A new leaf

By Jack Bess

An environmentally troubled site in Green Bay is being cleaned up by a "technology" dating back to the Garden of Eden.

Prairie Cascade Willows were planted in May 1996 at the Proctor & Gamble paper products plant, where an underground fuel oil line ruptured in the 1970s. The trees are slowly breaking down petroleum hydrocarbons in the soil at four "hot spots" within the 60-acre parcel.

The Green Bay site and several dozen others nationwide are part of an emerging science called phytoremediation, in which plants are used to remediate former industrial properties referred to as brownfields.

"It's new technology and very exciting, but it's not perfect for every site," said Eric Carman, a hydrogeologist at Arcadis Geraghty & Miller in Milwaukee who is working on the Green Bay project.

Taking root

Phytoremediation must be applied judiciously, depending on the type and amount of environmental contamination and the types of plants, which can either contain the contamination, degrade it or uptake it into the plant itself, Carman said.

Carman said poplar trees have been planted to cleanse groundwater contaminated with gasoline, while mulberries have been used to degrade a certain type of hydrocarbon.

In the case of Green Bay, the willows are not cleansing the hydrocarbons but secreting nutrients and sugars into their root zone, or rhizosphere, that help degrade the contamination in the soil, he said.

The breaking down of contaminants in the root zone was identified early on as an important mechanism in phytoremediation, said Daniel Zitomer, professor of civil and environmental engineering at Marquette University. In many cases, it's not the plant itself but the microorganisms, mostly bacteria, that live in the root zone that does the remediation, he said.

Trees of choice

Some plants are particularly suited for specific types of contamination, Zitomer added. Because willows respire a high volume of water through their systems, they can be used to consume very soluble pollutants, he said.

Grasses and plants can also be used to remediate contaminated sites. Indian mustard is a plant used to cleanse soil of metals, Carman said. What happens to these plants afterward will depend on the amount of contamination they are exposed to.

If the concentration of metals is low, the plants can be composted. If the amount is higher, the plants will be burned, but in some cases, metals can be recovered from the ash and re-used, he said. That underscores the durability and potential environmental problems caused by metals.

"That's why metals problems are tough to whip," Carman said.

Phytoremediation is a deceptively simple solution and involves understanding what specific plants do to certain pollutants. One plant might consume the contaminant and transform it into food, but another plant could convert it into something even more toxic. In that case, you have to make sure the site is fenced off and determine how you are going to dispose of the plants when they are done, Zitomer said.

Using trees is appropriate at sites where there are relatively low levels of pollution, certain types of compounds and shallow contamination of soil, say 5 feet deep, or close enough to the surface so that a tree's roots can reach it.

It wouldn't be appropriate where there is a high concentration of relatively toxic substances or deep groundwater pollution, Zitomer said.

With such a young science, there are still some unanswered questions awaiting study, such as what would be the consequences if an insect or animal were to nibble leaves that have absorbed a contaminant, Zitomer said.

But where phytoremediation has been used, the benefits are clear, Carman said. A formerly contaminated site beautified with trees improves public perception, he said.

In addition, the process is very cost-effective, said Carman, who estimated that phytoremediation could run 20 percent or less of the cost of treating a site with conventional technology. And Wisconsin alone has 12,000 sites in need of remediation, he added.

The process can even be a source of revenue, as in one case in Oregon, here trees used in phytoremediation were sold to paper mills, Carman said.

As for the Green Bay site, Carman said he is scheduled to take soil and tree samples in October to determine whether the process is finished or perhaps project a timeline when it will be complete.