SEI050 U.S. Department of the Interior Bureau of Land Management Wyoming State Office Buffalo Field Office January 2003 Final Environmental Impact Statement and Proposed Plan Amendment for the Powder River Basin Oil and Gas Project Volume 1 of 4 (WY -070-02-065) - 1 - MISSION STATEMENT It is the mission of the Bureau of Land Management to sustain the health, diversity, and productivity of the public lands for the usc and enjoyment of present and future generaticms. BLM/WY /PL-03/004+ I 3 I 0 - 2 - Chapter 3 — Affected Environment his chapter describes the affected environment for the project alternatives. TThe affected environment is the portion of the existing environment that could be affected by the project. The information presented here focuses on is­ sues identified through the scoping process and interdisciplinary analyses. The affected environment varies for each issue. Both the nature of the issue and components of the proposed project and alternatives dictate this variation. The following sections concentrate on providing only the specific environmental in­ formation necessary to assess the potential effects of the Proposed Action and alternatives. Groundwater This section describes the groundwater resource. More detailed information on groundwater resources in the Project Area is contained in the groundwater tech­ nical support document for the FEIS (AHA and Greystone 2002). Regional Characterization Groundwater resources are contained in permeable underground aquifers com­ posed of rock and sediments through which water can flow. Water moves slowly in aquifers in response to the prevailing hydraulic gradient, through tiny open spaces in the rock and sediment. Groundwater is replaced, or recharged, from precipitation that falls directly on the aquifers or by leakage through the beds of streams that intercept aquifers or from adjacent aquifers. Movement of ground­ water is from recharge areas down the hydraulic gradient to discharge areas. Aquifer permeability is directly related to the nature and type of porosity of the material that makes up the aquifer. Primary porosity is the open space between individual grains or rock clasts. Secondary porosity consists of joints and frac­ tures that form after a rock is consolidated (Whitehead 1996). Primary porosity is the porosity type of unconsolidated-deposit aquifers and consolidated sandstone aquifers in the PRB (Whitehead 1996). Coal aquifers in the PRB contain signifi­ cant secondary porosity. Davis (1976) describes groundwater resources as part of a hydrologic system. The components that describe a hydrologic system are the following: aquifer type (or geologic unit); water chemistry; confined (artesian) or unconfined conditions; and groundwater recharge or discharge areas. 3-1 PRB O & G FEIS - 3 - Chapter 3 — Affected Environment Aquifer Types The groundwater resources of the PRB are described by Whitehead (1996). Groundwater resources that are at or near the land surface within the PRB are contained in unconsolidated Quaternary alluvial or basin fill deposits or in semi- consolidated to consolidated lower Tertiary sandstones and coal beds that are the uppermost aquifers in the Northern Great Plains aquifer system. Clinker, which is also an aquifer, has formed from some of the lower Tertiary sediments (Heffern and Coates 1999). These Quaternary and Tertiary aquifers are described below in more detail. Quaternary Alluvial Aquifers Aquifers in stream-valley alluvium generally occur along rivers and major drain­ ages within the PRB. The groundwater resources contained in alluvial aquifers are described by Whitehead (1996). These unconsolidated deposits of silt, sand, and gravel occur as floodplains, stream terraces, and alluvial fans. Coarser allu­ vial deposits occur in valleys of the Belle Fourche, Cheyenne, Powder, Tongue, and Little Powder rivers and in the larger tributaries of the Powder and Tongue rivers. Alluvium overlying formations of Tertiary age in the central part of the PRB is mostly fine to medium grained (Hodson et al. 1973). The thickness of alluvial deposits within the Project Area is mostly less than 50 feet, but may be as much as 100 feet in some valleys near mountains (Hodson et al. 1973). Wells (1982) describes alluvial deposits as commonly 30 feet thick or less, but also reports that deposits 100 feet thick have been measured. Lowry et al. (1986) also describe alluvial deposit thickness and water yield from the PRB. The thickest and coarsest-grained alluvium occurs near the Bighorn Mountains along the western margin of the PRB, where saturated horizons are thick and high yields of water are possible. Mostly fine-grained alluvial deposits with a saturated thickness less than 20 feet occur distant from the mountains, resulting in low yields of water. Northern Great Plains Aquifer System The Northern Great Plains aquifer system is an extensive sequence of aquifers and confining units arranged in a stack of layers that may be discontinuous lo­ cally within the PRB, but that functions regionally as an aquifer system. This sys­ tem includes the lower Tertiary aquifers that are exposed