2010-02-09_PERMIT FILE - C1982056A (6)

Home Browse Search

does not form an aquifer in the Twentymile Park Basin. The relative position of these units within the stratigraphic sequence is shown in Figure 2, General SVatigraphic Column. • Twentymile Sandstone The Twentymile Sandstone aquifer is laterally continuous over most of the area and is effectively confined above and below by low permeability shale and interbedded siltstone/shale units. The unit is a massive, white, well-cemented, cross-bedded sandstone varying in thickness from ]00 to 200 feet. It is isolated hydrologically from the underlying Wadge Overburden sequence in the mine permit azea by a very low permeability marine shale, which has a thickness of up to 700 feet. Water levels of the Twentymile Sandstone have been monitored in the mine plan and are exhibited on Figure 3-a, Well Hydrogaph, for well 006-82-48A. Period of record water levels for the Twentymile Sandstone monitoring wells aze shown on Figures 3i, 3j, 3k, 3r and 3s. Water quality from the 81-30 and 006-82-48A wells completed in the Twentymile Sandstone aquifer indicate a chazacteristic sodium bicazbonate type water with a total dissolved solids concentration of about 300 mg/1, and this is depicted graphically in Figure 4-a, Ground water quality. Period of record water quality data for wells FBR11B and FBR2, 97012TH and 97013TH completed in this unit, show that TDS varies from 450 mg/1 to 540 mg/1 for well FBR 11-B (which will be abandoned in 2003 due to subsidence damage), and 928 mg/1 to 1040 mg/1 for well FBR-2, 480 mg/I to 530 mg/I for 97012Tm, and 590 mg/1 to 960 mg/1 for we1197013TM.. Trout Creek Sandstone The Trout Creek sandstone is the second major regional aquifer, which exists in the Twentymile Park Basin. The unit is light brown to light gray, fine grained, massive sandstone, which vazies in thickness from 50 to 400 feet. It has good lateral continuity throughout the basin and moderate values of hydraulic conductivity. • Water levels in the 006-83-48C well completed in the Trout Creek Sandstone approximately 2000 feet north of the underground operations have been measured on approximately a quarterly basis since early 1984, until it was abandoned in 1992. Water levels are plotted on Figure 3-h. Water levels in this well tend to rise during the period following spring snowmelt, which indicates that recharge to the aquifer occurs at this time. The water levels in the well are several hundred feet above the top of the Trout Creek showing that the aquifer is confined and under significant artesian pressure. Period of record water levels for the Trout Creek Sandstone monitoring wells are shown in Figures 3m, 3n, 30, 3p, 3q, 3t and 3u. Well 95H001 (figure 3p) has also exhibited a decline since 1995. This well was drilled to replace well 006-83-48C after the well was undermined and subsided. Mining of the EMD began in 1995, which is down dip of this well and the operation may have contributed to the declining water levels in this well. Well FBR-2 (figure 3q) located along the eastern permit boundary, exhibited consistent water levels until early 1999 and then dropped about 50 feet. Since that drop consistent water level trends have been recorded for the well. Well 97012TC (Figure 3t) has exhibited a steady decline in water level since monitoring began in the 1998 water yeaz. As indicated below, the Trout Creek Sandstone is an isolated aquifer. The declines in water level do not appear to be related to TCC mining activities. Well 001-83-107 (figure 3u) has exhibited a steady decline in water level since the eazly 1990's. This well is located down dip of the TCC bathhouse water well (001-83-106), figure 3m). Water level decline may be attributed to TCC's pumping of water from well 001-83-106. A sequence of low permeability siltstones, shales and coals, approximately 250 feet thick, sepazates the Trout Creek Sandstone from the overlying Wadge Overburden sequence, which includes the Wadge coal seam. Rechazge to the Trout Creek sandstone occurs along its outcrop, approximately 1.5 miles south of the southern permit boundary, as • shown on the Trout Creek Sandstone Potentiometric Surface Map (Map 33}. This is very little hydrologic connection between the Trout Creek sandstone and the Wadge coal seam in the mining azea. TCC has mined in the West, Southwestern and Eastern mining districts and this activity has not caused significant inflows from the Trout Creek sandstone into the mine workings. PR 03-06 2.04-20 05/07/03