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-36- All of the min The impact to dissolved soli The downstream Cd = discharges will enter the North Fork of the Gunnison River. he river was assessed by determining the increase in the total s concentration in the river using a standard mixing equation. concentration (Cd) is determined by: CuQu + C1Q1 + C2Q2 + C3Qg + ... CnQn Qu +Q1 + Q2 + 43 + ... Qn Where Cu is the upstream TDS concentration (mg/1 ), Qu is the upstream discharge (CFS), C1...Cn are the respective TDS concentrations of the mines contributing water to the river (mg/1) and QI...Qn are the respective discharges to the river (CFS). Data for the mines in the area used in the equation are presented in Table 7. The upstream TDS concentration (Cu) of the North Fork of the Gunnison River was assumed to be 95 mg/1. fie impact on the river was assessed at four different flows, a ten-year, seven day (Q7_10) of 32.5 CFS, the average minimum daily flow (1968-78) of 47.6 CFS, the average minimum monthly flow (1968-78) of 66 CFS and the average flow for the period of record (45.2 years) of 432 CFS. The downstream flow and TDS concentration was calculated for each flow (Table 8). The assessment indicates that the water quality would be degraded for all flows but the average flow. The assessment indicates that no significant impacts will occur on the North Fork of the Gunnison at the maximum extent of mining and no water uses will be precluded. Several conservative assumptions were made regarding contributions from the mines. The mine discharges and TDS concentrations were conservatively estimated and, therefore, the actual impact to the river will probably be less than the pro,)ected impact. If more reasonable assumptions were made based on a more extensive database amore precise estimate of the impact to the river could be made. Two parameters that could be changed in the assessment are the quantity of effluent from the Mt. Gunnison No. 1 Mine and the quality of the effluent from the Hawk's Nest Mine. If the quantity of discharge from the Mt. Gunnison No. 1 Mine is reduced to 719 gpm (1.605 CFS) and the TDS of effluent from the Hawk's Nest Mine is reduced to 2500 mg/1 the impact to the river will be reduced. The reduction of these values is reasonable; 719 gpm is the maximum anticipated inflow rate at the Mt. Gunnison Mine, while a TDS concentration of 2500 mg/1 is probably more representative of mine discharge water than the 7020 mg/1 value recorded after the water has been cycled through the wash plant at the Hawks Nest Mine. If these two assumptions are made the impact to the river is reduced (Table 9). No significant impacts to the system are anticipated and no water uses will be precluded on the North Fork of the Gunnison in this probable development scenario. I