1981-08-20_GENERAL DOCUMENTS - C1981038

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-24- For the ARCO Mt. Gunnison Mine, the worst case scenario would be if 224 acre- feet per year use for spraying the coal and 150 acre-feet per year use for potable water supply were entirely removed from the basin. On the average, this would amount to approximately 32 acre-feet of water each month. From Table 3, it appears (from column 5) that the months of August and September will be the critical months for removal of water from the watershed. In reality, most of the 150 acre-feet per year use of potable water will return to the North Fork of the Gunnison River. Consequently, ARCO's main concern will be the 224 acre-feet per year of water lost due to spraying activities. Since this amounts to less than 20 acre-feet per month, with some water being obtained from development of the mine, the net impact to downstream users should be minimal--assuming ARCO provides adequate storage for water to be used during dry months. The Bear, Somerset, and Blue Ribbon permit applications indicate that minor amounts of water may be removed from the basin due to their activities. Water will be removed from the basin through the spraying of coal and evaporation from sediment ponds, but this loss is expected to be minimal. Although avail- able data on these operations is not conclusive and additional monitoring data may be required, it appears that losses to the North Fork of the Gunnison River flows due to these mining operations will be minimal. The West-Central Colorado Coal Environmental Statement was reviewed for possible comparative purposes. However, the data was not used because information was in terms of broad generalizations or theoretical, unsubstantiated predictions. Water Quality Baseline Data The baseline water quality data for the North Fork of the Gunnison River is somewhat difficult to determine. The reason is a .Zack of data as published by the U.S.G.S. However, at the Somerset, Colorado gaging station on the North Fork of the Gunnison River, water quality data have been recorded since October, 1977. The values recorded are instantaneous values and do not reflect a long-term analysis. Table 6 illustrates how water quality parameters vary with respect to low flows, high flows, and flows existing during the middle of irrigation season when water is released from Paonia Reservoir. Impacts on Groundwater The six undergroun.d mines along the North Fork of the Gunnison may impact ground- water quantity by one or more of the following: - induced inflows into the mine through faults and fractures which provide communication between the mine and overlying and underlying aquifers and surface waters; - dewatering of the coal, roof and floo � with advance of mining; and - subsidence induced flows into the mine. 4 ��J)