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GENERAL40527
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Last modified
8/24/2016 7:59:42 PM
Creation date
11/23/2007 10:44:03 AM
Metadata
Fields
Template:
DRMS Permit Index
Permit No
C1980005
IBM Index Class Name
General Documents
Doc Date
8/10/1981
Doc Name
PROSOSED DECISION AND FINDINGS OF COMPLIANCE
Permit Index Doc Type
FINDINGS
Media Type
D
Archive
No
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l,'- <br />• • <br />VII. HYDROLOGIC IPfPACTS - GROUNDfdATER <br />(JUANTITY <br />The applicant has calculated the amount of groundwater inflow into the Wadge and Wolf <br />Creek pits on page 7-122. Assumptions used to arrive at these figures are listed <br />on page 7-120. The amount of inflow anticipated is extremely Low, due to the low <br />transmissivities of the strata affected, and because most of the inflow is expected <br />to evaporate. The inflow into the Wadge pit is estimated to be .82 acre ft./day. <br />The calculated dewatering tittn for the area of influence for each 120' x 120' Wadge <br />pit is 2Z days. Therefore, the total inflow into each Wadge pit should he 1.722 <br />acre feet. The inflow into the Wolf Creek pit is estimated to 6e 7.26 x 10-5 acre <br />feet/day. The dewatering time for the area of influence for each Wolf Creek pit <br />is so high that it will never be reached during the life of the mine. The active <br />pits at Seneca II are generally dry, and groundwater inflow has never been a problem. <br />Mining of the Seneca II site will change infiltration rates. Spoil underneath the <br />replaced topsoil will have permeability higher than the pre-mine sandstone due to <br />the increased volume of fractures. Compaction by machines used for regrading will, <br />however, temporarily decrease the infiltration of precipitation into the spoil and <br />will slightly increase surface water runoff. Infiltration and permeability will <br />increase from reclamation year I to year 5 as the vegetation density increases. <br />Reestablishment of soil structure will allow infiltration and permeability rates <br />to approach and possibly exceed pre-mine rates after 4 to 5 years. <br />Post-mining flow rates through spoil should be less restrictive than natural con- <br />ditions since there are no impermeable layers. Flow out of the spoil will depend <br />upon the pit base gradient, the composition of the spoil i.n the pit base, and <br />the hydraulic conductivity of the pit base material. In general, the pit base will <br />act as an impermeable layer causing water to move down di p. <br />Saturation of this reconstructed aquifer at the base of the spoil may not occur, <br />due to the steep grade and the lack of a barrier at the foot of the slope. Currently, <br />recharge infiltrates downward until it reaches the pit base, travels down gradient, <br />and is discharged. Data from a U.S.G.S. lysimeter study, indicates that the spoil <br />receives recharge during the spring snowmelt and retains a large portion of the <br />water through t1~e dry summer months. As shown on the potentiometric map (Exhibit 7-2), <br />the spoil and the strata at higher elevations is unsaturated, and are recharge areas. <br />Prior to mining, the aquifers in the area were under semi-confined conditions dis- <br />charging into structural lows (i.e. Hayden Syncline) rather than topographic lows <br />(i.e. Grassy Creek). After reclamation, the aquifer is unsaturated, flows down <br />gradient and discharges at topographic lows. The change is evident in LitCle Grassy <br />Creek. Directly upstream from the mining operations, Little Crassy Creek is ephemeral, <br />averaging only 6.9 gpm between 9/80 and 9/81 with no flow recorded during the sumrm r <br />months. As it passes through the mining operations and is discharged at fJPDES 002, <br />it becomes perennial, averaging 392.9 gpm and maintains flow during the dry months. <br />As mining progresses around the Tow Creek Anticline, the spring Located on the <br />unnamed tributary to Fish Creek will be destroyed. However, the same reconstructed <br />aquifer conditions are expected to occur in Fish Creek Basin as in Grassy Creek <br />Basin. Therefore, groundwater discharge should increase in Fish Creek Basin as a <br />result of increased permeability and unsaturated flow. <br />
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