2018-05-18_PERMIT FILE - C2009087A ‎(3)‎

• Permit No: C2009087A
• IBM Index Class Name: Permit File
• Doc Date: 5/18/2018
• Doc Name: Mitigation of the Impacts of Mining Operations
• Section_Exhibit Name: 2.05.6 Mitigation of the Impacts of Mining Operations
• Media Type: D
• Archive: Yes

RULE 2 - PERMITS
drainages. The potential flow increases will also be partially mitigated by operation of
sedimentation ponds that will retain disturbed area runoff, providing some buffering effect.
Although sedimentation ponds are integral to mitigating mining- related impacts on the surface
hydrologic system, operation of sedimentation ponds tends to reduce discharge flow volumes and
extend the period of effective flow for runoff from both snowrnelt and thunderstorm events. In
effect, sedimentation ponds function as limited capacity flood control structures reducing the
effective discharge rate for large volume flows through temporary storage and flow routing. Pond
surfaces are additionally subject to evaporation losses. With the exception of any ponds designed as
non - discharging structures (i.e., the Portal Sumps), the sedimentation ponds are designed to
gradually release impounded runoff following required detention for sediment control. Given
provisions for gradual discharge of any retained stonn flows, most of the runoff is returned to the
surface drainage system with only a short lag time corresponding to the design detention time for
each pond. Containment ponds are so small as to have a negligible effect relative to any reduction
in flow volume for the receiving drainage. Pecoco Pond and Ponds 002, 003, and 004 are already in
place, and permitted as permanent impoundments. Their effects on runoff timing and volumes are
reflected in the baseline conditions described in Section 2.04.7.
Operation of the drainage and sediment control network, and specifically the sedimentation ponds
that retain runoff from disturbance areas, has the potential to alter seasonal flow patterns in
receiving drainages. Because PSCM is located in a semi -arid climate, runoff contributions to most
of the drainages occur only as a result of snowmelt and large storm events. For the intermittent and
ephemeral drainages, any delayed discharge of runoff flows will represent a positive impact,
potentially extending the period of active flow. Minor alteration of the timing of runoff flows is
significant only if the delay and the corresponding volume of retained runoff reduces flows during
critical low flow periods or if the deferral of runoff discharge involves significant time delays. The
retained runoff volume for any of the proposed sedimentation ponds represents a relatively small
percentage of normal flow volumes for the receiving drainages and should not adversely impact
flow volumes even during low flow periods. As detailed in Exhibit 2.05.3 -E2, Drainage and
Sediment Control Plan, the Upper and Lower Sumps have a capacity of 1.2 ac -ft and 1.07 ac -ft
above their respective principal spillway inverts respectively, and receive runoff from 926 acres
and 1039 acres respectively, or 31% and 34% of the Little Grassy Creek watershed. The two Portal
Sumps are non - draining, can store up to 2.2 ac -ft, and receive runoff from only 13.2 acres (0.42 %
of the Little Grassy Creek watershed). Portal sump water will be used for underground mine
operations, or be pumped to the Coal Stockpile Sumps, and ultimately drain to Pond 002. By
comparison , the annual runoff leaving existing Pond 002 was 364 ac -ft in 2007, and 822 ac -ft in
2008. Additionally, operation of the sedimentation structures will not result in significant discharge
delays since the maximum design detention time does not exceed three days for the design storm
runoff volume and discharge will be occurring continuously once pond levels reach the design
discharge elevation for each pond. Exhibit 2.05.3 -E2, Drainage and Sediment Control Plan,
provides demonstration of the maximum design detention time.
The ponds have been designed such that discharge structures are above the design sediment level
and any runoff accumulations in the pond below this discharge elevation will be retained in the
pond as dead storage (note that dead storage will not affect pond capacity since the design sediment
storage capacity is being utilized for dead storage). The design sediment storage volume available
for temporary runoff storage represents a negligible amount relative to area stream discharge
volumes and so does not constitute a significant potential flow reduction. Note that under present
(pre- underground mining) conditions, Pond 002 operates with a permanent pool at or above the
TR -04 2.05 -87 Revision 04/12