2025-06-30_PERMIT FILE - C1981019 (6)

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RULE 2 PERMITS South Taylor/Lower Wilson – Rule 2, Page 19 Revision Date: 6/4/25 Revision No.: MR-264 General Groundwater Resource Information General Groundwater Characteristics – Colowyo Coal Company’s general area of operation lies within an area known as the Danforth Hills. Regionally, the Danforth Hills are bounded on the northeast by the Axial Basin Anticline, and on the southwest by the northeastern flank of the Piceance Structural Basin. The intensely folded strata of the Mesa Verde Group characterize the Danforth Hills. The groundwater in the general area occurs principally in alluvial material associated with the major stream valleys and to a lesser extent in the permeable and semi-permeable bedrock strata (CDM, 1985a). (The term ‘alluvial’ used in this permit application does not necessarily mean that all the materials in the valley bottom have been water deposited. In fact, the majority of these deposits are colluvial in nature). The principal geologic units in the area are the Williams Fork Formation, the Iles Formation, and the Mancos Shale. At the top of the Iles Formation is the Trout Creek Sandstone Member. The Williams Fork Formation consists of interbedded sandstone, siltstone, and shale with coal beds, and ranges up to 1,200 feet thick in the mine area. The Williams Fork Formation is not considered to be a major aquifer in the region (CDM, 1985a). The Twenty Mile Sandstone Member, which is considered an important aquifer within the Williams Fork Formation in northern Colorado, is not present in the Danforth Hills area (CDM, 1985a). A detailed description of the hydrogeologic units in the area is provided in Section 2.04.6. Groundwater in the bedrock is largely controlled by the existence of fractures instead of primary permeability within the rock strata itself. The low permeability and discontinuous and lenticular nature of the strata restricts the ability of the bedrock to store and transmit water. In addition to fracturing, structural features in the area influence the limited movement and occurrence of groundwater. Groundwater tends to occur in the synclinal axis of the folds in the area as a result of the increased fracturing in these areas and the general movement of groundwater in the down-dip and down-fracture direction (CDM, 1985a). General Groundwater Quantity – Groundwater monitoring in the general and South Taylor/Lower Wilson permit area has been conducted since 1983. Monitoring wells have been established in the alluvium, the Williams Fork Formation interburden and coal, and the Trout Creek Sandstone. A summary of the bedrock and shallow monitoring wells relevant to the permit revision is presented in tables 2.04.7-25 through 2.04.7-29 and are shown on Maps 10A and 10B. Locations of all known wells and test boreholes within the permit area and adjacent area are illustrated on Map 11B. Previous studies by CDM (1985a) and Dennis (2001, 2006) determined the hydraulic characteristics of the bedrock aquifers in the Williams Fork Formation and the Trout Creek Sandstone. The results of these studies are presented in Table 2.04.7-26 and discussed in detail below. The Trout Creek Sandstone is a moderately permeable confined aquifer and the Williams Fork Formation is mostly dry with a few, low permeability, discontinuous, and confined aquifers of limited extent. Alluvial aquifers have moderate to high permeability where encountered, with a wide range of hydraulic values encountered. There is little groundwater in the alluvium along Wilson Creek immediately below the Lower Wilson affected area. There is groundwater in the alluvium along the West Fork and main stems of Good Spring Creek below the South Taylor affected area. General Groundwater Quality – Bedrock water quality (Williams Fork Formation and Trout Creek Sandstone) in the general area was determined by previous investigations (CDM, 1985a; Dennis, 2001; Colowyo 1992). Data from these investigations indicate that the principal water type in the Williams Fork Formation is a calcium- or sodium-bicarbonate type water, containing only minor concentrations of