2025-05-05_PERMIT FILE - C1981010 ‎(24)‎

• Permit No: C1981010
• IBM Index Class Name: Permit File
• Doc Date: 5/5/2025
• Doc Name: AOC Variance Summary
• Section_Exhibit Name: Appendix B AOC
• Media Type: D
• Archive: No

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dominated by sagebrush and grass before mining. All areas designated for cropland have been
previously reclaimed. Areas of the L Pit and Ash Pit would be reclaimed to approved postmining land use
of rangeland open to grazing by both domestic species and wildlife. The AOC variance request does not
request an alternate land use post mining or changes to the approved revegetation for rangeland areas.
The AOC variance would have positive impacts on the post-mining land use. It is well known that
steepness of slope affects cattle distribution during grazing (Mueller 1965) and that the steeper the
slope, the more the cattle will congregate at the toe of the slope or on shallower areas of the slope. The
AOC variance will reclaim the L Pit to overall slopes of 4 to 14° and the Ash Pit to overall slopes of 4
to 14°. These gentler slopes would promote more even cattle distribution and result in a healthier overall
vegetative cover in the long term.
4.0 Watershed Improvements
The Regulations require that variances from AOC demonstrate that the watersheds of the area and
adjacent areas are improved by the post-mining topography variance including showing a reduction in
total suspended solids or other pollutants from pre-mining discharges or reduced peak flows and flood
potential and the total volume of flows during every season of the year will not vary to the extent it has an
effect on the ecology or existing or planned use of the surface water or groundwater.
Trapper contracted with Mattern to review the watershed improvements for the proposed AOC variance
post-mining topography. The Mattern report is contained as Attachment 2 to this document and also will
become part of Appendix B of the Trapper Mine Permit Application Package. The following discussions
provide information on pre- and post- mining watersheds for the L Pit and Ash Pit. The Division has
indicated that in order to make these demonstrations, a comparison of the changes in watershed areas
pre- and post-mining based on the proposed post-mining topography is required and that is included in
the discussion.
4.1 L Pit Watershed Improvements
The L Pit area of interest lies within four post-mine drainages; Flume Gulch (540.3 acres), Deal Gulch
(60.0 acres), and the Deacon and Jeffway Gulches (a combined 4.4 acres). The hydrologic evaluation
focuses on the Flume Gulch drainage since most of the L Pit lies within this drainage. Within Trapper’s
permit boundary there are multiple Flume Gulch sub-drainages referred to as Grouse, Sage, Oak, West
Flume, Middle Flume, East Middle Flume and East Flume. The L Pit includes East Middle Flume Gulch
and Middle Flume Gulch. Areas of the Flume Gulch watershed outside of the L Pit have been affected by
mining operations at the Trapper Mine over the last 30 years. Table 3 shows the changes in watershed
area for Flume Gulch within the Trapper Mine permit boundary. Figure 4 of the accompanying Mattern
report (Attachment 2) shows the similarities between pre- and post-mining drainage patterns.
Table 3 Flume Gulch Sub-Drainage Areas Within the Trapper Mine Permit Boundary
Sub-Drainage Pre-Mining Area (acres)
Post-Mining Area
(acres)
Grouse 207.6 252.3
Sage 188.7 109.5
Oak 378.1 439.1
West Flume 234.5 206.2
Middle Flume 621.7 614.9
East Middle Flume 143.3 419.0
East Flume 103.6 73.3
Total 1877.5 2114.3