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 • South Quarry—This drainage area includes the upper highwall and benches, as well as a portable crusher/temporary processing plant is located near Outfall 003. Runoff from this drainage reports to Outfall 003, which discharges into the existing UD#3 channel downstream of the site, which flows into Turkey Creek approximately 1 mile downstream of the outfall. In addition to runoff from the highwalls and benches, the South Quarry includes a localized depression at the bottom of the current excavation. Runoff also collects within this low point, where it is allowed to infiltrate or is pumped to Outfall 003 within 72 hours. As was discussed in Exhibit E—Reclamation Plan, Dewatering, if Phase IV mining activities are delayed or postponed, more permanent conveyance structures for runoff accumulating at this low point will be implemented. Of note, the mining plan includes the diversion of UD#1 across the South Quarry western face upper bench at El. 7,400. Once this is complete, surface water runoff from benches above this elevation will be collected in the diversion structure and conveyed to new outfall (Outfall 006) for discharge to UD#2. • Processing Area—This drainage area consists of the processing area, the upper portion of the haul road, and an undisturbed hillside to the south of the processing area. The runoff from this drainage area flows east, through the existing retention basin to Outfall 005. Outfall 005 discharges to an unnamed tributary of Turkey Creek. Of note, a recent stormwater evaluation was completed by Apex Companies, LLC (Apex) to assess detention pond sizing. It was determined that the stormwater retention basin for Outfall 005 was undersized and did not meet Urban Drainage and Flood Control District (UDFCD), now Mile High Flood District (MHFD), Water Quality Control Volume (WQCV) criteria for storage volume or operation. Due to this, Aggregate Industries, Inc. recently installed an additional retention basin in line and upstream of the existing basin to meet MHFD criteria (Apex 2020). • Haul Road —This drainage area consists of the haul road and undisturbed upslope areas along it. Flow from the drainage area upstream of the stormwater retention basin is routed to the retention basin located at the base of the haul road near CO-8. The retention basin discharges to Outfall 001 and flows into the roadside drainage channel along CO-8, which ultimately flows into an unnamed tributary of Turkey Creek. Runoff from the drainage area downstream of the sediment basin reports to Outfall 004 and flows into the drainage channel along CO-8 that ultimately flows into Strain Gulch. Of note, the retention basin was reviewed as part of the recent stormwater evaluation completed by Apex and was determined to be adequately sized (Apex 2020). Groundwater As was stated in Exhibit D— Mining Plan, it is anticipated that groundwater will be encountered in the South Quarry around El. 6,500, possibly impacting Phase III and requiring potentially significant dewatering activities to complete Phase IV mining activities. Temporary Phase III Groundwater Management As stated in the section titled Site Hydrology Overview, above, groundwater monitoring wells in the South Quarry (MW-2019-01 and MW-2019-02) will be used to determine if and when groundwater has been encountered during Phase III excavation. The Phase III mining plan September 2021 1 60