2017-03-23_PERMIT FILE - C1981019A

• Permit No: C1981019A
• IBM Index Class Name: Permit File
• Doc Date: 3/23/2017
• Doc Name: Permits
• Section_Exhibit Name: Volume 15 Rule 2
• Media Type: D
• Archive: Yes

RULE 2 PERMITS
Some of the seepage from the pit into the backfill may accumulate against the highwall of the pit since the
permeability of the unmined bedrock units is expected to be lower than that of the backfilled spoil
material. The amount of water that accumulates will depend on the quantity of water available in the pit
and the rate that the bedrock groundwater system recovers after dewatering wells are progressively turned
off as mining advances up -dip from north to south. Once wells are turned off, groundwater inflow to the
pit backfill may occur from lateral inflow from the bedrock units that are directly intersected by the mine
and from limited upward vertical flow from underlying bedrock units.
Once mining is completed the Collom pit will have a reclaimed surface area of approximately 825 acres
and a pit bottom that dips predominantly toward the north. The low point in the reclaimed pit surface
topography will be at its intersection with Little Collom Gulch at an elevation of approximately 7,300 feet
amsl. During the post -mining period, re -saturation of the reclaimed pit backfill will occur from bedrock
groundwater inflow from the pit walls, infiltration of direct precipitation on the backfill area, seepage of
surface water flowing over the backfill area, and groundwater inflow from the bedrock units underlying
the backfilled pit. The groundwater level will recover in the backfill until pre -mine water levels of 7100 to
7150 ft amsl are reached. These elevations would be below the Little Collom Gulch channel elevation of
7,300 ft amsl. Outflow will occur as bedrock groundwater flow in a down -dip direction to the north. Post
mining backfill static water levels may be elevated at times above pre -mine levels due to the higher
transmissivity of the backfill and infiltration of surface water runoff. It is highly unlikely that backfill
water levels would rise sufficiently to reach a level where a spring would emanate into Little Collom
Gulch.
The pre -mining bedrock groundwater elevation in the northern portion of the pit is in the range of 7100 to
7150 ft based on WMC (2005). This is likely the minimum groundwater level that will be re-established
in the backfill in the northernmost part of the pit. As described above, some re -saturation of the backfill
may occur during mining.
The pre -mining rate of groundwater flow from south to north through the bedrock units in the northern
part of the pit can be estimated based on the measured transmissivity in the bedrock, the hydraulic
gradient and the width of the flow zone, taken to be the east -west distance between the West Fork of Jubb
Creek and Collom Creek. The long-term pumping test reported in WMC (2005) measured a
transmissivity in this area of about 15 ft2/d, with about 10 ft2/d attributed to the F/G sequence and 5 ft2/d
to the bedrock units above the Fab coal. This transmissivity value represents a saturated thickness of
bedrock in the range of 200 ft (from elevation 6950 to 7150 ft). The hydraulic gradient in this area is
measured from wells and piezometers to be about 0.04 ft/ft. The width of the zone is about 10,000 ft.
This results in a pre -mining groundwater flow rate from south to north at the northern pit highwall of
about 50 acre -ft per year.
The hydraulic head in the backfill at the northern wall of the pit should re-establish itself to at least
elevation 7150 ft once equilibrium conditions are reached. At this hydraulic head, the post mining rate of
groundwater flow from south to north out of the backfill will be about equal to the pre -mining flow rate
and the post -mining groundwater flow system down -gradient of the mine will be essentially the same as
the pre -mining system.
The time for the pit backfill to re -saturate to the 7150 ft elevation at the north highwall is estimated based
on the volume of backfill in the pit up to the 7150 elevation and the estimated recharge rate to the
backfill. The bottom of the pit dips upward to the south at about 250 ft vertical distance per 2,000 ft
horizontal distance or at slope of about 0.125 ft/ft. The width of the pit is about 4,500 ft. This results in a
backfill volume of about 1.44 billion cubic feet. At a 20% porosity in the backfill, the volume of water
needed to saturate the backfill up to an elevation of 7150 ft is about 288 million cubic feet or about 6,610
Collom — Rule 2, Page 120 Revision Date: 11/16/16
Revision No.: MR -166