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-48- Cumulative Hydrologic Impact Assessment - Surface Water (Cont'd) For the ARCO Mt. Gunnison No. 1 Mine, the worst-case scenario would be if the 150 acre-feet per year of water used fcr spraying the coal and for the potable water supply were entirely removed fz~om the basin. On the average, this would amount to approximately 12.5 acre-feet of water each month. From Table 9, 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 water used for potable water will be returned to the North Fork of the Gunnison River. Consequently, ARCO's main concern will be the water lost due to spraying activities. ARCO will store water diverted from the North Fork in a storage pond adjacent to the river. The total estimated worst-case depletion (150 acre- feet per year) would amount to only 0.04 of the mean annual flow of the North Fork. (See materials submitted by ARCO Coal Company, December I5, 1981, in response to permit stipulation No. 7.) U.S. Steel has not submitted an estimate of the total water consumption in the Somerset Mine. The Division, therefore, has made the assumption that water consumption at the Somerset Mine is simu:lar to the proposed maximum water consumption i-n the Mt. Gunnison No. 1 Mine. This assumption is based on the close proximity of the mines, the similarity in mining methods, coal handling, manpower requirements, and total annual coal production. This estimated water consumption would amount to 150 acre-feet per year, or 0.041e of the mean annual flow in the North Fork of the Gunnison. The total estimated water consumed by all of the mines presently operating in the North Fork Drainage Basin is 556 acre-feet per year. This is approximately 0.14a of the mean annual flow of the river. This indicates that the total coal mining activities within the North Fork of the Gunnison Drainage Basin has a very minor impact on the quantity of water available in the Basin. Cumulative Impacts of the Blue Ribbon and Somerset Mines Since all anticipated mining at the Blue Ribbon Mine is within the Hubbard Creek drainage basin, the most significant cumulative impacts of all anticipated mining will be the combined impacts of the Blue Ribbon Mine and Somerset Mine on Hubbard Creek and on the downstream water users. Data collected for the Somerset Mine estimates the annual yield in Hubbard Creek to be 29,700 acre-feet/year (8~ of the annual yield of the North Fork of the Gunnison). The total depletion of the flow in Nubbard Creek due to the combined impacts of the Somerset and Blue Ribbon mines may be 0.09 cfs (39 gpm or 56,160 gpd), 0.07 cfs (31 gpm or 44,640 gpd) coming from inflow measure in the Somerset Mine adjacent to or under Hubbard Creek, and 0.02 cfs (8 gpm or 11,520 gpd) coming from withdrawal of water by the Blue Ribbon Mine. This total depletion amounts to about 0.18% of the average stream flow (49.3 cfs) and 3.90 of the los/ flow (2.23 cfs) in Hubbard Creek as measured above the Blue Ribbon Mine. Cumulative Hydrologic Effects - Finding An assessment of the probable hydrologic consequences of the proposed mining oper- ation and an assessment of the probable cumulative impact of all anticipated mining in the area on the hydrologic balance has been made. Pursuant to Rule 2.07.6(2)(c), the Division finds that the proposed operation has been designed to prevent material damage to the hydrologic balance outside the proposed permit area.