INFLUENCES OF WATERSHED CHARACTERISTICS ON MERCURY LEVELS IN WISCONSIN RIVERS

Total and monomethyl mercury were measured at 39 river sites in Wisconsin during fall 1992 and spring 1993. Using a Geographic Information System (GIS), we delineated watersheds with unique and homogeneous physical characteristics. Mean unfiltered total Hg (Hg-T) was higher in spring (7.94 ng L(-1)) than in fall (3.45 ng L(-1)). Major differences in Hg-T yields were observed among various land-use groupings. In wetland/forest watersheds, elevated Hg-T fluxes were associated with the filtered phase, while in agricultural watersheds, increased Hg-T fluxes were due to particle loading. Monomethylmercury (MeHg) yields from wetland/forest sites were higher than agricultural/forest sites and agricultural only sites. Percent wetland surface area was positively correlated with MeHg yield. These results identify the importance of land use and land cover in influencing Hg concentrations, speciaton, and transport in rivers.