USE OF MULTIDISCIPLINARY AND VISUAL APPROACHES TO UNDERSTAND FRACTURE-FLOW CONTAMINANT TRANSPORT IN AN UNDERGROUND GOLD MINE

The Buckhorn Mine is an operating underground gold mine in northern Washington State that has been out of compliance with its discharge permit since shortly after it began operating in 2008, largely due to failure of the dewatering system to retain mine-related contaminants within the designated capture zone. The dominant groundwater and surface water contaminants are nitrate/ammonia (from blasting), sulfate (from sulfide oxidation), and chloride (from cement curing and ion exchange water treatment); As, Cu, Mn, and Zn are elevated relative to baseline values in some groundwater locations from current and historic mining. Fracture flow through faulted metamorphic terrane with a partial glacial cover complicates environmental assessment, and a combination of stratigraphic, geochemical (waste testing), hydrologic, water quality, groundwater flow modeling, and geophysical approaches, and a 3D graphical representation of the mine, have been used to discern sources and pathways and identify approaches for a clean closure. Although several lithologies have high acid-generation potential and the one available waste rock leachate sample had a pH <5, mine water is currently basic (pH >12), due to cemented waste rock in underground workings. Streams and groundwater have moderate alkalinity. Seasonal groundwater levels show rapid response to snowmelt infiltration and slow response to dewatering, partially explaining the lack of hydrologic control. The appearance of nitrate in streams over a mile from the mine has been linked to movement of stored mine water along major faults using a combination of water quality data and a 3D graphical representation that shows mine sumps puncturing faults. A short video tour of the underground, including faults, monitoring wells, and mine workings will be shown. Ground-based geophysical surveys conducted by the mine’s consultants using electromagnetic induction (EM-31 and -34), electrical resistivity imaging, and VLF radio methods serve as a proxy for missing waste rock leachate data and show high-solute waters moving into the shallow subsurface. The dominance of transport along faults highlights the importance of pre-mining geophysical characterization and improved methods for mine water capture in fractured bedrock, which is present at many if not most hardrock mines.