2021-09-07_REVISION - M1973021 (4)

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Aggregate Industries, Morrison Quarry Amendment No.7 to Permit M-1973-021 FN Morrison,CO appropriate Best Management Practices (BMPs) in accordance with the Site Stormwater Management Plan (Ver. 6, June 2021). Slopes steeper than 1HAV may not be feasible to install BMPs but will be graded to the nearest stormwater outfall or to the proposed reservoir/dewatering intake point in the South Quarry. Channelizing and capturing surface runoff within the quarry area would further reduce impacts to areas outside of the already impacted mining activity. A diversion channel planned to be installed along the El. 7,040 contour of the South Quarry conveying UD#1 to UD#2 towards Turkey Creek (shown in Exhibit F.3 and Appendix 2) would remain in place following reclamation. This diversion channel is designed to handle a 100-year storm event, and more information regarding its design can be found in Appendix 2. To the extent possible, stormwater flow below this new diversion will be directed towards a proposed stormwater outfall to be located east of the South Quarry draining into UD#3. This proposed outfall is described in more detail in the following Dewatering section. Dewatering To handle stormwater accumulations within the quarry base after Phase III, South and West Quarry mining operations have lowered benches to El. 6,500. Aggregate Industries, Inc. proposes installation of a horizontal drain running from a sump location within the South Quarry through the eastern wall and daylighting in either UD#2 or UD#3 as proposed new Outfall 007. Dewatering would include installing a borehole approximately 2,000 feet under the eastern wall of the South Quarry. The outlet of the borehole could consist of an 8-inch stainless steel casing standpipe to be able to handle up to 45 feet of head pressure. The remainder of the hole would be unlined and drilled at a 6.75-inch diameter capable of handling flows up to 540 gallons per minute. Note that this proposed borehole is conceptual, and actual dimensions, flow capacities, alignments, and other design criteria would be determined following extensive investigations. Hydrogeologic and geotechnical investigations will be key to providing the necessary data for drainage analysis, unlined borehole stability, borehole alignment, and other key components of the design. It is assumed the outlet would be a free-flowing pipe, but if necessary, a wellhead complete with valve control and pressure gauge could be installed to regulate flows from the base of the South Quarry during heavy precipitation events. A riprap rundown would be installed at the outlet of the discharge to dissipate energy and reduce the likelihood of erosion in the immediate area. Because flow from UD#1 is already planned to be channeled to UD#2 (described in section titled Stormwater and Erosion Control Features, above), it is assumed that UD#3 (previously UD#1 prior to mining) would be able to handle additional stormwater flow generated solely within the South and West Quarries. As with the proposed deepening of the South Quarry noted in Exhibit D— Mining Plan, Phase IV — South Quarry Deepening, until such time that appropriate hydrogeologic and geotechnical investigations are performed and the horizontal drain can be designed and installed, active pumping within the Phase III area of the South Quarry would continue to discharge to Drainage Outfall 003. September 2021 1 29