MERCURY AND METHYLMERCURY DYNAMICS IN UPLAND LANDSCAPES AT SLEEPERS RIVER, VERMONT USA

Total mercury (HgT) and methylmercury (MeHg) concentrations in streamwater have been monitored in forested and agricultural watersheds of the Sleepers River Research Watershed in northeastern Vermont, USA, since 2000. Our primary objectives were to assess the conditions and mechanisms responsible for HgT movement from upland to lowland environments, and to assess whether uplands are a source of MeHg. Samples were taken during hydrologically active periods, primarily at snowmelt and during a few summer storms. Total Hg concentrations in unfiltered water varied from 0.2 to 94 ng/L and methylmercury ranged from <0.04 to 2.0 ng/L. The highest concentrations of both HgT and MeHg occurred during high flows in association with elevated particulate organic carbon (POC) concentrations. Most of the annual export of HgT and MeHg, from all landscape types, occurs in the particulate phase (>0.45 µm) during brief high-flow episodes. At nearly all sites, HgT and MeHg concentrations were positively correlated to each other and to stream discharge, suggesting that HgT and MeHg have a common source, presumably from in-stream or near-stream organic matter. This coupled transport behavior of HgT and MeHg is in contrast to observations in Europe where HgT concentrations increase but MeHg concentrations decrease (dilute) during high flows. Our results indicate that not only are uplands important sources of HgT to methylating environments downstream, but also that significant methylation takes place in the upland landscape itself. Methylation and demethylation incubations confirmed the potential for significant net methylation in some upland settings, particularly in headwater stream sediments that become anoxic in summer.