Seasonal influences on partitioning and transport of total and methylmercury in rivers from contrasting watersheds

Seven Wisconsin rivers with contrasting, relatively homogeneous watershed composition were selected to assess the factors controlling mercury transport. Together, these watersheds allow comparisons of wetland, forest, urban and agricultural land-uses. Each site was sampled nine times between September 1993 and September 1994 to establish seasonal signatures and transport processes of total mercury (Hg-T) and methylmercury (MeHg). Our results clearly show that land use and land cover strongly influence mercury transport processes. Under base-flow conditions, unfiltered MeHg yield varies by a factor of sixteen (12-195 mg km(-2) d(-1)), and increases with the fraction of wetland area in the watershed. Elevated mercury yields during high flow are particle-phase associated in agricultural sites, but filtered-phase associated in wetland sites. Methylmercury represented less than 5% of total mercury mobilized during the spring thaw across all watersheds. Autumn MeHg yield was generally 11-15% of Hg-T in wetland influenced watersheds, though a maximum of 51% was observed. In some cases, single high-flow events may dominate the annual export of mercury from a watershed. For example, one high-flow event on the agricultural Rattlesnake Creek had the largest Hg-T and MeHg yield in the study (107 and 2.32 mg km(-2) d(-1), respectively). The mass of mercury transported downstream by this single event was an order of magnitude larger than the eight other (non-event) sampling dates combined. These results underscore the importance of watershed characteristics and seasonal events on the fate of mercury in fresh water rivers.