1981-08-20_GENERAL DOCUMENTS - C1981038

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