-  EPA
     EPA/600/F-01/012 December
                                                       Office of Research and Development
MASS BALANCE: ADDING IT ALL UP

Pollutants enter the Great Lakes and other aquatic
ecosystems from the air, land, and water. Scientists seek
to understand how pollutants are cycled within aquatic
systems and their foodwebs, and to determine effects of
pollutants on living organisms. Solving this puzzle involves
collecting a variety of environmental samples, and using
mathematical models to analyze the relationships among
the sample sites.

The amount of pollutant that enters a lake equals the
amount of that  pollutant leaving, remaining in, or breaking
down in the lake. This mass balance of sources and fates
of pollutants is  a valuable tool. It enables resource
managers to design cost-effective strategies to minimize
risks to human  and ecosystem health. The Mid-Continent
Ecology Division is using a mass balance approach to
study two pollutants of concern in Lake Michigan:
mercury and atrazine.

Atrazine

Atrazine was selected for study in Lake Michigan
because of its high use in the watershed, presence in the
lake, and suspected potential toxicity. Atrazine is an
herbicide used  to control broadleaf weeds in corn and
other crops nationwide. It accounts for 26% of the total
herbicide applied within the Lake Michigan watershed.
Atrazine enters the lake primarily through runoff and
precipitation. Little is known about this pesticide's fate
once in the lake, but it is thought to be harmful to aquatic
organisms.

Though atrazine degrades  rapidly on land, it can remain in
water for years. The amount of atrazine in the water
increases during times of greater agricultural use and
rainfall. Both small and large plants in the lake
(phytoplankton  and macrophytes) are vulnerable to
herbicides. However, bottom-dwelling invertebrates,
zooplankton, and fish may  be affected by the changes in
their food supply or shelter through harmful to plant
communities. Atrazine also raised concerns as a
potential human carcinogen and endocrine disrupter.

Mercury

Mercury was selected for study in Lake Michigan
because of its persistence  in the environment, known
toxicity, and bioaccumulative properties. Environmental
mercury results from both natural and anthropogenic
(human-caused) events. Mercury occurs naturally as an
element in some minerals, and often occurs in coal and
ore deposits. Anthropogenic activities contribute an
estimated 11,000 tons of mercury to the air, water, and
soil each year. Sources include fuel combustion,
mining and manufacturing activities, and agricultural
and municipal wastes. Mercury is used in batteries,
thermometers, switches, fluorescent lamps, and
commercial chemicals.

A strong neurotoxin, mercury contributes to many
serious human and animal health problems worldwide.
As methyl mercury, it is of particular concern because
of its ability to bioaccumulate.  Methyl mercury harms
organisms as its concentration in tissue increases and
interferes with cellular processes.

People around Lake Michigan are developing a
Lakewide Management Plan (LaMP) to define and
reduce the amount of mercury and other priority
pollutants in the lake. The LaMP involves government
agencies (federal, state, local, and tribal), academia,
industry, and environmental groups working together to
form concise strategies to confront pollution issues in
the Lake Michigan basin. Atrazine is listed in the LaMP
as an "emerging pollutant" with the potential to harm the
lake. Completing a mass balance model for both
mercury and atrazine will provide a tool for use  in the
LaMP process.

Study of these chemicals is part of a larger effort of the
Lake Michigan Mass Balance Project that focuses on
four specific pollutants: PCBs, trans-nonachlor,
atrazine, and mercury. These pollutants were chosen
from a list of chemicals of concern in Lake Michigan to
represent a broad  range of pollutants from a variety  of
sources. Most are persistent in the environment, and
many bioaccumulate in living tissue. Thus, they pose a
threat to the lake's precious wetlands,  wildlife, and
fisheries.

Contact: Russell Kreis (734) 692-7615

National Health and Environmental Effects
Research Laboratory
Mid-Continent Ecology Division
6201 Congdon Blvd.
Duluth, MN 55804
Environmental Education and Scientific
Outreach

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