FACTORS CONTROLLING MERCURY TRANSPORT IN AN UPLAND FORESTED CATCHMENT

Total mercury (Hg) deposition and input/output relationships were inve stigated in an 11-ha deciduous forested catchment in northern Vermont as part of ongoing evaluations of Hg cycling and transport in the Lake Champlain basin. Atmospheric Hg deposition (precipitation + modeled v apor phase downward flux) was 425 mg ha(-1) during the one-year period March 1994 through February 1995 and 463 mg ha(-1) from March 1995 th rough February 1996. In the same periods, stream export of total Hg wa s 32 mg ha(-1) and 22 mg ha(-1), respectively. Thus, there was a net r etention of Hg by the catchment of 92 % the first year and 95 % the se cond year. In the first year, 16.9 mg ha(-1) or about half of the annu al stream export, occurred on the single day of peak spring snowmelt i n April. In contrast, the maximum daily export in the second year, whe n peak stream flow was somewhat lower, was 3.5 mg ha(-1) during a Janu ary thaw. The fate of the Kg retained by this forested catchment is no t known. Dissolved (< 0.22 mu m) Hg concentrations in stream water ran ged from 0.5-2.6 ng L-1, even when total (unfiltered) concentrations w ere greater than 10 ng L-1 during high flow events. Total Hg concentra tions in stream water were correlated with the total organic fraction of suspended sediment, suggesting the importance of organic material i n Hg transport within the catchment. High flow events and transport wi th organic material may be especially important mechanisms for the mov ement of Hg through forested ecosystems.