2021-12-22_PERMIT FILE - C1981019A

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RULE 2 PERMITS found at the Colowyo Mine. For South Taylor, the lowest seam currently considered to be mined is the G8/9 seam. At Lower Wilson, the I seam is the main seam to be underground mined. The South Taylor and Lower Wilson mining areas have similar lithology to that found in the current Colowyo mine. However, the mining areas are located topographically and structurally higher than what is encountered at the Colowyo mine. Therefore, the elevations of the base of the coal seam floor to be mined in both of these areas are generally higher than the old surface elevation of the already mined East and West Pits. The topographic high and geologic structure in the South Taylor mining area (an anticline) provides little recharge area for the underlying beds and seams. The Trout Creek sandstone, a major regional aquifer, and the overlying coal sequence, outcrop west and southwest of these areas and thus are above any sources for continuous water recharge. Thus, there is no source to contribute to artesian conditions in the coal seams to be mined. Monitoring well MWST-06-04 has water at an elevation of 7,915 feet (below the floor of the G9 seam), and piezometer PST -06-17 did not encounter water at or above the G8/9 seam. These boreholes are shown on Map 10. Perched bedrock groundwater will be found during mining activities. This has been confirmed by water monitoring during drilling, short-term air lift tests, and an examination of the geophysical logs. This water will contribute to a significant inflow of water during the boxcut and initial mining, but should cease rather rapidly (2 years ±). No artesian groundwater conditions exist above the Km marker bed, a proven regional aquiclude (confining layer), within the mining areas. Groundwater Quantity — Transmissivity of the perched zones of the interburden within the Williams Fork Formation ranged from 79 square feet per day (ft2/d) to 1,930 ft2/d. Corresponding hydraulic conductivity ranged from 3.43 feet per day (ft/d) to 80.4 ft/d (CDM, 1985a; Table 2.04.7-26). The range in values is indicative of the variability in the degree of fracturing in the formation. Published transmissivity for the upper Williams Fork Formation is 33 to 95 ft2/d (Robson & Stewart, 1990). The Fes, coal seam hydraulic transmissivity was 4.3 to 5.7 ft2/d and the conductivity was measured to be 0.24 to 0.29 ft/d (CDM, 1985a). The Trout Creek Sandstone just south of the Lower Wilson box cut was found to be dry (Well 84 -B -TC). Other wells north of the Lower Wilson mine that intersected saturated portions of the Trout Creek closer to the axis of the Collom Syncline had transmissivity that ranged from 2.06 to 4.57 ft2/d and had hydraulic conductivity ranging from 0.086 to 0.29 ft/d. All of these values are considered to be indicative of moderate permeability, and typical of relatively clean and/or fractured sandstone lithologic units (CDM, 1985a). Storativity values for the Williams Fork Formation and Trout Creek Sandstone could not be estimated as part of the CDM studies, however, observations made during drilling and monitoring suggest that the water -bearing intervals are of limited saturated thickness and under confined conditions with significant artesian pressure (CDM, 1985a). Storativity values (unitless) for the Fb coal seam ranged from 10-3 to 10-7, which are indicative of confined conditions. Storativity values for the Trout Creek Sandstone was about 10-2. In addition to the bedrock wells, studies of the alluvial/colluvial aquifers in the general area have also been conducted. CDM slug -tested the wells to determine the hydraulic characteristics of the shallow aquifers (CDM 1985a). A slug test consists of the introduction or removal of a known volume of water or slug then subsequently measuring the water level through time in the well as the water level declines. For each of the wells, a slug -in and a slug -out test were performed. The results of the slug tests performed on the relevant alluvial wells are presented in Table 2.04.7-27. In 1985, monitoring well A-5 was destroyed South Taylor/Lower Wilson — Rule 2, Page 21 Revision Date: 4/7/17 Revision No.: RN -07