Lake Michigan Mass Balance

The Mass Balance Concept

The development of effective strategies for toxics management requires a quantitative understanding of the relationships between contaminant sources, concentrations, sinks, and effects in the ecosystem. The mass balance approach integrates environmental monitoring, load estimation, and research efforts within a modeling framework that is compatible with both scientific and management objectives. Mass balance was chosen for Lake Michigan in order to understand the complex pathways through the system so that effective management strategies can be designed to reduce the threats from toxic chemicals. A key to mass balance is the estimation of the magnitude of mass fluxes (the movement of chemicals between environmental media such as water, air, and sediment). Computer models evaluate environmental data to estimate the the flow of pollutants between water, air, and sediment and the food web. 

Based on these estimates, the mass balance can determine how changes in inputs from atmospheric deposition or tributary streams affect concentrations and inventories of toxics. Thus, the mass balance can serve as a useful tool to understand the current environmental concentrations of toxic chemicals and predict future concentrations based on various management scenarios.

LMMB Modeling Objectives:

1. Provide a consistent framework for integrating load estimates, ambient monitoring data, process research efforts, and prior modeling efforts, leading to a better understanding of toxic chemical sources by media, transport, fate, and bioaccumulation in Lake Michigan
2. Estimate the loading of priority toxics, solids, and nutrients from major tributaries to Lake Michigan for the duration of the Enhancement Monitoring Program
3. Estimate the atmospheric deposition and air-water exchange of priority toxics, including spatial and temporal variability over Lake Michigan
4. Calibrate and confirm mass balance models for priority toxics using Enhancement Monitoring Program data, based upon models for hydrodynamic and sediment transport, eutrophication/organic carbon dynamics, toxics transport and fate, and food web bioaccumulation
5. Evaluate the magnitude and variability of toxic chemical fluxes within and between lake compartments, especially between the sediment and water column and between the water column and the atmosphere
6. Apply the mass balance models to forecast contaminant concentrations in water and sediment throughout Lake Michigan, based upon meteorological forcing functions and future loadings based upon load reduction alternatives
7. Predict bioaccumulation of persistent toxic chemicals through the food web leading to top predator fish (lake trout and coho salmon) for specific fish populations in the lake to relate mass balance predictions of water and sediment exposure to this significant impaired use
8. Estimate (quantify) uncertainty associated with estimates of tributary and atmospheric loads of priority toxics, and model predictions of contaminant concentrations
9. Identify and prioritize further monitoring, modeling, and research efforts to address additional toxic substances, further reduce uncertainty of predictions, establish additional cause-effect linkages (such as ecological risk endpoints and feedbacks), and evaluate additional source categories in the watershed

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