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does not forth an aquifer in the Twen[ymile Park Basin. The relative position of these units within the stratigraphic • sequence is shown in Figure 2, General stratigraphic Column. Twentvmile 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 100 to 200 feet. It is isolated hydrologically from the underlying Wadge Overburden sequence in the mine permit area 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 Hydrograph, for well 82-84A. Period of record water levels for the Twentymile Sandstone monitoring wells are shown on Figures 3i, 3j, 3k, and 31. Water quality from the 81-30 and 82-48A wells completed in the Twentymile Sandstone aquifer indicate a characteristic sodium bicarbonate 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 FBRI 1 B and FBR2, completed in this unit, show that TDS varies from 470 mg/I to 540 mg/I for well FBR 11-B and 940 mg/I to 1040 mg/I for well FBR-2. 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 varies 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 006-83-84C 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 and area 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, and 3q. A sequence of -ow permeability siltstones, shales and coals, approximately 250 feet thick, separates the Trout Creek Sandstone from the overlying Wadge Overburden sequence, which includes the Wadge coal seam. Recharge 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 area. 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. Water quality data from the 001-83-106 well indicates that the Trout Creek sandstone baseline water quality is a sodium bicarbonate type with moderate sulfate concentrations and very low concentrations of calcium and magnesium as depicted graphically in Figure 4-a, Ground Water Quality Data. Total dissolved solids concentrations are in the order of 900 mg/I. Period of record date for well 001-83-106 shows a total dissolved solids concentration of 330 mg/I to 960 mg/, well 95M001 shows a range of 450 mg/1 to 1010 mg/l, and well FBR 2-E shows a range of 820 mg/I to 1010 mg/I. Wadge Overburden . The Wadge Overburden consists of a sequence of siltstones, shales, coal and fine-grained sandstones. The sequence varies in thickness from 50 to 100 feet, averaging approximately 75 feet. The Wadge Overburden is characterized by Permit Renewal No. 3 "'.04-20 12/10/98