2022-12-30_REVISION - M1977300 - Laserfiche WebLink

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_`I�wCOLORADO LEGACY LAND Surface Water Locations: CLL proposes to limit the surface water monitoring to the locations within the permit boundary of the site: • SW-AWD: Data from this upgradient sample location show background water chemistry. • SW-BDIS: Data from this sample location show water chemistry below the two waste rock piles,including they South Waste Rock Pile (SWRP) expansion. • SW-BOS: Data from this sample location show the cumulative effects of all surface water sources but before any shallow groundwater may be forced to surface by bedrock rising near BPL. • SW-BPL: Data from this sample location shows all shallow groundwater cumulative site impacts to surface water at the property boundary. These four sites are justified because as was established in AM-06, groundwater well MW-15 demonstrates an inward gradient to the east of the mine pool from MW-15 toward the mine pool, indicating that there is not a groundwater migration path to the east of the mine to surface water. To allay concern about any other groundwater migration route toward Ralston Creek, CLL is adding the new monitoring well in the vicinity of current MW7/MW-12 which will further confirm an inward gradient from all sides of the mine. Appropriate and sufficient environmental monitoring is provided by shallow groundwater measurements near the waste rock piles (MW-00, MW-0, MW19 and MW-20), deep bedrock groundwater sampling(MW-13,MW-14,MW-15,MW-16,MW-17,MW-18 and the new proposed well at MW-7/MW-12 area)as shown in Figure 3,and surface water sampling from within the CLL property is sufficient for all known potential contamination migration routes.Figure 4 shows the current surface water sampling locations. Figure 5 shows the proposed surface water sampling locations. Updated Analytical Sample Suite: CLL began the quarterly sampling program in January 2019 (Technical Revision 26)by monitoring for a robust list of metals,nutrients,radionuclides,and general chemistry parameters. This program provided sufficient data to assess which analytes are present or may be a potentially site related constituent that should be considered for continued monitoring during closure. Table 2 summarizes the current analytical sampling suite, surface water and groundwater(monitoring well)monitoring data collected by CLL since taking ownership of the site in 2018,associated surface water and groundwater water quality standards, and recommendations for tailoring the analytical suite to focus on site-specific parameters. Stormwater Management Plan: The alluvial valley excavation project began in the spring of 2018. In Technical Revision #14, the previous operator,Cotter Corporation,presented the baseline scope for the excavation project. This document initially identified 33,000 to 54,0000 cubic yards (CY) cubic yards of soil for removal and onsite disposal in the Minnesota Adit, In the end, more than 60,000 CY were loaded into the Minnesota Adit in the large open hole present in the mine.During excavation activities,CLL has identified additional contaminated soils in the alluvial valley at depth. These additional materials were not identified because they were previously partially covered by a layer of clean fill material. In 2020,CLL submitted Application Amendment 5 (AM-05)to permit the disposal of additional materials in the Former Black Forest Mine. The Former Black Forest Mine provided an additional approximately 15,000 cubic yards of capacity for the disposal of alluvial soils. In 2021, CLL conducted additional test pitting to further refine the vertical extent of contamination at the site. These test pits indicate that an additional 4,500 cubic yards of material beyond the capacity of the Former Black Forest Mine are present in the alluvial valley. In 2022,CLL submitted Application Amendment 6 (AM-06)to permit the expansion of the South Waste Rock Pile (SWRP) to the south / south-east in front of the former (and now filled) Black Forest Mine. However, a stormwater management plan was not provided given the limited time remaining in the review period for AM- 6. CLL is now submitting a stormwater management plan as part of this technical revision (Attachment C) as required by the decision document for AM-06. PAGE 2 OF 23