Excerpted from the March/April 1999 issue
Excerpted from the March/April 1999 issue
On the Origin of Metallic Species
Heavy Metals in Lake Superior's Watersheds
All mercury is not created equal. Neither is all zinc, lead, or copper. The toxicity of these heavy metals depends upon their forms, or "species." A free metal ion, for example, is generally more toxic than a metal ion associated with a large molecule because the free ion tends to cross an organism's cellular membranes more easily.
With support from UW Sea Grant, researchers David Armstrong, Martin Shafer, and James Hurley, all of UW-Madison, are analyzing the species, bioavailability, and origins of a dozen potentially toxic metals that wash into Lake Superior from its watersheds each year.
The scientists are also examining how physical, geological, and biological features of the lake’s watersheds affect the transport of metals.
For example, certain species of some metals adhere strongly to the dissolved organic carbon (DOC) that vegetation produces when it decays. Watersheds that produce large amounts of DOC—such as wooded or wetland watersheds—would flush these metals more rapidly than those that produce little DOC.
The chemical properties of soils and rocks also influence the amounts and species of metals that reach the lake.
The researchers are also investigating how much of the metals in the watersheds originate from human versus natural sources. Armstrong says one important source is the worldwide burning of fossil fuels and waste. This combustion produces heavy metals as byproducts and emits them to the atmosphere. Some of these eventually fall to earth around (and on) Lake Superior. Smaller amounts originate naturally from weathering of rocks and soils in the drainage basin itself.
A preliminary finding of the project is that 40% to 70% of the annual total of heavy metals reaching Lake Superior washes in during heavy rains and the spring snow melt.
Measuring these high flow events presented logistical challenges to the researchers.
"We’re here in Madison, and the streams we’re sampling are up around Lake Superior. So the team that’s been working on this would have contact with the gauging station [up there] so that we could watch the water flow. When it began to rise, the team would jump in the car and race up there to sample," Armstrong said. "We’re pretty happy, though—we caught most of them near peak flows."
The researchers have overcome technological challenges as well. Some heavy metals exist in such small quantities in water and soil that samples can be contaminated simply from contact with containers or workers’ clothing. By adopting very sensitive analytical methods and clean techniques, the research team can detect traces as scant as a few parts metal per trillion parts water.
As the project enters the second of its two years, the researchers continue to gather knowledge that will aid regulators and managers in developing realistic standards governing trace metal pollution in Lake Superior and elsewhere.
- John Karl
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Last updated 28 March 2000 by Karl
All contents copyright 1999 University of Wisconsin Sea Grant Institute
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