2021-08-09_PERMIT FILE - C1981019A ‎(5)‎

• Permit No: C1981019A
• IBM Index Class Name: Permit File
• Doc Date: 8/9/2021
• Doc Name: Volume 1 Rule 4
• Section_Exhibit Name: 4.01 Through 4.30
• Media Type: D
• Archive: Yes

RULE 4 PERFORMANCE STANDARDS

Rule 4 Performance Standards 4-15 Revision Date: 12/21/20
Revision No.: TR-143
under the sandstones. No toxic concentrations of acid forming materials have been found in the
overburden, reclaimed slopes or surface and ground water system associated with the Colowyo
Mine. No adverse effects on groundwater quality are expected to occur due to mine excavations
or backfilling.

4.05.12 Protection of Groundwater Recharge Capacity

The reclamation plan as described in 2.05.4 will return the disturbed lands to approximately the
pre-mining condition; therefore, ground water recharge capacity is expected to approximate the
premining condition. Also, because of the minimal existence of ground water in the mining area,
the mining operation and subsequent reclamation should have no adverse effect on the existing
ground water recharge capacities.

The ground water monitoring plan is further documented in 4.05.13. The mine has established
Point of Compliance locations for alluvial and valley fill aquifers. Please refer to Volume 2C,
Exhibit 7, Item 19 for a description of the alluvial aquifer investigation report and the points of
compliance wells for Goodspring, Taylor, and Wilson Creeks. There are not established points of
compliance for any regional aquifers because of a lack of ground water. The following is provided
to document this:

The aspect of monitoring ground water is dependent on whether or not there is a
continuous, non-perched ground water layer/zone to monitor. Since active mining at
Colowyo Mine began 30 years ago, the mining zone in both the East and West Pits have
not encountered any significant ground water, except for perched ground water. Therefore,
the following is presented to clarify the ground water conditions at the Colowyo Mine.

Geology/Topography

The Colowyo Mine is located in the Williams Fork Formation of the Cretaceous Mesaverde
Group. The Williams Fork Formation is comprised of discordant beds/units of sandstone,
siltstone and mudstone and coal seams, with an approximate thickness of 1,200 to 1,300
feet in the mine area. Deposited in a deltaic environment, the beds vary in thickness and
lateral extent throughout the Colowyo Mine. The numerous coal seams also vary in
thickness and lateral extent. The sandstones tend to be very fine grained to fine grained
and poorly sorted, with various amounts of silt and clay. For the siltstones and mudstones,
theses units contain various amounts of finer and coarser materials. The total mined
sequence in both pits is up to 450 feet thick and is comprised principally of mudstones,
siltstones and coals, with sandstone layers being least prevalent.

The Williams Fork Formation conformably overlies the Iles Formation. At the top of the
Iles Formation is the Trout Creek Sandstone (TCSS). The TCSS is a massive, white to
light gray, very fine to fine grained, moderately well sorted sandstone with a thickness of
between 50 and 70 feet and is approximately 1,200 to 1,300 feet below the Colowyo Mine.
This is the only mapped continuous unit in the area of the Colowyo Mine and has been
noted as being an excellent marker bed for correlation work of the coal seams. Beneath
the TCSS, the Iles Formation is comprised of sandstones, siltstones and marine shales.