Sources and pathways for Mercury to Michigan's inland lakes

Mercury has a unique geochemical cycle due to its ability to exist as a gas at Earth surface conditions. As a result Hg can be transported across a region (e.g., the Midwest) or across the globe. The importance of local watershed scale versus regional/global sources is still debated. The over-arching hypothesis driving this research is that local scale sources and watershed scale processes have more impacts on the rate of Hg accumulation in the aquatic environment than regional to global scale sources. To evaluate this hypothesis, 210 Pb dated mercury sediment chronologies were compared among inland lakes within the State of Michigan and investigated using the following approaches: (1) examine the spatial and temporal trends of Hg loading to the environment to discriminate regional and local scale processes, (2) use modern rates of recovery to identify those watershed attributes that influence the transport of Hg through the watershed and (3) use diagnostic ratios of polycyclic aromatic hydrocarbons (PAHs) to infer sources of Hg contamination.;Results of temporal and spatial analysis show that (1) mercury accumulation rates for many lakes are still increasing; (2) focusing corrected anthropogenic Hg inventories are similar to those determined from Great Lakes sediments suggesting a regional source and (3) anthropogenic accumulation rate profiles are similar by sub-region and share episodic accumulation events indicative of local scale sources.;Contemporary recovery rates were calculated for mercury based on concentration and accumulation rate profiles. These recovery rates were then compared to watershed attributes. Agricultural, forested and wetland attributes were all found to be significantly correlated. However, those cover types found along highly dynamic watershed flowpaths were found to be most significant.;Mercury and PAHs can share common sources and pathways resulting in similar environmental fates. Diagnostic ratios of PAHs are often used to differentiate source of these contaminants to the environment and may be useful to identify sources for Hg. Ratios of PAHs indicate that a variety of sources exist including biomass (e.g., grass or coal) and petroleum (e.g., vehicle or natural gas) combustion. Sources are unique on regional to sub-regional scales with biomass combustion a dominant source for lakes in the southeastern portion of the Lower Peninsula.;Deer Lake, an US EPA Area of Concern, in Michigan's Upper Peninsula was contaminated with Hg as a result of a mining related laboratory activities. Sediment cores collected from the lake revealed that the natural recovery remediation strategy employed was resulting in deposition of "clean" sediments and Hg concentrations should reflect those of nearby lakes in ca. 10y. Most recently a terrestrial pathway for Hg was identified using Al:Hg ratios which could hamper the process of delisting Deer Lake as an Area of Concern.;The results of this research support the hypothesis that local scale processes are more significant than regional to global sources for Hg to Michigan's lakes. This has important implications for the development of new environmental legislation limiting Hg output from coalfired power plants and other energy generating facilities.