THE INFLUENCE OF LAND USE/LAND COVER ON THE ACCUMULATION OF MERCURY IN INLAND LAKES

The importance of local watershed scale versus regional/global sources is still debated. The hypothesis driving this research is that watershed scale sources, currently and historically, have had significant influence on mercury accumulation in inland lakes and these sources will continue to contribute to mercury loadings after regional and global sources have been reduced. To evaluate this hypothesis, ²¹⁰Pb dated mercury sediment chronologies were compared among 20 inland lakes within the State of Michigan representing a broad geographic range and varied watershed land use characteristics. Results 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; 3) temporal change in Hg loading is not gradual but includes episodic pulses of increased loading; 4) some common episodic pulses can be related to events such as volcanism and war time activities, however, many pulses are not common among lakes, and the magnitude of the pulses are greater in recent sediment and 5) recent atmospheric inputs do not fully explain the anthropogenic inventory as predicted by mass balance modelling. Both common and different episodic accumulation events among the watersheds and modelling results support the hypothesis that watershed processes may be as or even more important than regional/global sources. Multivariate statistical analysis of sediment metal concentrations reveal that mercury may be entering Michigan lakes through non-atmospheric pathways supporting the watershed level source hypothesis. Ongoing studies combining mercury accumulation rates and temporal and spatial patterns with GIS delineated land use/land cover within the watershed are attempting to identifiy important watershed attributes influencing mercury accumulation. Such an approach has been shown to be sucessful for Pb.