title: Comparison of mercury mass loading in streams to atmospheric deposition in watersheds of Western North America: Evidence for non-atmospheric mercury sources.
abstract: Annual stream loads of mercury (Hg) and inputs of wet and dry atmospheric Hg deposition to the landscape were investigated in watersheds of the Western United States and the Canadian-Alaskan Arctic. Mercury concentration and discharge data from flow gauging stations were used to compute annual mass loads with regression models. Measured wet and modeled dry deposition were compared to annual stream loads to compute ratios of Hg stream load to total Hg atmospheric deposition. Watershed land uses or cover included mining, undeveloped, urbanized, and mixed. Of 27 watersheds that were investigated, 15 had some degree of mining, either of Hg or precious metals (gold or silver), where Hg was used in the amalgamation process. Stream loads in excess of annual Hg atmospheric deposition (ratio>1) were observed in watersheds containing Hg mines and in relatively small and medium-sized watersheds with gold or silver mines, however, larger watersheds containing gold or silver mines, some of which also contain large dams that trap sediment, were sometimes associated with lower load ratios (<0.2). In the non-Arctic regions, watersheds with natural vegetation tended to have low ratios of stream load to Hg deposition (<0.1), whereas urbanized areas had higher ratios (0.34-1.0) because of impervious surfaces. This indicated that, in ecosystems with natural vegetation, Hg is retained in the soil and may be transported subsequently to streams as a result of erosion or in association with dissolved organic carbon. Arctic watersheds (Mackenzie and Yukon Rivers) had a relatively elevated ratio of stream load to atmospheric deposition (0.27 and 0.74), possibly because of melting glaciers or permafrost releasing previously stored Hg to the streams. Overall, our research highlights the important role of watershed characteristics in determining whether a landscape is a net source of Hg or a net sink of atmospheric Hg.
authors.affiliation: U.S. Geological Survey, California Water Science Center, 6000 J Street, Placer Hall, Sacramento, CA 95819, United States. Electronic address: joed@usgs.gov.
authors.forename: Joseph
authors.initials: J
authors.lastname: Domagalski
authors.affiliation: U.S. Geological Survey, California Water Science Center, 6000 J Street, Placer Hall, Sacramento, CA 95819, United States.
authors.forename: Michael S
authors.initials: MS
authors.lastname: Majewski
authors.affiliation: U.S. Geological Survey, California Water Science Center, 6000 J Street, Placer Hall, Sacramento, CA 95819, United States.
authors.forename: Charles N
authors.initials: CN
authors.lastname: Alpers
authors.affiliation: U.S. Environmental Protection Agency, Office of Environmental Assessment, EPA-Region 10, 1200 6th Ave., Suite 900, Seattle, WA 98101, United States.
authors.forename: Chris S
authors.initials: CS
authors.lastname: Eckley
authors.affiliation: U.S. Geological Survey, Forest and Rangeland Ecosystem Science Center, 3200 SW Jefferson Way, Corvallis, OR 97331, United States.
authors.forename: Collin A
authors.initials: CA
authors.lastname: Eagles-Smith
authors.affiliation: U.S. Geological Survey, Oregon Water Science Center, 2795 Anderson Ave., Suite 106, Klamath Falls, OR 97603, United States.
authors.forename: Liam
authors.initials: L
authors.lastname: Schenk
authors.affiliation: U.S. Geological Survey, Oregon Water Science Center, 2130 SW 5th Ave., Portland, OR 97201, United States.
authors.forename: Susan
authors.initials: S
authors.lastname: Wherry
keywords: Stream loads; Urban contaminants; WNAMS; Western North American Mercury Synthesis; historical mining