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the BA, the Craig Generating Station consumes coal at a rate of 4.8 mtpy, and the Hayden <br /> Generating Station uses coal at a rate of 1.75 mtpy (i.e.,the Craig Generating Station consumes <br /> 73.3 percent of total coal consumed at both generating stations). Using these numbers, the Craig <br /> Generating Station would contribute 3.75 kg of local mercury (0.733 x 5.13 kg); and the Hayden <br /> Generating Station would contribute 1.37 kg of local mercury (0.267 x 5.13 kg). Applying <br /> percentages to the sourced coal that is consumed in the three units at the Craig Generating <br /> Station estimates Trapper's contribution to locally deposited mercury. Trapper sourced coal that <br /> is combusted in Unit 91 and Unit 92 contributes 19.16 percent of the mercury emitted from the <br /> Craig Generating Station, and other coal that is consumed in Unit 91,Unit 92, and Unit 93 <br /> contributes 80.84 percent of the mercury emissions. Relating these percentages to the local <br /> mercury deposition from the Craig Generating Station indicates that Trapper Mine coal results in <br /> 0.72 kg of locally deposited mercury (3.75 kg x 0.1916); and other coal results in 3.04 kg of <br /> locally deposited mercury. It is worth noting that, due to the prevailing winds generally being <br /> west to east in the action area,more of the mercury emitted by the Craig Generating Station is <br /> likely to be deposited east of Craig than to the west(i.e., further upstream along the Yampa and <br /> White Rivers than endangered fish critical habitats). <br /> It is also important to note that the calculations above are in reference to wet deposition of <br /> mercury. Some research has shown that dry deposition can be equal to or greater than wet <br /> deposition. Research has shown this rate to be anywhere from 0.8 to 4.8 times higher in the <br /> central and eastern United States (Zhang et al. 2012). The rate of dry deposition is highly <br /> dependent on the meteorological conditions and the chemical speciation of the mercury. <br /> Although most all of the sites analyzed in Zang et al. (2012)were in the eastern United States <br /> with more precipitation than that experienced in western Colorado, one site analyzed was in Salt <br /> Lake City,Utah. At that site,total mercury was 2.5 times that of the wet deposition of mercury <br /> (Zhang et al. 2012). Applying this factor to the Trapper Mine coal that is combusted at the Craig <br /> Generating Station provides a total mercury deposition of 1.8 kg (0.72 kg x 2.5) in the Action <br /> Area from the proposed action. <br /> We see that estimating the amount of mercury locally deposited from the combustion of the <br /> proposed Trapper coal using emissions data from the Craig Generating station (and assumptions <br /> regarding local deposition), results in a smaller estimate of locally deposited mercury (0.187 <br /> kg/yr)than that obtained from the estimate using the mercury deposition data at the MDN site <br /> (and associated conservative assumptions regarding mercury sources) (1.8 kg/yr). In terms of <br /> volume, this translates to between 0.47 and 4.4 ounces of mercury per year, if it were to be <br /> consolidated. <br /> 4.1.2 Selenium <br /> In addition to mercury emissions from the combustion of coal, another element known to be <br /> emitted is selenium. Selenium, a trace element, is a natural component of coal and soils in the <br /> region. While it may be released during combustion, it is not monitored at coal combustion <br /> stations to the same degree as mercury. No estimate as to the amount of selenium emitted <br /> annually and potentially deposited into the area was made in the BA. <br /> When selenium is present in flue gas after combustion, it tends to behave much like sulfur and is <br /> removed to some extent via the Sulfur dioxide (SO2) air scrubbers in place and also absorbs onto <br /> 47 <br />