients are infered to be 120 feet/mile to the southwest to approximately 60 feeUmile to the southwest. Recharge to the Middle Sandstone probably occurs in the southern portion of the permit area at the outcrop. The aquifer may also be recharged [o a lesser degree by vertical leakage from underlying and overlying aquifers. Natural discharge from the Middle Sandstone is probable down dip to the west-northwest toward the deeper portions of the structural basin. Discharge may also take place as vertical leakage from the aquifer to adjacent formations. Twentymile Sandstone The Twentymile Sandstone lies approximately 165 to 246 feet above the Middle Sandstone and between 370 to 380 below the "P" coal seam (previously mined at the No. 9 Mine). The Twentymile Sandstone averages about 120 feet thick. This sandstone unit is composed of fine-grained sandstones cemented with calcium carbonate. For information regarding the structure of the Twentymile Sandstone, refer to the Structure and Isopach Map of the Twentymile Sandstone (Map 14). The Twentymile Sandstone forms relatively broad outcrop areas near the central L J Permit Renewal No. 3 2.04-13 Revised 7/IO/00