The Mt. Zirkel Wilderness Area (MZWA) straddles the Continental Divide in the Routt Mountain Range in Northwest Colorado. The Wilderness lies above the Yampa River Drainage Basin (Yampa Valley), which extends to the west ~165 km along the course of the Yampa River. The Yampa Valley lies within Routt and Moffat Counties and contains the major population centers of Steamboat Springs (pop. ~7,500), Hayden (pop ~1,600), and Craig (pop. ~8,500), Colorado. The permanent population of the Yampa Valley approaches 27,100, which is substantially augmented during winter by skiers and during summer by outdoor enthusiasts. The major economic interests in the Yampa Valley include tourism, cattle ranching, coal mining, and electric power generation.
Two coal-fired power plants are located within the Yampa Valley. The Hayden Station, in Hayden, CO, is ~30 km west of the closest Wilderness boundary and consists of one 184 MW and one 262 MW generating units with separate stacks. Prior to release, emissions are routed through hot-side electrostatic precipitators to remove primary particles. The Craig Station, in Craig, CO, is ~60 km west of the closest Wilderness boundary and consists of three 450 MW units, each with a separate stack equipped with precipitators. Units 1 and 2 stacks, also known as the “Yampa Project” are proceeded by wet scrubbers to remove sulfur dioxide with ~67% efficiency. The newer Unit 3 passes emissions through a dry sulfur dioxide scrubber with >85% efficiency.
On July 14, 1993, the Rocky Mountain Region of the United States Forest Service (USFS) certified an impairment of visibility in the MZWA. Ely et al. (1993) compiled technical information from color slides, meteorological data analysis, emissions inventories, and dispersion modeling that provided a basis for the certification. The USFS named the Hayden and Craig generating stations as possible contributors to visibility impairment. Since current information was insufficient to reasonably attribute observed visibility impairment to specific sources, a “reasonable attribution” study, the Mount Zirkel Visibility Study (MZVS), was conducted.
The study objectives were to:
- Determine the extent of visibility impairment, if any, within the Mount Zirkel Wilderness Area.
- Determine whether the cause of or contribution to any visibility impairment within the MZWA may be reasonably attributed to emissions from any source or group of sources.
- Determine a reasonable assessment of the relative contribution of emissions from each source or group of sources to visibility impairment.