CLIMATE-INDUCED MOBILIZATION OF RARE EARTH ELEMENTS IN HIGH ALTITUDE STREAMS, COLORADO: POTENTIAL ROLE OF COMPLEXATION IN HYDROUS METAL OXIDE PRECIPITATION

Pyrite weathering, the biogeochemical process creating acid rock drainage (ARD) and acid mine drainage (AMD), is commonly associated with the mobilization of hydrous metals. Drier conditions attributed to a warming climate have accelerated this process. The chemical parameters for pyrite weathering have the ability to mobilize various metals, including rare earth elements (REEs). Recently, an increase in REE concentration was discovered in a tributary leading to the Dillon Reservoir, CO- the source of drinking water for Denver, Colorado. The humic fraction of dissolved organic matter (DOM) can form complexes with REEs and can also be sorbed by hydrous metal oxides. To study this relationship and the fate and transport of REEs we chose five sites in the Colorado Mineral Belt with distinct biogeochemical environments. These sites vary in dominant types of hydrous metal oxides, DOM sources, such as above tree lines, subalpine forests or wetlands, and known REE concentrations in country rock or water. For example, Little Sayres Gulch flows from above tree line through a subalpine forest, has abundant iron oxide precipitation, and a cerium concentration of 377 𝜇m/L. To gain an understanding of the DOM-REE relationship at each site we (1) identified geochemical parameters, particularly REEs, summed and individual concentrations within stream water and flocculant found on the stream bed (2) used these geochemical parameters to assess the connections and patterns between changes in REE-DOM complexes and their sorption onto hydrous metal oxides. This research advances our understanding of how REEs behave in aqueous environments and will be useful in addressing issues related to increasing REEs concentrations. In particular, there are currently no drinking water standards regarding REEs.