2022-12-09_PERMIT FILE - M2022001 ‎(2)‎

• Permit No: M2022001
• IBM Index Class Name: Permit File
• Doc Date: 12/9/2022
• Doc Name Note: 6th Response
• Doc Name: Adequacy Review Response
• From: Civil Resources, LTD
• To: DRMS
• Email Name: PSH
• Email Name: JLE
• Media Type: D
• Archive: No

CIVIL RESIVURCES, LLC
ENGINEERS & PLANNERS
MEMORANDUM
TO: Robert Lembke
Red Tierra Project File—Section 20 Gravel Mine
FROM: Civil Resources, LLC
DATE: October 14, 2022
RE: Modifications to Model in Response to DRMS Comments
This memorandum summarizes the modifications to the Section 20 Groundwater Model Report dated August 2022 in response
to a Division of Reclamation Mining and Safety (DRMS) comment that the Monarch DENM Gravel Mine, west of the site, be
included in the Section 20 groundwater model (Adequacy Review dated September 12, 2022). Model modifications include:
➢ Added the Monarch DENM Gravel Mine slurry walls to the mitigation scenarios.
➢ An underdrain was included on the Monarch DENM Site.
➢ Due to the additional obstruction,the eastern mitigation system of an underdrain or dewatering wells pumping 2.5-3.25
cfs was modeled to be constructed at the same time as the Cell 4 slurry wall rather than during the construction of the
Cell 5N slurry wall.
➢ Approximately 1,900 feet of underdrain was modeled to be constructed at the same time as the Cell 6 slurry wall to run
between the Cell 6 slurry wall and Weld County Road 42. This drain was sized to remove 3.3 cfs from the model via
discharge to the river which will require coordination with the Monarch DENM ownership group.
The recommendations presented in the August 2022 report mitigated the groundwater mounding modeled to occur with full
build-out of Section 20 slurry walls and the related rise in groundwater in the area of the Town of Gilcrest east of the Section 20
Site. The addition of the Monarch DENM resulted in further obstruction of groundwater flow around the west side of Section 20
resulting in additional regional mounding that requires a second underdrain system that conveys the flow to the river and limits
the upstream groundwater rise via a westward draining underdrain beginning near the southwest comer of the Section 20 Site
and carrying approximately 3.3 cfs to the South Platte River(approximately 1,900 feet long).
Attachments:
- DRMS Response Letter
- Revised Groundwater Model Report
JIUnited Water&San-1411Sechon 201DRMSIGroundwate6MEMO Groundwater Model Modifications docx
8308 COLORAD❑ SLVD, SUITE 200 0 FIRESTONE, COLORADO SOSO4 9 PHONE: [phone]