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-13- <br /> �-� No mine discharge is expected durinq mining operations, since mining is <br /> progressing downdip from the portal opening and the applicant anticipates <br /> consuming all the groundwater that collects in the mine, plus all water <br /> pumped into the mine from the Stevens Gulch well field for dust control <br /> purposes. <br /> Assessment of Probable Cumulative Impact <br /> Orchard Valley and Area Mines <br /> The Orchard Valley mine currently operates approximately 2 miles North of <br /> Paonia, Colorado. In order to assess the cumulative impact of all anticipated <br /> mining in the general area on the hydrologic balance, the other coal mines <br /> along the North Fork of the Gunnison River must be evaluated as well, given <br /> the limits of available data. <br /> Water Balance <br /> A water balance for the North Fork of the Gunnison River has been performed <br /> for the watershed whose boundaries are illustrated in Figure 1. Within these <br /> boundaries are the following mines: <br /> 1. ARCO Coal - Mt. Gunnison No. 1 Mine (under construction) <br /> 2. Colorado Westmoreland - Orchard Valley (existing) <br /> 3. Western Slope Carbon - Hawk's Nest (existing) <br /> 4. U.S. Steel - Somerset (existing) <br /> 5. Bear Coal Co. - Bear Mine (existing) <br /> 6. Western Associated Coal - Blue Ribbon (existing) <br /> The boundaries can be effectively divided into two sections; namely, North of <br /> the North Fork of the Gunnison River and South of the North Fork of the Gunnison <br /> River. Table 1 identifies how much area exists in each section, along with <br /> approximate average elevations. <br /> In order to perform a mass balance on this area, it is necessary to determine <br /> how much precipitation occurs annually. One technique that can be used is to <br /> utilize existing precipitation data. Unfortunately, only two precipitation <br /> stations exist for the watershed; namely, at Paonia and at Wilcox Ranch. <br /> Since these are located near the North Fork of the Gunnison River, it was felt <br /> that precipitation values obtained would not adequately represent the total <br /> watershed. Therefore, isohyetal lines were used to determine average precip- <br /> itation and total amount of water that falls on the watershed. Figure 2 <br /> represents mean annual precipitation and Figure 3 illustrates May to September <br /> precipitation for the years 1931 to 1960. All values were obtained from maps <br /> drawn by the U.S. Weather Bureau. Table 1 summarizes precipitation data for <br /> both the North And South sections of the watershed. <br /> Outflow from the system will occur by surface flows, groundwater flows, evapo- <br /> transpiration, and diversions. Various techniques exist to determine how much <br /> evapotranspiration occurs. For this study, the technique proposed by Hamon <br /> (1961) was utilized to determine a value for potential evapotranspiration (PET) . <br /> The advantage of utilizing Hamon's equation is that the value determined for <br />