GENERAL48922

• Permit No: C1980001
• IBM Index Class Name: General Documents
• Doc Date: 10/29/1981
• Doc Name: Proposed Decision and Findings of Compliance
• Permit Index Doc Type: FINDINGS
• Media Type: D
• Archive: No

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alluvial aquifers could occur. No impact is anticip,~tcd on the major bedrock aquifers
in the basin. It is recommended that a ycound water model be developed for the Twenty
Mile Park basin that would allow the Division to make more quantitative predictions
regarding the cumulative hydrologic consequences of mining.
VIII. ALLUVIAL VALLEY FLOORS (AVF) (2.n6.R, 4.24)
The applicant has presented sufficient information to justify that Trout Creek and
Oak Creek are alluvial valley floors. Deposits of unconsolidated alluvial materials
were identified along both creeks, sufficient water is available in both drainages
to support agricultural activities and alluvial wells and vegetative species indicate
the presence of subirrigation. Current crop production on the Trout Creek AVF was
determined to be significant to the individual Farming units in the civinity of
the mine. Land use on the Oak Creek AVF, in the vicinity of the mine, is undeveloped
rangeland and is not significant to farming. Map 2.5-21 of the permit application
delineates the boundaries of both alluvial valley floors.
Trout Creek Findi
1. The proposed surface coal mining operations will not interrupt, discontinue,
or preclude farming on the identified alluvial valley floor.
None of the proposed mining operations would be located within the alluvial valley
floor. Therefore, the AVF itself would not be physically disturbed. The ALT
associated with Trout Creek, is currently being used For irrigation activities, and the
associated hay production was determined by the applicant to be significant to the
individual farming units.
Because the mining limits do not impinge upon the alluvial valley floor boundary,
direct impacts resulting from the physical disruption of alluvial and/or channel
materials will not be realized. However, secondary affects, including the dewatering
of the alluvium and water quality impacts due to the leaching of dissolved salts
from the spoil material, have been identified and have the potential to impact the
AVF. The significance of these secondary impacts are discussed below.
2. The proposed operations will not materially da:naye the quantity and quality
of water in surface and underground water systems that supply those alluvial valley
floors or portions of alluvial valley floors.
As mentioned above, two secondary impacts to the Tmut Creek AVF are possible as
a result of the close proximity of the mining operations to the designated AL'F.
The significance of each of these secondary impacts is discussed accordingly.
Dewa terin4 of the Alluvium
This impact could occur in both the !Jest Ridge and the h[offat Area under those
circumstances where the floor of the box cut exists at an elevation below the water
table elevation in the alluvium. A reverse gradient would thereby exist, allowing
alluvial ground water to enter the pit Following migration through the undisturbed
overburden which lies between the box cut and the alluvium. In the !•toffat Area, the
down-dip limit of mining is approximately 400 feet from the edge of the alluvium;
in the West Ridge area, this distance is approximately 50 feet. The applicant has
quantified the amount of water which could enter the pits via this mechanisum (see
page 2.5-92 of the permit application). In the [Jest Ridge area, the rate of inflow
would be about 29 gpm, while in the Moffat Area, the figure drops to about 16 gpm.