Fate and Transport of Dust-borne Trace Metals and Solutes during Snowmelt Runoff in the Provo River, Utah

Abstract:

Aeolian dust is an important physical and chemical flux to mountain snowpack, with potentially significant contributions of trace metals and solutes to mountain streams during snowmelt. However, it is difficult to differentiate element contributions from dust relative to inputs from soil erosion and weathering. Sr isotopes show promise as a tracer of the soluble fraction of dust from snowpack to snowmelt runoff. We analyzed ⁸⁷Sr/⁸⁶Sr ratios, Sr and trace metal concentrations, and other parameters in bulk snowpack (wet and dry deposition), dust, and snowmelt runoff in the upper Provo River watershed in the Uinta Mountains (Utah, USA) over three years (2013-2015). In 2014 snowpack and dust samples had the same ⁸⁷Sr/⁸⁶Sr ratio (~0.7110), indicating that dust was responsible for the majority of Sr deposition to snowpack. Preliminary results using a two end-member mixing model (end-members of bulk snowpack and river baseflow) indicate that dust contributed up to 40% of Sr riverine load during peak runoff in 2014. In contrast, in 2013 there were no visible dust layers in the snowpack and mixing models indicate a different Sr source (e.g., soil water). Analyses are still underway on the 2015 samples. Sequential leaching experiments on dust samples suggest that Sr is likely associated with the highly soluble carbonate mineral fraction in dust, but other elements such as Cr, Cu, Pb, and U are associated with the relatively insoluble organic fraction. Additional work is underway to investigate transport of the less soluble dust-borne metals during snowmelt by complexation with organic matter or other mechanisms. These results suggest that dust on snowpack is an important but variable source of soluble elements during snowmelt, and that dust should be considered when investigating solute loads in mountain streams.