PERMFILE112544

• Permit No: C1981022
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Doc Name: Cumulative Hydrologic Impact Analysis ‎(CHIA)‎
• Section_Exhibit Name: Exhibit 2.04-E4 Part 10
• Media Type: D
• Archive: No

All of these coal mines have life-of-mine areas within a 247-square-mile
subwatershed of the 978-square-mile North Fork of the Gunnison River drainage
basin and have been considered as the mines in the general area for this
•
DESCRIPTION OF THE HYDROLOGIC ENVIRONMENT
study. All mining operations in the general area are underground operations
with the exception of Terror Creek Loadout which is strictly a loading
facility. The life-of-mine areas of these mines are plotted on Figure 1.
This assessment is divided into the three major subsections: Description of
the Hydrologic Environment, Probable Hydrologic Consequences of the Somerset
Mine, and Cumulative Hydrologic Impact Assessment.
The first part of this assessment describes the hydrology of the drainage
basin of the North Fork of the Gunnison River and the mining activity
occurring in the basin. This includes: 1) a description of the regional
geology; 2) a description of the ground water regime; 3) a description of the
surface water regime; and 4) a discussion of precipitation and runoff in the
basin.
REGIONAL GEOLOGY
The Somerset Coal Field lies on the southeast margin of the Piceance Basin and
just south of Grand Mesa. The sedimentary strata exposed in the Somerset Coal
Field dip at 3 to 5 degrees to the north and northeast, and range in age from
late Cretaceous to early Tertiary.
.` Coal is produced from the Mesaverde Formation, a 2,500-foot-thick sequence of
sandstone, shale and coals overlain by the Ohio Creek conglomerate and
underlain by the Mancos Shale (Figure 2>. The Mesaverde Formation is composed
of four members in order of decreasing age - the Rollins Sandstone, the Lower
and Upper Coal members and the Barren member.
Only minor faulting of limited vertical displacement has been observed in the
existing Blue Ribbon, Bear and Hawk's Nest Mines. However, in the Orchard
Valley Mine, a fault with a displacement of fifty (50> feet was encountered
during mining and drill hole data indicates the presence of-other faults in
the life-of-mine area with similar displacements. One major fault has been
encountered in the Somerset Mine. The faults which have been encountered in
existing mines tend to be high-angle normal faults.
The steep slopes of the stream valleys and the instability of the rock strata
in the North Fork drainage basin have contributed to numerous landslides, mud
flows and rock falls. These mass wasting features have been mapped by
W.R. Junge of the Colorado Geological Survey and published as an open file
report entitled "Geologic Hazards, North Fork Gunnison River Valley, Delta and
Gunnison Counties, Colorado."
Geologic units exposed in the North Fork Drainage Basin consist of Late
Cretaceous to Early Tertiary Age sedimentary strata, Tertiary Age igneous
intrusives, and Quaternary Age alluvial and coliuvial deposits. The units of
the Late Cretaceous in the general area are described below in ascending
order. A stratigraphic coiumn representing the geology of the coal member of
~~ ~ the Mesaverde Formation in the permit area can be found on Figure 2.
-2-