2024-10-28_REVISION - M2022009 (3)

Home Browse Search

CIVIL RESOURCES, LLC ENGINEERS & PLANNERS October 9, 2024 Colorado Division of Reclamation and Mining Safety Mr. Patrick Lennberg 1313 Sherman Street, Room 215 Denver, CO 80203 RE: Technical Revision 2—Monarch DENM Mine(Permit M-2022-009),Weld County, Colorado Dear Mr. Lennberg: Monarch Mountain Minerals and Aggregates, LLC (MMMA) requests a Technical Revision 2 (TR-02) to Permit M-2022- 009 to revise the mining plan, reclamation plan, and associated maps to add an additional slurry wall lined cell north of Weld County Road 42 (WCR-42). Background The Monarch DENIM Mine was initially approved with four (4) slurry wall lined cells and three (3) unlined cells. After approval of the 112 permit the property owner negotiated an agreement with PDC Energy for an oil and gas pad on the property. The future well pad will be located between Cell 3 and Cell 4. The pad has reduced the size of these two cells. In order to offset the reduction in mined material, MMMA is proposing to slurry wall line the stockpile area north of WCR-42. Proposed Changes to the Mine Plan The additional slurry wall lined cell will be the third slurry wall constructed and will be referred to as Cell 26 (see attached Map C-5). The construction of the slurry wall is approximately 10 to 11 years out from being constructed and will be approximately 5,187 linear feet. Assuming approximately fifty (50) feet to bedrock and a five (5) foot key the approximate curtain area of the proposed slurry wall is 285,285 square feet. The proposed Cell 4 slurry wall will be reduced in length to approximately 7,036 linear feet. Assuming the same depth to bedrock and key the new Cell 4 slurry wall curtain area is approximately 386,980 square feet. An underdrain has been added to the southeast corner of the Cell 2B slurry wall. The underdrain is approximately 970 linear feet of 12-inch slotted pipe. This additional underdrain is still approximately 10 to 11 years out from construction and will be bonded prior to construction. Changes to the Groundwater Model The Cell 2B slurry wall was added to the existing groundwater model for the Site and the Section 20 Gravel Mine. The additional impervious area resulted in a slight rise to the southeast of the cell, approximately 0.8 feet at PZ-20 from the previous mitigation scenario. This is above the mitigation cut-off set in the Groundwater Monitoring Plan. In order to mitigate this additional rise an additional section of underdrain was placed along the eastern corner of Cell 2B. This underdrain will need to be able to intercept and carry 1.1 cubic feet per second (CFS) of groundwater. The underdrain will flow towards a manhole where it will join the flow from the Cell 3 underdrain. From the manhole the water will discharge to the unnamed slough which flows to the South Platte River(see attached maps TR-1 and TR-2) Table 1 (below) depicts the groundwater level at the surrounding piezometers premining base condition and compares them to the fully constructed Site with Cell 2B slurry wall and underdrain (Model 2). As shown in Table 1,the rise from the base condition with the addition of the Cell 2B slurry wall and underdrain is less than 2 feet at the monitoring well 8308 COLORADO BLVD 9 SUITE 200• FIRESTONE, CO 80504 • PHONE: 303.833.1416