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Lake Michigan Mass Balance Study - Overview

LMMB Study
Overview Contaminants

The Issue: Are sediments, air, land, and water sources or pathways of contamination that affect the integrity of the ecosystem?

Sediments, air, land, and water continue to be sources or pathways of contamination that affect the integrity of the Lake Michigan ecosystem. While regulatory and remediation programs reduce pollutant sources, ongoing releases and the region’s legacy of contamination continue to serve as sources of pollutants. The findings of the Lake Michigan Mass Balance Study will allow decision-makers to better understand pollution pathways, permitting them to develop more effective policies to deal with pollution issues and pathways.

The mass balance approach is based upon the principle of conservation of mass, which states that the mass of a chemical contained in the lake is equal to the amount entering the system, less the amount leaving and chemically changed in the system.

What is the Lake Michigan Mass Balance Study?

The Lake Michigan Mass Balance (LMMB) Study is an enhanced monitoring and modeling project which developed a scientific base of information to inform LaMP policy decisions and to better understand the science of pollutants within an ecosystem. The LMMB Study’s specific objectives are:

  1. To identify relative loading rates of four categories of pollutants - polychlorinated biphenyls (PCBs), mercury, atrazine, and phosphorus - entering Lake Michigan from major media (air, tributaries, and sediments).
  2. To establish baseline loading estimates in 1994-1995 against which to gauge future progress
  3. To develop the predictive ability to determine the environmental benefits of specific load reduction scenarios for toxic substances and the time required to realize those benefits through the use of models
  4. To improve our understanding of key environmental processes governing the movement of pollutants through the lake (cycling) and availability to fish and plant life (bioavailability) within this large freshwater ecosystem

The LMMB Study focused on constructing mass balance models for a limited group of pollutants. Polychlorinated biphenyls (PCBs), atrazine, mercury, and phosphorus were selected for inclusion in the LMMB Study because these pollutants pose or were anticipated to pose a risk to aquatic and terrestrial organisms (including humans) in the Lake Michigan ecosystem. These pollutants were also selected to cover a wide range of chemical and physical properties and represent other classes of compounds which pose current or potential problems. Once a mass balance for selected pollutants is established, additional contaminants can be modeled, given sufficient data.

Figure 1. Lake Michigan Mass Balance Modeling Framework

The LMMB Study used an integrated, multimedia mass balance modeling approach to evaluate the sources, transport, and fate of contaminants in the Lake Michigan ecosystem (Figure 1). The modeling framework is a series of coupled and/or linked models which integrates the physical, chemical, and biological components of the system and accounts for the dynamic interactions and processes in the system. The mass balance approach is based upon the principle of conservation of mass, which states that the mass of a chemical contained in the lake is equal to the amount entering the system, less the amount leaving and chemically changed in the system. In the Lake Michigan system, pollutant inputs may come from atmospheric deposition, tributary loads, and from sediments within the system. Pollutants may leave the system through discharge through the Straits of Mackinac, permanent burial in bottom sediments, and volatilization to the atmosphere. Pollutants within the system may be transformed through degradation or stored in the ecosystem compartments such as the sediment, water column, or biota, including humans.

Figure 2. Lake Michigan Mass Balance project water spatial resolution/segmentation schemes

The modeling construct was applied to the study contaminants, where appropriate, and used three different spatial resolutions (Figure 2). Modeling results will be provided for each of the contaminants at the highest resolution that is presently available. The mass balance was primarily designed to provide a lakewide perspective of contaminant sources, fate, transport and effects. However, with the present spatial resolution design, selected aspects of the contaminants can be addressed on a finer scale. Information regarding Lake Michigan tributaries will be provided from samples collected only from tributary mouths.

Sample Design and Sample Collection

To characterize Lake Michigan, over 200 locations (stations) were sampled during the course of the project (Figure 3). Samples were collected for air, water, sediment, tributary mouths, and biota. Over 35,000 samples were collected for the Lake Michigan Mass Balance during the 1994 and 1995 sampling seasons. The study produced approximately 1,000,000 analytical data points. The field sample collection methods and the laboratory methods used in analyses are documented in the Lake Michigan Mass Balance Methods Compendium.

Figure 3. Lake Michigan Mass Balance survey stations

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