156 - The influence of seasonal water-level fluctuations on porewater redox chemistry and methylmercury production in wetlands downstream of a historical mining district

Understanding the drivers of methylmercury (MeHg) production within a watershed can help guide landscape-scale management decisions aimed at decreasing the impacts of mercury (Hg) pollution. Sediment and soil redox conditions can influence MeHg production and are affected by topographical, seasonal, and climatic variables. The objective of this study was to identify the connection between water-level fluctuations, sediment redox chemistry, and MeHg production. The study area was the lower basin of the Coeur d’Alene River (Idaho, USA), located downstream of a historical silver mining district. Field measurements focused on collecting sediment and porewater across a gradient of permanently inundated and seasonally inundated wetlands as well as proximate terrestrial soils during the spring and fall. In addition, dataloggers were used to measure daily changes in sediment redox conditions and water level fluctuations within wetland areas over multiple years. Samples were analyzed for total-Hg (THg), MeHg, organic carbon, reduced iron, sulfur/sulfide, among other ancillary parameters. The results showed the importance wetland microtopography (hummocks versus hollows) and seasonal water level fluctuations on MeHg concentrations. Areas with the highest MeHg production tended to be those that were seasonally inundated with water compared to terrestrial soils (typically unsaturated) or permanently wet sediments. The results indicate that landscape-scale hydrological and topographical variables can be influential factors on the production of MeHg. Because Hg pollution is distributed broadly across landscapes, management actions aimed decreasing the conditions favorable to methylation may help decrease MeHg concentrations.

Authors: Chris Eckley, Todd Luxton, Rick Wilkin, Mark Johnson, Kim Prestbo, David Leptich, Matt Noerpel, David Van de Reit, Steve Sluca, Jenny Goetz