Big answers found on microscopic level

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Brent Lewis is digging deep, but not too deep, to study seasonal variability of heavy metals in coastal wetland sediments along Saginaw Bay.

"The main goal of the project is to examine the chemical and physical processes that control metal cycling in the sediments and their potential for remobilization into the waters of the Bay," said Lewis, associate professor of Environmental Chemistry at Kettering University.

Beginning in March 2003 and continuing through February 2005, Lewis will use microelectrode technology to sample sediments on a microscopic level to determine what makes them contain or re-mobilize heavy metal pollutants in an area that is in close proximity to the water source for southeastern Michigan residents.

His research is part of a two year, $63,000 grant from the Michigan Seagrant Program, part of the National Oceanographic and Atmospheric Administration (NOAA) National Seagrant College Program. There are five Seagrant projects in Michigan including Lewis' heavy metals and coastal wetlands project.

"Part of what we will look at in the Saginaw Bay wetlands is the forms that the metals are in," said Lewis, "for example, at the surface where you have oxygen present, the heavy metals like mercury, cadmium, lead, and zinc are generally associated with oxides. As you go deeper down in the sediment, bacteria uses up the oxygen and those oxides then dissolve - that releases the metals back into the water," he said.

"The form the metal is in controls whether it stays in place or whether it is re-mobilized or re-dissolved into the water column and moves," Lewis said.

When sediments are dredged, metals that have been locked in place are brought up and exposed to air, which change their forms to metal sulfides. The transformation releases the metals allowing them to dissolve and travel elsewhere.

The dissolved free ion is the toxic form. When locked into the sediments the oxide is inert and it is not going to bother aquatic organisms, but if brought up by dredging, a big storm, lots of boat traffic or a change of seasons, it can become a problem for aquatic life. "One of the issues we're looking at in Saginaw Bay is how do things change from summer to winter?" said Lewis.

"We will sample three or four wetland sites on the eastern shore of the Saginaw Bay," said Lewis. "This is by far the largest watershed in Michigan, it drains the Flint, Shiawassee, Tittabawassee and Cass rivers, and at the head of the bay there is Bay City with heavy industry."

"One of the sites we'll be sampling is very close to a Consumers Energy coal fired power plant - burning coal is a source of atmospheric metals, so we'll expect to see higher metals because of the industry and the power plant and then we'll sample a couple of stations further down the bay three or four times over the course of a year," Lewis said.

Using the microelectrodes, designed at the University of Delaware, to measure the sediment on a sub millimeter resolution, Lewis and his students will be able to go down about half a millimeter at a time. That small of a sample range is very important for wetlands sediments because the chemistry changes within the upper four to five centimeters.

Researchers are unable to get as accurate a sample by traditional methods in which they can only analyze 25 millimeter slices. "You miss most of the chemistry," said Lewis, "microelectrodes allow you to get very fine scale resolution.

"We'll go down slowly, taking our first measurement slightly above the mud. This allows us to measure oxygen, manganese, sulfur and iron, all in a single measurement," he said. "Sediment is not all just solid, it is very small solid grains. Between the grains is the porewater. What the microelectrodes do is measure the chemical concentrations in the water between the sediment grains," said Lewis.

On sub-millimeter resolutions, the changes in concentration with depth are correlated to the chemical reactions and biological reactions going on, so by being able to measure these vertical profiles researchers can identify the major chemical reactions that are happening. Those reactions change over time, either seasonally, or with physical disruption, type and density of vegetation in a wetlands.

Most of the equipment Lewis will use is hand built, custom-designed equipment. He and his team will build a platform they can set in the marsh to make in situ measurements and take cores to bring back to the lab. Once in the lab they will section the cores, extract the porewater from them and measure metals for both the porewaters and solids, looking at the different mineral phases the metals are in.

"In terms of metal pollution one of the things the EPA is interested in is developing Sediment Quality Criteria (SQC)," Lewis said. "They've done for organic contaminants, but it's hard to do for the metals. What we would like to have is some fairly straightforward, simple-to-apply measurements that will outline a quantitative measure to identify hot spots for management purposes," he said.

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"I'm interested in the chemistry," he said, "it's a process-oriented study; what are the chemical, physical biological processes that can be applied in a general sense, not just for Saginaw Bay but for any fresh water wetlands?"

In the long term, Lewis would like to use Kettering's new Environmental Scanning Electron Microscope to do major element analysis. "I would also like to try to develop a collaboration with an aquatic toxicologist at another university to follow up on our collections with some actual toxicity measurements," he said.

For more information about the National Seagrant College program, visit the NOAA website.

Written by Dawn Hibbard
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