SPATIAL AND TEMPORAL PATTERNS OF MERCURY LOADING IN MICHIGAN’S INLAND LAKES

Improved understanding of the sources and pathways of Hg is still needed to help solve continuing environmental problems and evaluate the effectiveness of environmental legislation. The relative importance of regional versus watershed sources and atmosphere versus terrestrial pathways is debated. Our working hypothesis is that as environmental regulations effectively reduce regional atmospheric sources for contaminants in the Great Lakes region, spatial and temporal loading patterns as indicated in sediment chronologies will reveal the importance and mechanisms of watershed scale processes on Hg cycling.

To evaluate this hypothesis, Hg sediment chronologies were compared among inland lakes within the State of Michigan. Sediment cores were collected from twenty-two inland lakes representing a broad geographic range and varied watershed land use characteristics. Cores were collected from the deepest portion of the lakes and sectioned in the field. Sedimentation rates and focusing factors were estimated using ²¹⁰Pb and confirmed by ¹³⁷Cs radionuclide and stable lead profiles. Sediment ages were estimated using constant rate of sedimentation, rapid steady state mixing, or constant flux:constant sedimentation models. Mercury concentrations were analyzed on freeze dried sediment using a Lumex thermal decomposition atomic absorption spectrophotometer. Results show that 1) Hg 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; and 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. Results are consistent with the hypothesis but indicate that watershed scale processes have been important historically. The continuing increase of Hg loading was not expected.