GENERAL54873

• Permit No: C1980007
• IBM Index Class Name: General Documents
• Doc Date: 6/23/1981
• Doc Name: Proposed Decision and Findings of Compliance
• Permit Index Doc Type: Findings
• Media Type: D
• Archive: No

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The propsed Mt..Gunnison number one mine will be an underground mine
within the F~coa1 seam. Mining is to progress from the portals in the
valley of the North Fork of the Gunnison River up the dip of the F seam
under the watershed of Minnesota Creek. The extraction of coal will be
by a combination of long wall, pillaring and'room and pillar methods.
Controlled subsidence will occur during Long walling and pillaring.
Development waste rock and processing waste from the Mt. Gunnison mine
will be disposed of in two disposal. piles, a small initial pile located
on the bench below the portals, which will be active during the first five
year permit period, and a larger pile located above the portal bench,
which will be acti-ve until succession of mining.in the F seam.
Load out facilities for the Mt. Gunnison. Mine will be Located along the
North Fork of the Gunnison River at the present site of the Bear Mine.
B - Effects of Mining on Ground Water
As the Mt. Gunnison No. 1 Mine begins operation, waters from the F seam,
saturated beds in the Barren Member immediately above the F seam and some
fractures will be intercepted and appear as mine inflows. As mining in the
F seam expands, it is Zikely that continually greater mine-inflows will be
encountered as the F seam, Barren member and fractures are dewatered. As
larger blocks of the area influenced by mining become dewatered, inflow
into the mine will likely decline until equilibrium is reestablished between
the various components, including mine inflow, aquifer recharged spring
discharge.
To estimate mine inflows, exclusive of spurious inflows encountered in
fracture zones, the applicant assufied the mine to be a single well source
and applied the straight line solution technique of Jacob and Lohman.
Using this procedure, and assuming~'uncoafined conditions in the aquifer,
the applicant determined the maximum inflow to the mine, again excluding
fracture flows, to be 45 gpm by year 5, with the average inflow over the
life of the mine to be 30 to 70 gpm. However, in making this determination,
it is not clear whether all restrictions on use of the Jacob-Lohman equation
were realized. This equation is an approximation valid only when the variable
of integration "u" is Less than .01. Insufficient data and explanation are
given in ~tfie permit to check these calculations. Furthermore, the trans-
missivity, "T", and storativity, "S", values use in these calculations are
not valid, (See section IV. Hydrologic Balance of this permit). To estimate
the extent of the cone of depression caused by mining, a variation of Jacob's
Modified Non-equalibrium equation was utilized. Using a transmissivity of
10 g/d/ft and a storage coefficient of .001 (confined conditions), it was
determined that the cone would expand 0.2 miles in 1 year and 0.4 miles in
5 years. However, in conducting these calculations, the basic assumption
of Jacob's technique (that u ~ .Ol) has been violated. In both the first year
and fifth year determinations, the value of the variable of integration "u"
exceeds the value allowable under the qualifying assumptions for use of the
modified non-equilibrium technique. In addition, the slope and boundary
conditions in the "aquifer" have hot been appropriately addressed. Again,