INFLUENCES OF WATERSHED CHARACTERISTICS ON MERCURY LEVELS IN WISCONSIN RIVERS

Total and monomethyl mercury were measured at 39 river sites in Wiscon sin during fall 1992 and spring 1993. Using a Geographic Information S ystem (GIS), we delineated watersheds with unique and homogeneous phys ical characteristics. Mean unfiltered total Hg (Hg-T) was higher in sp ring (7.94 ng L(-1)) than in fall (3.45 ng L(-1)). Major differences i n Hg-T yields were observed among various land-use groupings. In wetla nd/forest watersheds, elevated Hg-T fluxes were associated with the fi ltered phase, while in agricultural watersheds, increased Hg-T fluxes were due to particle loading. Monomethylmercury (MeHg) yields from wet land/forest sites were higher than agricultural/forest sites and agric ultural only sites. Percent wetland surface area was positively correl ated with MeHg yield. These results identify the importance of land us e and land cover in influencing Hg concentrations, speciaton, and tran sport in rivers.