Mercury methylation trends pre and post refilling in a Northern Appalachian impoundment

Abstract:

High rates of atmospheric deposition of mercury (Hg) in the northern Appalachian Mountain region of the eastern United States have led to an accumulation of anthropogenic Hg in soils and the aquatic food chain. Much concern is focused on methyl mercury (MeHg) in surface waters as it is the most bioavailable species. The bacterial transformation of organic Hg to MeHg has been suggested to increase when oxidized soils or sediments become reduced, such as during flooding. The refilling of a Northern Appalachian impoundment has been tracked in this study in order to examine a potential increase in methylation rate. The sediments in this 0.29 km² constructed impoundment lay dry for 6 years before dam reconstruction allowed refilling. Since being drained, redox concentration and depletion colors have appeared in the formerly gley sediments suggesting intense oxidation/reduction events. Dry sediment (soil order Entisols) Hg concentrations were typically highest in the upper horizon of the sediments and coincided with the highest concentrations of organic carbon. The ratio of MeHg to total Hg (THg) in the soils/sediments was used as an indicator of methylation rate. The highest ratio of MeHg/THg before the refilling were found in the deepest part of the former lake, were the sediments still periodically held rainwater. The sediments will be sampled again in late summer 2014 to evaluate the change in methylation rate with lake refilling. A potential change in stream water MeHg concentration in the outflow versus inflow determines if the restored lake may act as a source of MeHg to downstream environments. Results from this sampling of post-refilling water and sediments will be evaluated during the fall.