INFERRING SOURCES OF MERCURY TO MICHIGAN'S INLAND LAKES USING STABLE LEAD ISOTOPES AND POLYCYCLIC AROMATIC HYDROCARBONS

The predominant pathway of mercury (Hg) to the aquatic environment is through atmospheric transport and deposition, and due to its geochemical characteristics Hg can travel great distances. Therefore Hg is considered a global pollutant. It becomes difficult then to determine the source of Hg to any particular region using Hg concentration alone. The sediment profiles of the stable lead (Pb) isotopic ratios and polycyclic aromatic hydrocarbons (PAH) have frequently been used to determine source or source areas for Pb to the environment. Since Hg and Pb have similar sources (e.g., fossil fuel combustion) Pb isotopes and PAH could help to resolve sources of Hg to the environment. The hypothesis driving this research is that local-scale sources contribute significant loadings of Hg to inland lakes. To test this hypothesis sediment cores were collected from the deepest portion of six inland lakes of Michigan and dated with ²¹⁰Pb to compare depositional histories of Hg, Pb and PAH. Lead isotopes and concentrations were determined with ICP-MS using microwave assisted nitric acid digested sediment. Whereas, total Hg was determined with a Lumex TD-AAS using freeze-dried sediment. Sediment concentrations of PAH were determined with GC-MS after dichloromethane (DCM) Soxhlet extraction and elution with hexane followed by 20% DCM in hexane. Ratios of specific PAH suggest that the combustion of coal and other fuels are important anthropogenic sources for Michigan lakes. However, the relative contribution of these fuel sources varies across the state. Pb isotopes indicated a shift from background ratios to ratios associated with Pb rich ore-bodies and fossil fuel combustion. However, abrupt changes in the isotopic ratios of Pb, ratios different than to be expected for fossil fuel combustion, coincide with episodic Hg accumulation events. These coincident geochemical indicators suggest a watershed pathway for Hg loadings to lakes. This implies that as Hg emissions decline, as a result of environmental legislation, the watershed will play a greater role in the loading of Hg to inland lakes.