Lake Michigan Mass Balance

The Lake Michigan Mass Balance Project

Certain toxic substances accumulate or persist in the Great Lakes because, unlike rivers that are constantly flushed with cleaner waters, lakes act as "pollutant sinks". A drop of water entering Lake Michigan today will remain in Lake Michigan for an average of 100 years before it either evaporates or washes into Lake Huron through the Straits of Mackinac. For a particle of soil, the retention time is much, much longer. Unless a pollutant naturally breaks down into harmless components, it persists as a threat to the environment. Human health concerns center on those persistent chemicals that also collect or "bioaccumulate" in fish and/or other aquatic organisms. Certain bioaccumulative chemicals of concern (BCCs) are present in Lake Michigan fish at concentrations that may exceed U.S. Food and Drug Administration (FDA) tolerances or human health guidelines. This situation has resulted in closures of some commercial fisheries and posted warnings about fish consumption.

The Lake Michigan Mass Balance Study (LMMB) focuses on four chemicals: PCBs, trans-nonachlor, atrazine, and mercury. These substances are being studied because they are representative of classes of pollutants (eg: pesticides, herbicides, metals, etc.) of environmental significance in Lake Michigan and throughout the Great Lakes.

The Lake Michigan Mass Balance Workplan identifies four specific objectives:

1. Identify Chemical Loading Rates
To identify relative loading rates of critical pollutants from major media (air, tributaries, sediment resuspension) to the Lake Michigan Basin in order to better target future load reduction efforts.
2. Establish Baselines
The LMMB loading rates will establish a baseline against which to gauge progress in meeting reduction goals.
3. Predict Benefits
The mass balance models will deliver predictive ability to resource managers to assist in choosing management strategies for Great Lakes toxic chemicals. Specifically, managers will determine the environmental benefits of specific load reduction scenarios for toxic substances and the time required to realize those benefits.
4. Understand Ecosystem Dynamics
To improve our understanding of key environmental processes governing contaminant cycling and availability within relatively closed ecosystems.

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