17.24.304(1)f HYDROLOGIC AND GEOLOGIC DESCRIPTION 1.0 INTRODUCTION This section contains a description of the geologic and hydrologic regimes within and adjacent to the proposed Bull Mountains Mine No. 1 permit area in Musselshell and Yellowstone Counties, Montana. The study area includes the permit, mine plan, and adjacent areas. The adjacent area is defined as that region located within 3 miles in a downgradient direction of the permit area and one mile in all other directions. Regional background information is presented on: • precipitation. • topography and physiography. • geology, including stratigraphy and structure. • groundwater. Geologic and hydrologic baseline conditions for the.study area are defined including descriptions of hydrogeologic units in terms of: • geology. • baseline water levels. • aquifer hydraulics. • groundwater quality. Conclusions are drawn using a conceptual hydrogeologic model that explains the interaction of hydrogeologic units and the controlling mechanisms for occurrences of springs. In addition, surface water baseline water quality and quantity are described in detail. 2.0 BACKGROUND INFORMATION 2.1 PRECIPITATION Precipitation data have been collected at the National Weather Service station in Billings, Montana since January 1911 (Table 304(6)-lA) and since June 1914 at the Montana State Cooperative station in Roundup, Montana (Table 304(6)-1B). Annual precipitation totals for Billings and Roundup for the period 1959 to 1988 are presented graphically on Figures 304(6)-1A and 304(6)-1B, respectively. For the period of record, the long term average annual precipitation for the Billings station is 13.55 inches and 12.07 inches for the Roundup station. Most of the precipitation falls between April and October. Slagle (1986) has used an average annual precipitation amount of less than 14 inches for the Bull Mountains Area, which corresponds favorably with the long term data records for Billings and Roundup. Within the study area, precipitation data are collected continuously at three meteorologic/air quality monitoring stations (Map 304(5)-2): • the Carlson site, installed in March 1989; • the Johnson site, precipitation data only, installed in October, 1989; and • the Dunn Mountain site, precipitation data only, installed in July 1991. Bull Mountains Mine No. 1 304(1)f-1 Permit Update, May 2010 304(1)f Text DLG 3-15-11.doc The precipitation data for these sites are tabulated in Tables 304(6)-2A, 2B, and 2C. Figures 304(6)-2A, 2B, and 2C and Figures 304(6)-2D, 2E and 2F graphically present the daily precipitation and the total monthly precipitation for the Carlson, Johnson and Dunn Mountain sites, respectively. Climatological data, are discussed in more detail in 17.24.304(1)h. 2.2 PHYSIOGRAPHY AND TOPOGRAPHY The study area lies within the Bull Mountains, which are part of the Northern Great Plains physiographic province, The topography of the study area consists of gently sloping valleys bounded by moderately steep to steep,ridges capped by isolated sandstone and clinker mesas. Elevations in the study area range from approximately 3700 to 4700 feet above mean sea level. Surface slopes vary from zero to 15 percent in the vicinity of the proposed surface facilities and up to 50 percent or more in the area of Dunn Mountain and the other mesas and ridges in the southeastern and central portions of the study area. The premining topography is. shown on Map 304(6)-1. Differential erosion of rock of varying hardness and resistance is the main process active in forming the present landscape. The underlying rocks are composed of interbedded shales, claystones, siltstones, coals, and sandstones; however, the high mesas and ridges are capped by "clinker". Clinker is a term used to describe the baked sedimentary rocks resulting from burning of underlying coal beds. The shales and claystones tend to be easily eroded, while the sandstone and clinker are more resistant to erosion. Sheet and rill erosion are active geomorphic processes in the upper drainage basins, and mass wasting occurs locally along the steep-walled ridges. The study area lies within the drainage basins of the Yellowstone River and the Musselshell River. Dunn Mountain is on the divide between these two rivers. The lands to the north drain to the Musselshell River while the lands to the south drain to the Yellowstone River. The permit area, including the area of surface disturbance, is located within the drainage area of Rehder Creek, a tributary of Halfbreed Creek and the Musselshell River. All of the basins in the study area are discussed in detail in the Surface Water Section below. 