burners. Within Coconino County, visibility is generally better in the Page/Glen Canyon area at the north end of the county than at the Grand Canyon and areas further south (Sunset Crater, Walnut Canyon, Wupatki National Monuments). Visibility in the Page/Glen Canyon area averages 3.5 deciviews above natural background conditions, within less than one deciview of the best visibility listed in the contiguous U.S. (The DV scale is near zero for a pristine atmosphere, with a change of one deciview being a just noticeable change.) In 2012, a fish consumption advisory was issued for striped bass in lower Lake Powell, raising concerns about mercury emissions from the plant. NGS emissions, however, were found to contribute less than 2% of atmospheric deposition of mercury on the Colorado River watershed. Atmospheric deposition, furthermore, accounts for only a part of the mercury loading, much of it coming from natural geological deposits. The natural weathering of rock in the Green River catchment, for example, alone contributes about 40% of the mercury loading in Lake Powell. Updated Emissions Requirements The EPA's Mercury and Air Toxics Standard (MATS) that took effect in 2015 required that the plant reduce mercury emissions to () or on a gross basis. On January 4, 2013, Department of the Interior, the EPA, and Department of Energy announced plans to jointly develop a plan for the future of the plant that maintains its energy, water, and economic benefits. The plan would outline short term investments in the plant to meet environmental goals, and long term plans for transitioning to cleaner energy options, such that short term and long term plans work together. On January 17, 2013, the EPA proposed a BART ruling to further reduce NOx emissions:The Environmental Protection Agency (EPA) is proposing a source-specific federal implementation plan (FIP) requiring the Navajo Generating Station (NGS), located on the Navajo Nation, to reduce emissions of oxides of nitrogen (NO X) under the Best Available Retrofit Technology (BART) provision of the Clean Air Act (CAA or Act) in order to reduce visibility impairment resulting from NGS at 11 National Parks and Wilderness Areas. NGS, which was built over 35 years ago, is the largest coal-fired power plant in the West in terms of generating capacity. It is central to the economies of the Navajo Nation and Hopi Tribe and supplies power to the states of Arizona, Nevada, and California. Electricity produced by NGS is also