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REV88289
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Entry Properties
Last modified
8/25/2016 3:10:29 AM
Creation date
11/21/2007 10:41:10 PM
Metadata
Fields
Template:
DRMS Permit Index
Permit No
M1974015
IBM Index Class Name
Revision
Doc Date
9/20/1999
Doc Name
WESTERN MOBILE BOULDER INC PN M-74-015 LYONS PIT PERMIT AMENDMENT 006 ADEQUACY REVIEW
From
DMG
To
WESTERN MOBILE BOULDER INC
Type & Sequence
AM6
Media Type
D
Archive
No
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<br />26. As required by Rule 6.4.7(6), please identify all known aquifers that may be directly or indirectly <br />afFected by the proposed mining and reclamation activities at the Lyons Pit-Rockin' WP operation. <br />The applicant should also characterize the geology and depths of the aquifers as well as pertinent <br />aquifer characteristics, if known. <br />27. On page G-5, the applicant states that in order to ensure that the proposed mining does not adversely <br />impact the prevailing hydrologic balance in the vacinity of the Lyons site, Western Mobile will <br />employ a slurry wall and compaction of backfilled slopes as mitigation techniques. Page G-2 of the <br />application states that ground water recharge occurs from the south and west and that discharge of <br />ground water is into the St. Vrain and the South Branch. At times of low ground water conditions <br />the St. Vrain may be recharging the ground water. Page G-1 states that the site is characterized by <br />flood plain features. Page G-2 also states that the ground water table is typically two to three feet <br />below the ground surface on the average, but it may rise to the surface during the spring and early <br />summer runoff periods or fall as low as six feet below the ground surface during the late fall and <br />winter. Finally, the January 28, 1997 RMC letter to Scott Gudahl states that surface and ground <br />water features north of the valley will be protected by a slurry wall constructed pazallel to St. Vrain <br />Creek and the South Branch. <br />With this information, it appears that there is a strong hydrologic connection between the St. Vrain <br />(and South Branch) and the surrounding flood plain, i.e.: riparian, wetland, subirrigated meadow <br />complex. The applicant will need to explain how the normal and essential hydrologic functions of <br />recharge and discharge for this alluvial-ground water system will occur following installation.of the <br />slurry wall and what long-term impacts or changes to the system might be expected. In particular, <br />will the reclaimed floodplain complex be able to resume a naturally occurring hydrologic <br />interchange with the main stem and south branch of the St. Vrain for the 2200 linear feet of channel <br />passing through or adjacent to the site? Also, the applicant should explain how the upgradient and <br />downgradient ends of the slurry wall will be keyed or configtued into the surrounding alluvium. <br />28. The applicant will need to cleazly identify on an appropriate map the location of all proposed slurry <br />walls. [n addition, the applicant will need to provide specific infotmation regarding the construction <br />of the slurry walls including the depth, thickness, description of bedrock key cuts, construction <br />materials, construction methods, and quality control/quality assurance procedures. The applicant <br />will need to supply cross sectional engineering designs for the features noted above. The applicant <br />will also need to indicate the linear footage of slurry wall to be constructed and provide a time <br />schedule for when various portions of the slurry wall will be completed. <br />29. The applicant has referenced probable impacts to ground water relating to mine dewatering and final <br />reclamation activities via the January 28, 1997 RMC letter to Scott Gudahl. These probable impacts <br />are based on RMC's experience with other pits sealed with either liners or slurry walls. In order to <br />accurately evaluate the impacts to surface and/or ground water at the Lyons Pit-Rockin' WP site and <br />surrounding area, the Division requests that the applicant provide a detailed hydrogeologic <br />evaluation of the expected impacts to surface andlor groundwater during both mining and during <br />final reclamation. This evaluation should include the depth and lateral extent of the cone(s) of <br />depression expected during the various phases of the mine plan when dewatering is employed. The <br />evaluation should also include an inventory of all wells (regardless of use) that occur within the <br />cone(s) of depression and an assessment of the impact expected to each particular well. The well <br />inventory should include the following information for each well: owners name and address, <br />5 <br />
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