GENERAL55781

• Permit No: C1981016
• IBM Index Class Name: General Documents
• Doc Date: 3/14/1983
• Doc Name: PROPOSED DECISION AND FINDINGS OF COMPLIANCE
• Permit Index Doc Type: FINDINGS
• Media Type: D
• Archive: No

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Two other possible surface water effects: increased infiltration in the
areas overlyiny undergrouno workings and increased erosion from disturbed
areas, were briefly considered but then deleted from further analysis.
Although subsidence might logically increase infiltration rates in an
undermined watershed (and therefore decrease runoff), the Division is
unaware of any literature documenting that this effect actually occurs.
Surface flow from disturbed areas would carry higher sediment
concentrations than natural runoff. However, kule 4.05.2 requires that
all disturbed flow pass through a properly designed sediment control
pond. Effluent from the pond is required to meet NPDES limitations prior
io discharge. During operation, monitoring and inspection by the
Division ensures that the sediment control system is maintained to
function as designed. At the close of operations, surface disturbances
would be reclaimed and the sediment control system would be removed
following the ensuing liability period. As a result, the increased
sediment load carried by disturbed flows would not be expected to reach
receiving streams.
Discharge of Mine Water from Underground Workings
Underground mines in the North Fork area are generally dry. The only
operation to discharge mine water from the underground workings on a
continuing basis is the Somerset Mine. The Somerset Mine discharges this
water into Bear Creek at a site approximately 3/4 mile upstream from its
confluence with the North Fork of the Gunnison River. The current pe rniit
application anticipates a discharge rate of 115 gpm (0.25 cfs). keview
of NPDES monitoring reports on file with the Division indicate that the
discharge has averaged 107 gpm (0.24 cfs) over the past 3 years. The
data indicate that the discharye rate has increased over the past 2
years. It is unknown whether or not this trend will continue.
Since Bear Creek is an ephemeral stream, it is appropriate to consider
the effects of this discharge on the North Fork of the Gunnison. The
mass balance - mixiny model (report by Water, Waste, and Land, Ltd.,
1980) presented below was used to estimate the downstream concentrations.
QuCu + QmCm
Cd= Qu + Qm
where Co =downstream concentrations in the river,
Cu = upstream concentration,
Qu = upstream river discharge,
Qin = mine discharge, and
Cm = concentration in the mine discharge.
U.S. Steel Somerset anticipates a mine discharge of 0.25 cfs, and NPDES
records indicate that past discharges have averaged a total dissolved
solids concentration of approximately 3,500 mg/l. Data from the U.S.G.S.
gaging station at Somerset shows the lowest monthly mean flow of the
North Fork of the Gunnison kiver to be 45.2 cfs for the water year 1979.
Using this low flow value for Qu, a very conservative estimate can be
made. Water quality data from the North Fork that same year (1979) show
an average TDS concentration of 95 mg/l.