2010-12-17_REVISION - C1981019 (127)

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Mine water management 97 Table 7.6 Summary of estimated dewatering rate Dewatering installation c 1 Estimated production rate North side wells (water supply) 60 to 90 Dewatering wells (number varies with mining) 30 to 150 Horizontal drains 10 to 80 Vertical drains 20 to 100 Spoil drains and in -pit sumps 50 to 200 Total 170 to 620 7.3.4 Water balance and water management considerations A preliminary estimate of the average water balance for the Option 2 mine plan has been prepared using the same components as listed in Section 7.2.4 for Option 1. This balance is for the period when the dewatering production rates in Table 7.6 are applicable (i.e., once the mine extends at least 200 ft below the water table). During the first two years of mining, production rates from the dewatering system will be small, with only minor amounts of perched water flowing into the pit. It is assumed that the north side water supply wells will be installed to produce about 60 to 90 gpm of water for mine use. During years 2 through 6 (or longer, depending on the actual advance rate of the mine) water production rates will increase as the mine floor extends below the water table and dewatering wells are added to the advancing face. Once the mine advances to the central portion of the footprint area, water production rates will probably become relatively stable through the remaining mine life. The Option 2 water balance estimate for the period after dewatering conditions have stabilized (i.e., after the mine floor extends about 200 ft below the water table) indicates the following: Water available Total non - contact water: 90 to 240 gpm Total contact water: 80 to 380 gpm Water used Potable water: 5 to 10 gpm Water for dust control: 400 to 600 gpm Maximum surplus: 215 gpm Maximum makeup: 440 gpm 2572 -R2 Colowyo Coal Company Water Management Consultants