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2019 Plan of Operations Modification American Gypsum Company, LLC <br /> 3.0 OPERATIONAL AND BASELINE ENVIRONMENTAL <br /> INFORMATION <br /> 3.1 Air Quality <br /> The closest Clean Air Act Class I areas are Flat Tops Wilderness to the northwest, Eagles Nest <br /> Wilderness to the east, and Maroon Bells Snowmass Wilderness to the south. The closest air <br /> quality monitoring stations are in Rifle, Colorado. <br /> The Gypsum Plant operates under Colorado Department of Public Health and Environment <br /> Operating Permit 950PEA041, first issued in June 1997, revised April 2019, and expiring 01 April <br /> 2024. Permits would be revised with the appropriate regulatory agencies as necessary to <br /> accommodate the proposed mining activities described herein. <br /> 3.2 Climate and Hydrology <br /> The area is classified as Zone VI climate, a continental subarctic climate (Dfc) according to the <br /> Koppen climate classification system (Brittannica, 2016). This describes an area of less than <br /> 12 inches of annual precipitation (usually falling as snow), low humidity, and long, cold winters. <br /> Average maximum temperatures range from 340F (January) to 85°F (July). Average minimum <br /> temperatures range from 40F (January) to 460F (July). The site receives an annual average <br /> precipitation of 11 inches (Western Regional Climate Center, 2016). <br /> Normal precipitation in the area produces virtually no surface runoff. Most of the stormwater is <br /> absorbed by dry soils and geologic fractures. Short-term runoff does occur during rare major <br /> storms. <br /> The 10- and 100-year 24-hour precipitation depths were obtained from the National Oceanic and <br /> Atmospheric Administration (NOAA)Atlas 14 (Perica, et al., 2013) and were assumed to follow a <br /> Natural Resources Conservation Service (NRCS) Type II distribution. Precipitation depths for <br /> design storm events are shown in Table 3.1. <br /> TABLE 3.1: NOAA DESIGN STORM DEPTHS <br /> PrecipitationStorm Event Depth <br /> 10-year 24-hour 1.47 <br /> 100-year 24-hour 2.21 <br /> There are dry creek beds within the proposed disturbance limits that flow during major storms, <br /> which are rare at the Mine. During normal precipitation events, most runoff infiltrates into dry soils <br /> or geologic fractures. These dry creek beds rarely flow with appreciable water. Stormwater from <br /> Am Gypsum Poo Mod ISSD 20190909 September 2019 (11] <br />