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~_ ~~ -l_ra'1 f -45- ~ ~/~~II-UU The effects of the Eagle No. 5 and No. 9 Mines were obtained, in part, from the applicant's permit application. Some effects, not protected by the applicant, were based upon worst-case assumptions. Mine water discharge from the Eagle No. 5 Mine was assumed to remain constant over the life of the operation, discharging at a rate of 4.46 cfs and having a TDS concentration of 800 mg/1. (The Eagle No. 5 Mine has already mined beneath the Williams Fork and Yampa Rivers, and the proposed activities mine away from the rivers). Following the mining operation, some affected ground water would discharge to the stream/alluvial systems. This discharge has not been quantified, but is assumed to be 4.46 cfs as a worst case. (Inflow rates will diminish as the mine workings fill and overlying aquifers recharge.) The quality (TDS) of this discharge is protected to be 1,100 mg/1 based upon observations at the inactive Middle Creek mine which had a minewater discharge quality during operations similar to the Eagle No. 5 Mine. (The Middle Creek mine is also located in a ground water discharge area at the confluence of several perennial streams.) The Eagle No. 9 Mine is still under development, and the applicant has. proposed partial extraction beneath the Yampa River and Big Bottom area. The current discharge rate (250 gpm) per area mined (81 acres) was extrapolated for the life of mine area (1993 acres) to obtain a maximum discharge rate for the Eagle No. 9 Mine in the CHIS. Mine water discharges from the Eagle No. 9 Mine are protected to reach 74.63 cfs. This discharge is expected to have a total dissolved solids concentration of 800 mg/1 during mining. Following mining, some affected ground waters would discharge to the surface water systems. On a worst-case basis, the post-mining discharge is expected to be 14.63 cfs. The dissolved solids (TDS) concentration of this discharge is expected to be 1,100 mg/1. This concentration for TOS is the concentration obtained for post-mining discharges from the Middle Creek mine near Steamboat Springs, Colorado, and spoil water discharges from the Empire No. 1 strip pit. The Middle Creek mine had mine discharge TDS concentrations during operation similar to those in mine discharges from the Eagle Mines. Therefore, the mine discharge TDS concentrations for the Eagle No. 9 Mine is expected to increase to a level similar to that experienced in the Middle Creek mine. The spring discharge from the Williams Fork pit No. 1 has been protected by the applicant to be 0.35 cfs with a TDS concentration of 900 mg/1. Over the long term, the Division has protected a continued deterioration of water quality from this spring. Eventually, the total dissolved solids concentration should be similar to that of spoil water in the area (3,200 mg/1). Using the above values for short-term and long-term effects, and the mass balance equation from the Bishop et al (1982) study, a portion of Table 6 in the Bishop et al. (1982) study was revised (Table 2) to include the effects of the additional mines identified.