GENERAL35604

• Permit No: C1981038
• IBM Index Class Name: General Documents
• Doc Date: 7/21/1988
• Doc Name: PROPOSED DECISION & FINDINGS OF COMPLIANCE FOR RN1
• Permit Index Doc Type: Findings
• Media Type: D
• Archive: No

• -36-
All of the mine discharges will enter the North Fork
The impact to the river was assessed by determining
dissolved solids concentration in the river using a
fie downstream concentration (Cd) is determined by:
CuQu + C1Q1 + C2Q2 + C3Q3 + ... CnQn
Cd =
Qu +Q1 + Q2 + Q3 + ... Qn
of the Gunnison River.
the increase in the total
standard mixing equation,
where Cu is the upstream TDS concentration (mg/1 ), Qu is the upstream
discharge (LFS), CI...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 1. 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.
The 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 LFS, the average minimum monthly flow (1968-78) of 66 CF5 and the average
flow for the period of record (45.2 years) of 432 CFS. The downstream flow
and 7DS concentration was calculated for each flow (Table 8).
fie 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 projected impact. If more reasonable assumptions were made based on
a more extensive database a more 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 LFS) and the TOS 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.
CJ