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2016-08-08_GENERAL DOCUMENTS - C1981019
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2016-08-08_GENERAL DOCUMENTS - C1981019
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Last modified
8/24/2016 6:25:34 PM
Creation date
8/9/2016 8:42:50 AM
Metadata
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Template:
DRMS Permit Index
Permit No
C1981019
IBM Index Class Name
General Documents
Doc Date
8/8/2016
Doc Name
Proposed Decision and Findings of Compliance
From
DRMS
To
Tri-State Generation and Transmission
Permit Index Doc Type
Findings
Email Name
JRS
RDZ
Media Type
D
Archive
No
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<br /> <br /> <br /> <br />PR-04, Colowyo Coal Mine C1981-019 43 August 2016 <br /> <br />the effects of mining by approximately 450ft of vertical separation including a laterally <br />continuous impermeable smectite layer (the KM). In order to verify the prediction of no impact, <br />a monitoring well will be drilled to the Trout Creek Sandstone, as dictated by the following <br />stipulation: <br /> <br />STIPULATION 18: COLOWYO COAL COMPANY WILL CONSTRUCT A <br />GROUNDWATER MONITORING WELL COMPLETED IN THE TROUT CREEK <br />SANDSTONE. FOLLOWING COMPLETION OF THE WORK, A MINOR REVISION WILL <br />BE SUBMITTED TO UPDATE THE PERMIT APPLICATION PACKAGE WITH A <br />COMPLETE DESCRIPTION OF THE WELL, AND TO UPDATE TABLES 2.04.7-38, -39, -40 <br />AND -41, AND MAPS 10B AND 11C. THE REVISION WILL BE SUBMITTED NO MORE <br />THAN 90 DAYS AFTER WELL COMPLETION. THESE ACTIONS MUST BE <br />COMPLETED BEFORE ANY DISTURBANCE ASSOCIATED W ITH THE COLLOM PIT <br />BOX CUT IS ALLOWED TO OCCUR. <br /> <br /> <br />No impacts to other water users are expected, since all are well beyond the predicted cone of <br />depression from pit dewatering. <br /> <br />Following mining the pit will be backfilled with spoil material. Compared to the surrounding <br />bedrock, the backfilled pit will be more isotropic and more permeable, which will make the unit <br />as a whole more transmissive of groundwater. This will result in a reduced saturated thickness <br />for an equivalent groundwater flowrate, and, in general, a relative lowering of the groundwater <br />level. Since the bottom of the pit slopes to the north, and flow out of the pit to the north will be <br />constrained by the lower permeability of the undisturbed bedrock units, the water level is <br />expected to re-establish to pre-mining levels (approximately 7150ft above sea level) in 30-130 <br />years, depending mainly on the rate of infiltration. The development of a spoil spring is unlikely. <br />These predictions will be tested against data from a future monitoring well to be drilled into the <br />spoil at the lowest point. <br /> <br />Continued operations in the active pits from 1981 to present support the assertion that little water <br />exists above the coal strata to be mined. What water does exist has only been experienced under <br />perched conditions. Seeps encountered during operations have been consumed by the face or <br />evaporated from the floor of the pit. Excavation to the lower coal seams has produced dry <br />conditions, indicating there is no lateral or vertical continuity in the connection between the Iles <br />and the Williams Fork Formations. The probability of aquifer cross-contamination is therefore <br />negligible. <br /> <br />The quality of ground water within the permit site has been rated poor by the USGS and <br />designated for limited agricultural use only. The chemical composition of the area's water was <br />tested by the USGS in 1978 and found to be somewhat saline, alkaline and hard (see pages
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