2.3 REGIONAL AND LOCAL GEOLOGY 2.3.1 Stratigraphy The stratigraphy of the area is relatively uncomplicated and resembles that of the Northern Powder River Basin. The subsurface section is nearly complete from the Precambrian to, the recent Quaternary with the exception of Silurian, Permian, Triassic and Upper Tertiary aged rocks, which are missing due to nondeposition or erosion (Map 304(6)-2). The Bull Mountains are composed of sedimentary rocks of the Tertiary Fort Union Formation. This formation contains all of the commercial coal within the Bull Mountains. The Fort Union has been divided into three members; in ascending order, these are: • the Tullock member, which is 400 to 500 feet thick in the study area. It consists of yellowish and darker colored sandstones and shales and contains no commercial coal beds. • the Lebo Shale, which consists of 200 to 300 feet of olive green and gray sandstone and shale. To the southeast, in the Northern Powder River Basin, it contains minor coals. In the Bull Mountains area, however, it is less carbonaceous, and the few coal beds are thin, high in ash, and not of Bull Mountains Mine No. 1 304(1)f-2 Permit Update, May 2010 304(1)f Text DLG 3-15-11.doc commercial value. The Lebo Shale member is less resistant to erosion than the overlying Tongue River member and tends to form gently rolling grass covered slopes. • the Tongue River member, which is composed of light-gray to brownish-gray sandstones and siltstones, light-buff to dark-gray shaley siltstones and shales, brown to black carbonaceous shales, and coals. Several thin beds of buff colored limestone also have been observed in the upper portion of the Tongue River (Woolsey, 1917). The thick sandstones were deposited by fluvial processes as point bars and channel deposits, while the siltstones, shales, and claystones were deposited by flood plain, overbank, and lacustrine processes. The coals were deposited in peat swamps and the limestones probably were deposited in shallow lakes. All of the rocks outcropping within the study area belong to the Tongue River member of the Fort Union Formation. Twenty six coal beds in the Tongue River member, ranging in thickness from approximately one foot to 15 feet have been named and mapped in the Bull Mountains (Woolsey, 1917). Numerous additional thin coal beds occur in various parts of the Bull Mountains basin and are of no commercial value. Only the Roundup, Carpenter, McCleary, Dougherty, Mammoth, and Rehder have been or are now of economic importance. The coal of primary interest for the Bull Mountains Mine No. 1 is the Mammoth coal. Relatively thin valley alluvium, colluvium and mesa forming clinker are other lithologic units exposed in the Bull Mountains. The alluvium, exposed in some of the drainage bottoms,is of Quaternary Recent age. The clinker, exposed along the ridge and mesa tops, is of Quaternary or late Tertiary age. 2.3.2 Structure The Bull Mountains are underlain by a gently plunging syncline in the western portion of the Bull Mountains Basin (Map 304(6)-2). The axis of the syncline, based on the Mammoth coal structure, trends northwest-southeast through the central, and topographically highest, portion of the study area. The deepest portion of the syncline is located about 8 miles north-northwest of the P.M. Mine (Connor, 1988). Thickness of the overburden increases rapidly with distance from the outcrop. 2.4 REGIONAL GROUNDWATER 2.4.1 Introduction Regional aquifers of interest can be divided into: • the Tongue River member of the Fort Union Formation and the Madison Formation, which will be directly affected by mining. • regionally significant aquifers including the alluvium of major streams and rivers, the Cretaceous lower Hell Creek-Fox Hills, and the Tullock aquifer, which will not be impacted by mining. 2.4.2 Aquifers Affected or Potentially Affected by Mining The regional aquifers of primary concern to the mining operation are: • the Tongue River member of the Fort Union Formation, which contain most of the currently used water resources in the study area: and Bull Mountains Mine No. 1 304(1)f-3 Permit Update, May 2010 304(1)f Text DLG 3-15-11.doc • the Madison Group, proposed as a source of water for the surface and underground mining facilities. A brief description of these aquifers is as follows: Tongue River Member: Groundwater flow is complex in the shallow, Tongue River member aquifers, as it radiates outward from an apparent local recharge area in the Bull Mountains region (Figure 304(6)-3) (Slagle, 1986). Within the mine plan area, groundwater flow in these aquifers is predominantly to the north-northwest, following the synclinal structure. The hydraulic gradient ranges from 0.002 to 0.008 feet/feet (ft/ft). The Tongue River member exhibits the greatest potential for water production, as it lies at the shallowest depth, and contains the largest percentage of sandstones and coals (Slagle, 1986). Heterogeneities within this member make depth to water and probable yields difficult to predict: however, well yields average approximately 8 gallons per minute (gpm) (Slagle et al., 1985). The overburden, the Mammoth coal, and the underburden are all included in the Tongue River member.