at the surface in the PRB and underlying, deeply buried regional aquifers that are stacked with inter­ vening confining layers. The deeply buried aquifer systems are composed of up­ per Cretaceous sandstones and coals, lower Cretaceous sandstones, upper Paleo­ zoic limestones and dolomites, and lower Paleozoic sandstones, limestones, and dolomites (Whitehead 1996). These deeply buried regional aquifers are strati- graphically below, isolated from, and older than the aquifers that may be affected by CBM development in the PRB and are not described further. PRB O & G FEIS 3–2 - 4 - Chapter 3 — Affected Environment Lower Tertiary Aquifer System The lower Tertiary aquifer system consists of semi-consolidated to consolidated Oligocene to Paleocene sediments (Whitehead 1996). The Oligocene White River Formation is present in the Project Area only as isolated erosional rem­ nants, such as Pumpkin Buttes in southwestern Campbell County (Lewis and Hotchkiss 1981), and is not described further. The lower Tertiary aquifers consist of sandstones and coal seams contained in the Eocene Wasatch Formation and the Paleocene Fort Union Formation (Whitehead 1996). Both of these geologic units are continental deposits consisting of sand­ stones, siltstones, claystones, and beds containing lignite and subbituminous coal. Stratigraphically, from youngest to oldest, the Lower Tertiary Aquifer System consists of the Wasatch aquifers, the Fort Union aquifers contained in the Tongue River member of the Fort Union Formation, the Lebo confining layer, and the Tullock aquifer. Clinker has been formed from these geologic formations in loca­ tions where these sediments have been altered in place by spontaneous combus­ tion of coal beds (Coates and Heffern 1999). Clinker plays an important role as an aquifer in the storage and flow of water within the PRB. Rainfall and snowmelt infiltrate rapidly in clinker exposure ar­ eas. The stored water is discharged slowly to springs, streams, and aquifers, which helps maintain flow in perennial streams during dry periods (Heffern and Coates 1999). Clinker outcrops cover about 460 square miles of the Project Area and are concentrated in the following areas: along the eastern boundary of the Project Area in the Rochelle Hills, within the Powder River Breaks in the north­ ern portion of the Project Area, within the Tongue River Breaks north of Sheri­ dan, within the Lake DeSmet area north of Buffalo, and within the Felix coal out­ crop area west of Gillette and northeast of Wright (Heffern and Coates 1997). Wasatch Aquifers The Wasatch Formation consists of fine- to coarse-grained, lenticular sandstone interbedded with shale and coal (Hodson et al. 1973). Minor constituents include coarse conglomerates occurring along the western margin of the PRB, carbona­ ceous shales, and thick coal beds (Seeland 1992). Sandstone layers comprise an estimated one-third of the sequence and are important PRB aquifers. High per­ centages of sand (from 30 to 50 percent and more) have been documented along a trend paralleling the western margin of the PRB, beginning east of Buffalo and west of the Powder River and continuing toward the southeast (Seeland 1992). Wasatch coal beds are thickest in the central and western portions of the PRB (Seeland 1992). Locally, in the northwest part of the PRB near the Bighorn Mountains, the Wasatch is divided into two conglomeratic members. The Wasatch Formation is as much as 1,800 feet thick in the southern portions of the PRB (Keefer 1974). Southeast of Buffalo, the maximum preserved thickness of the Wasatch Formation is about 3,000 feet (Seeland 1992). 3–3 PRB O & G FEIS - 5 - Chapter 3 — Affected Environment Fort Union Aquifers The Fort Union Formation yields water from fine-grained sandstone, jointed coal, and clinker overlying the Lebo confining layer (Zelt et al. 1999). The Sandstone content of the Fort Union aquifer ranges from 21 to 91 percent and is hydrologi­ cally confined, except near the land surface (Hotchkiss and Levings 1986). The Fort Union Formation is as much as 3,900 feet thick in the southern part of the PRB (Hotchkiss and Levings 1986). Numerous thick and laterally widespread coal beds occur within the Fort Union Formation and are important PRB aquifers (Lewis and Hotchkiss 1981). The thickness of the Fort Union coal aquifers varies greatly within the PRB. The maximum thickness of a single Fort Union coal seam is less than 25 feet along the western margin of the PRB and in the northern portion of PRB in southeast­ ern Montana. The maximum thickness of a single Fort Union coal seam is more than 100 feet near Wright and extending west and northwest of Wright, within the central portion of the PRB in Wyoming (Seeland 1992). Lebo Confining Layer The lower Paleocene Tullock member of the Fort Union Formation is partially isolated and confined by the overlying Lebo member (Brown 1993). The Lebo confining layer generally retards water movement (Hotchkiss and Levings 1986). The Lebo confining layer consists predominantly of dark shales containing dis­ continuous zones of white calcareous banding (paleosol horizons).