Watersheds are major reservoirs of metals, and rivers draining these watersheds are major sources of metals that enter Lake Superior. This project is assessing key factors controlling the mobility and fluxes of a dozen different metals in representative tributaries of Lake Superior. Researchers are studying trace metal transport and relationships to stream geochemistry, watershed characteristics, and hydrologic events. They are also examining the relative importance of natural and human-produced sources. The use of newly developed "ultra-clean" sampling and analysis methods is enabling the researchers to measure metal concentrations and forms present in tributaries throughout the Lake Superior Basin.

Update - February 1999

Field studies to date have focused on the collection of samples during key hydrologic regimes in each of six representative Lake Superior watersheds. Preliminary results indicate that 40%-70% of the annual export of many metals occurs during spring melt, with most of the remainder occurring during summer and fall events. These data indicate hydrologic events may dominate annual export loading, particularly for particle-associated metals but also for certain metals associated with dissolved organic carbon (DOC). The samples were subjected to a variety of chemical and physical separation techniques designed to elucidate the phase-association of a dozen environmentally relevant trace metals.

Experiments with DOC-associated metals demonstrated that the qualitative and quantitative association with DOC is strongly predicted by watershed characteristics. Speciation of silver and lead is highly coupled to colloidal iron. Related work shows that the fraction of total metal that is labile, or "free," is frequently small. These results have important implications for short-term bioavailability; however, the longer-term fate of complexed metals is still open to question. Experiments now underway will address the availability of metals associated with particles and differentiate anthropogenic and background sources of metals during distinct hydrologic periods.