Sonia A. Nagorski

Arsenic mobilization in the hyporheic zone of a contaminated stream

Arsenic behavior was examined in a contaminated stream by sampling the dissolved (<0.45 μm) arsenic and metals in surface water, shallow hyporheic zone water, and adjacent ground water. Surface water was oxic and slightly basic, and ground water was anoxic and acidic. Hyporheic zone water had pH values of 6–7, dissolved oxygen concentrations mostly between 0 and 3 mg L−1, and mean concentrations of most metals inbetween surface and ground water sample concentrations. However, arsenic and iron were enriched in the hyporheic zone. Most of the hyporheic zone dissolved arsenic was in the form of As(III), which is considered to be more toxic to some organisms than As(V). In the oxic surface water, 20% of the total dissolved As was found to occur in its reduced form. We hypothesize that upon burial and reduction of Fe-oxyhydroxides in the streambed, sediment-bound arsenic is transferred into the dissolved phase as As(III), and it is subsequently released into the surface water, where it does not immediately reoxidize. A continual flux of reduced As to the surface water maintains As(III) concentrations above that expected in oxygenated surface waters.

Spatial distribution of mercury in southeastern Alaskan streams influenced by glaciers, wetlands, and salmon

Southeastern Alaska is a remote coastal-maritime ecosystem that is experiencing increased deposition of mercury (Hg) as well as rapid glacier loss. Here we present the results of the first reported survey of total and methyl Hg (MeHg) concentrations in regional streams and biota. Overall, streams draining large wetland areas had higher Hg concentrations in water, mayflies, and juvenile salmon than those from glacially-influenced or recently deglaciated watersheds. Filtered MeHg was positively correlated with wetland abundance. Aqueous Hg occurred predominantly in the particulate fraction of glacier streams but in the filtered fraction of wetland-rich streams. Colonization by anadromous salmon in both glacier and wetland-rich streams may be contributing additional marine-derived Hg. The spatial distribution of Hg in the range of streams presented here shows that watersheds are variably, yet fairly predictably, sensitive to atmospheric and marine inputs of Hg.

Continuous proxy measurements reveal large mercury fluxes from glacial and forested watersheds in Alaska

In this study, a stream from a glacially dominated watershed and one from a wetland, temperate forest dominated watershed in southeast Alaska were continuously monitored for turbidity and fluorescence from dissolved organic matter (FDOM) while grab samples for unfiltered (UTHg), particulate (PTHg), and filtered mercury (FTHg) where taken over three 4-day periods (May snowmelt, July glacial melt, and September rainy season) during 2010. Strong correlations were found between FDOM and UTHg concentrations in the wetland, temperate forest watershed (r2=0.81), while turbidity and UTHg were highly correlated in the glacially dominated watershed (r2=0.82). Both of these parameters (FDOM and turbidity) showed stronger correlations than concentration-discharge relationships for UTHg (r2=0.55 for glacial stream, r2=0.38 for wetland/forest stream), thus allowing for a more precise determination of temporal variability in UTHg concentrations and fluxes. The association of mercury with particles and dissolved organic matter (DOM) appears to depend on the watershed characteristics, such as physical weathering and biogeochemical processes regulating mercury transport. Thus employing watershed-specific proxies for UTHg (such as FDOM and turbidity) can be effective for quantifying mercury export from watersheds with variable landcover. The UTHg concentration in the forest/wetland stream was consistently higher than in the glacial stream, in which most of the mercury was associated with particles; however, due to the high specific discharge from the glacial stream during the melt season, the watershed area normalized flux of mercury from the glacial stream was 3-6 times greater than the wetland/forest stream for the three sampling campaigns. The annual specific flux for the glacial watershed was 19.9gUTHgkm-2y-1, which is higher than any non-mining impacted stream measured to date. This finding indicates that glacial watersheds of southeast Alaska may be important conduits of total mercury to the Gulf of Alaska.

Mercury and water-quality data from Rink Creek, Salmon River, and Good River, Glacier Bay National Park and Preserve, Alaska, November 2009-October 2011

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