Application of copper and zinc isotopes to fingerprint metal sources and identify attenuation mechanisms in the Abandoned Waldorf Mine Area, Clear Creek County, Colorado
Acid rock drainage (ARD) is caused by the oxidative weathering of sulfide minerals. Sulfur is oxidized by dissolved oxygen or Fe(III) and sulfate ions, metals, and protons are released into the environment. Hence, the pH of nearby water and soils often decreases in response to ARD while metal concentrations increase. Because a large part of the Rocky Mountains of Colorado are highly mineralized (i.e., the Colorado Mineral Belt), ARD is a serious problem in this region. Moreover, the many abandoned mining areas in the Rocky Mountains that include waste rock piles, tailings impoundments, and surface disturbances add to the problem because they expose enormous quantities of sulfide-rich rocks to the atmosphere. The Colorado mineral belt hosts many streams that have concentrations of metals that are above aquatic life and sometimes even human health water quality standards (EPA) In 2007, the Colorado Division of Reclamation, Mining and Safety (CDRMS) estimated that the Colorado mineral belt contained more than 17,000 hazardous abandoned mines. ARD is a substantial problem even outside Colorado because the Rocky Mountains' snow pack, river basins and aquifers supply water for over sixty million people in the western United States (Bales et al. 2006).^ Infiltration of surface water through mine wastes (tailings, tunnels, waste rock piles, etc.) can be a major source of metal contaminants in a watershed (McDougal and Wirt, 2007). The identification of metal loading pathways is critical for understanding how metals impact terrestrial and aquatic ecosystems and for designing useful remediation strategies (Liu, F., Williams M. W. and Caine, N., 2004). However, in many cases, particularly in mountain watersheds with limited infrastructure and monitoring capacity, metal concentrations alone are not adequate to identify metal loading sources or transport and attenuation mechanisms. In these cases where hydrology and geochemistry are too complex for a basal approach we suspect that transition metal isotopes may prove to be a useful chemical tool. The stable isotopes of some transition metals can fingerprint sources while others are reflective of the mechanisms during the processes that control their transport, including redox reactions, precipitation, and adsorption.^ The Abandoned Waldorf Mine area in Clear Creek County, Colorado, is a good example of a mountain catchment severely impacted by metal contamination where the utility of metal isotopes for unraveling these complexities can be directly evaluated. The site hydrology and metal loading pathways are extremely complex (EPA) and there is limited infrastructure (like monitoring wells) for a thorough hydrogeochemical investigation. Finally, copper (Cu) and zinc (Zn) are present at elevated concentrations over most of the site, and their isotopic compositions can be used to investigate metal cycling. At the Waldorf site contaminated water discharges from the abandoned Wilcox tunnel, as well as from springs at the toe of a waste rock pile. The Wilcox tunnel, established about 1902, is located at the foot of Mount McClellan and leads to approximately 30,000 feet of underground workings. Metal-rich waters from the waste rock pile and tunnel flow through a groundwater-fed wetland that includes ephemeral alpine streams and pools before ultimately discharging to Leavenworth Creek. Leavenworth Creek contributes drinking water for the city of Georgetown, Colorado, and is a tributary of Clear Creek, which supplies water for Golden, Colorado (Malem, 2006).^ In this study we measured the Cu and Zn isotopic compositions of water, mineral and ore concentrate samples collected from the abandoned Waldorf mine area in Clear Creek County, Colorado. The objectives of our study were to use the metal isotopic signatures to identify and track metal loading sources and metal transport and attenuation mechanisms in the watershed. To this end, samples were collected from the Wilcox tunnel, waste-rock pile and wetland areas in and around the abandoned mining area. We additionally measured Zn isotopes in soil pore waters collected from multiple depths at three locations within the wetland. This work will directly benefit remediation efforts in the region and will serve as a case study for the application of these isotopic tools in other systems. ^
Aranda Luna, Suzan, "Application of copper and zinc isotopes to fingerprint metal sources and identify attenuation mechanisms in the Abandoned Waldorf Mine Area, Clear Creek County, Colorado" (2010). ETD Collection for University of Texas, El Paso. AAI1483852.