Mercury is a worldwide contaminant derived from natural and anthropogenic sources. River systems play a
key role in the transport and fate of Hg because they drain widespread areas affected by aerial Hg deposition,
transport Hg away from point sources, and are sites of Hg biogeochemical cycling and bioaccumulation. The
Madison and Missouri Rivers provide a natural laboratory for studying the fate and transport of Hg contributed
by geothermal discharge in Yellowstone National Park and from the atmosphere for a large drainage
basin in Montana and Wyoming, United States of America (USA). Assessing Hg in these rivers also is important
because they support fishery-based recreation and irrigated agriculture. During 2002 to 2006, Hg concentrations
were measured in water, sediment, and fish from the main stem, 7 tributaries, and 6 lakes.
Using these data, the geothermal Hg load to the Madison River and overall fate of Hg along 378 km of the
Missouri River system were assessed. Geothermal Hg was the primary source of elevated total Hg concentrations
in unfiltered water (6.2–31.2 ng/L), sediment (148–1100 ng/g), and brown and rainbow trout (0.12–
1.23 µg total Hg/g wet weight skinless filet) upstream from Hebgen Lake (the uppermost impoundment). Approximately
7.0 kg/y of geothermal Hg was discharged from the park via the Madison River, and an estimated
87% of that load was lost to sedimentation in and volatilization from Hebgen Lake. Consequently, Hg concentrations
in water, sediment, and fish from main-stem sites downstream from Hebgen Lake were not elevated
and were comparable to concentrations reported for other areas affected solely by atmospheric Hg deposition.
Some Hg was sequestered in sediment in the downstream lakes. Bioaccumulation of Hg in fish along
the river system was strongly correlated (r2=0.76–0.86) with unfiltered total and methyl Hg concentrations
in water and total Hg in sediment.