New Tool to Track Sources and Exposure Pathways of Mercury in the Environment — Application for Predatory Fish in the Great Lakes

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The fingerprinting tool includes novel analyses of stable mercury isotopic signatures in lake sediments to infer relative contributions of mercury from key end-member sources. The tool works by identifying the different types of mercury atoms present in the Great Lakes. Each mercury atom has a certain mass, which can range from 196-204. The relative abundance of each of these masses is different in each of the mercury sources, and these differences provide the ability to "fingerprint" the mercury source contributions.

Surface sediment samples collected from 58 locations throughout the Great Lakes during 2012-2014 were analyzed for stable isotopes of mercury and used to determine sources. The tool was applied to the entire Great Lakes ecosystem to distinguish among mercury derived from atmospheric deposition, watershed runoff, and industrial point-source inputs. The results show novel insights into mercury sources across the Great Lakes, with Lakes Superior and Huron dominated by atmospheric inputs. Lakes Erie and Ontario dominated by industrial and watershed sources, and Lake Michigan showing about equal contributions from all three source categories.

Mercury is a persistent pollutant that is distributed globally. Previous USGS and other research has shown that human-induced sources of mercury (atmospheric deposition, watershed inputs, or direct inputs through point source discharges) are the dominant sources of mercury to the environment when compared to natural emission sources such as volcanoes and oceans. Understanding the sources contributing to mercury has been an active area of research at USGS for the past 25 years. This study advances this research by providing a new tool to more accurately define mercury sources.

Environmental Health Implications

With this new fingerprinting tool, resource managers can now better understand what mitigation strategies will be most effective for reducing mercury loading and exposure to fish and wildlife. The results from the source model were extended to recent results from mercury isotope work in measurements in predatory fish (Lake trout and Burbot) to ascertain the relative importance of the end-member sources to the food web. Comparison of the mercury signatures in predatory fish from the three lakes (Michigan, Superior, and Ontario) reveals that bioaccumulated mercury in these predator fish is more similar to atmospherically derived mercury than to mercury in a lake's sediment. The study findings suggest that atmospherically derived mercury many be a more important source of methylmercury to higher trophic levels (predator fish) than mercury stored in legacy sediments in the Great Lakes. In aquatic systems microbially mediated processes can transform mercury to methylmercury, a more toxic form that is biomagnified in aquatic food webs. Bioaccumulated methylmercury is a potential threat to aquatic ecosystems and humans health through fish consumption. Providing a better understanding of sources is a key component of managing environmental health risk.

This research was funded by the USGS Ecosystems Mission Area’s Environmental Health Program (Contaminant Biology and Toxic Substances Hydrology), and the U.S. Environmental Protection Agency (EPA) Great Lakes Restoration Initiative, EPA Great Lakes Fish Monitoring and Surveillance Program.