2024-05-23_REVISION - M2022009 ‎(2)‎

• Permit No: M2022009
• IBM Index Class Name: Revision
• Doc Date: 5/23/2024
• Doc Name Note: Groundwater Monitoring and Anaylisis Plan
• Doc Name: Adequacy Review Response #2
• From: Civil Resources
• To: DRMS
• Type & Sequence: TR1
• Email Name: RMN
• Media Type: D
• Archive: No

1.0 INTRODUCTION
The Monarch DENM Gravel Mine (Site)is located west of State Highway 60 with Weld County Road 40.5 to the south
and is bounded by the South Platte River to the west and north. More specifically, the Site is comprised of portions of
Section 18, Section 19, and Section 30, Township 4 North, Range 66 West of the 61h Principal Meridian.
The Site is permitted through the Colorado Division of Reclamation, Mining, and Safety as a construction aggregate
material mine. The approved mine plan calls for five (5)soil bentonite slurry walled cells that will encompass most of the
Site excluding approximately 15 acres along the eastern side of the Site along Highway 60 which will wet mined and
backfilled with wash fines and waste material. Historically, the Site has been utilized for pivot irrigated and dry land
farming.
This Groundwater Monitoring Plan has been created referencing the"Groundwater Monitoring: Sampling and Analysis
Plan Guidance Construction Materials and Hard Rock Sites" document from the Colorado Division of Reclamation,
Mining, and Safety (DRMS). This plan is intended to replace the"Groundwater Monitoring Plan" letter that was
submitted to DRMS during Adequacy Review and dated June 1St, 2022 and is being submitted as a Technical Revision
JR)to the DRMS 112 permit.
From the date of TR approval, this document will supersede any groundwater monitoring or sampling plans submitted
prior to this final document.
2.0 EXISTING GROUNDWATER CONDITIONS
Groundwater is measured in seven (7) piezometers that were drilled at the Site in October 2021 by Universal Drilling.
The piezometers were constructed with 20 feet of 2-inch slotted PVC pipe at bedrock with 2-inch PVC solid pipe above
continuing to 2.5 feet above surface. As the sonic drill pipe was removed, the borehole annulus was allowed to collapse
with native sand. The remaining open hole was filled with drill cuttings to approximately 5 feet below surface. The top 5
feet of the annulus was filled with bentonite chips. The piezometers were completed with steel monuments to protect the
well casing. Well Construction Reports as well as the Well Permits are compiled in Appendix A and piezometer locations
are shown in Figure 1.
Monthly groundwater elevations have been measured at the Site starting in November 2021 and typically occur at
depths usually ranging from 4.6' to 29.3' feet below surface with shallower groundwater to the north and west of the Site
(see Figure 1). The prevailing groundwater flow direction is subparallel to the South Platte River, north-northwest
reflecting the Site topography. Groundwater in the area is tributary to the South Platte River located west and north of
the Site. The observed hydraulic gradient across the Site was approximately 0.4%.
The depth to groundwater during the initial measurement period varied from season to season and annually. These
variations are typical for alluvial aquifers and are due to a multitude of factors including river conditions, precipitation, and
irrigation. The hydrographs presented below depict the monthly changes in groundwater elevation from 2021 to the end
of 2023. During the monitoring period groundwater elevations were lower in the late winter/early spring and peaked in
later summer/early fall which is typical for the area.
Higher groundwater elevations than those observed during this monitoring period would have been expected as a result
of historic agricultural irrigation in the area.
Monitoring and Analysis Plan—Monarch DENM Gravel Mine -1 - March 2024