Atmospheric Deposition of Mercury and Its Impact on Aquatic Ecosystems Draining Forested Watersheds in Pennsylvania

Elizabeth W. Boyer, Christopher Grant, James A. Lynch and David R. DeWalle,
School of Forest Resources
Pennsylvania State University

Mercury emissions to the atmosphere from coal-fired power plants and other sources such as waste incineration can be deposited to the landscape in precipitation and in dry fallout. Some mercury reaches nearby streams through direct deposition or subsurface flow processes where it can accumulate in sediments and biota. Pennsylvania is one of the leading states in terms of total mercury emissions from all sources, and receives among the highest levels of wet mercury deposition of any location in the northeastern USA. Human exposure to mercury occurs primarily through fish consumption, and currently fish eating advisories due to mercury contamination have been posted for over 877 stream miles and 28 lakes across Pennsylvania. Mercury levels in stream water are believed to be a key indicator for concentrations of mercury available for uptake by biological organisms. We measured total mercury in precipitation at 9 locations across Pennsylvania from which we present variation in mercury deposition and initial assessments of factors related to patterns and trends. Further, we collected baseline data on mercury in streams draining 40 forested watersheds throughout the state, spanning gradients of atmospheric deposition, climate and geology. These watersheds are minimally disturbed (e.g., no direct impacts of agriculture, urbanization, or mining, and no point sources of Hg). Total and methyl mercury concentrations were measured in stream water under base-flow conditions, and total mercury was measured in stream-bed sediments, stream-bed mosses, and in fish tissue from brook trout. The highest mercury concentrations were found in mosses and sediments, suggesting their potential utility as indicators for understanding long term watershed responses. Mercury concentrations in fish samples are very low with respect to human health concerns. However, mercury levels in fish samples suggest potential ecosystem level concerns in some of the watersheds, with concentrations in fish approaching the US Fish and Wildlife Service advisories for birds and wildlife that eat fish. We comment on the relevance of our results to policy considerations in the state, and make recommendations for further monitoring and assessment of the effects of mercury deposition in Pennsylvania and the northeast.