The U.S. Army Corps of Engineers has turned to an unlikely ally in its attempt to rid Washington's Spring Valley of contaminated soil -- ferns.

That's right: The feathery plants that decorate countless gardens were discovered a few years ago to have an unusual talent beyond just sitting there and looking pretty. A certain kind was found to soak up arsenic from soil like a horticultural sponge.

The Corps of Engineers decided to test the fern's effectiveness in the Spring Valley neighborhood, which was built on land the Army used for experimenting with chemical agents and munitions during World War I. About 2,800 ferns were planted in May at three sites, part of the Corps of Engineers' investigation and cleanup of an area that includes American University, embassies and hundreds of residences.

The study marks the first time the Corps of Engineers has tested the ferns in this capacity in a residential area. The ferns were used by the agency at the Army's Picatinny Arsenal in Dover, N.J., at a former apple orchard that had elevated levels of arsenic from insecticides. In a two-year study, the ferns reduced the arsenic level in the soil by 25 percent, a Corps of Engineers scientist said.

In Spring Valley, soil samplings taken at hundreds of properties found about 150 with elevated levels of arsenic, a known carcinogen. The Corps of Engineers did extensive remediation on 26 of the properties, including soil removal, replacement and landscaping, in 2002 and 2003. Crews have been working to clean up the soil at the remaining properties, a process that officials said could stretch to 2009. They are working on properties in groups, starting with those with the highest levels of arsenic.

Corps officials said they know that the work is messy and disruptive, because crews dig up everything in people's yards, including trees and driveways, to remove and replace the soil. That's why they said they decided to test the ferns in a $130,000 study. If successful, the ferns could be used to remove arsenic from soil in some cases in a less disruptive manner, cutting costs associated with the restoration, officials said.

"We hope to be able to remediate properties with less disruption to the homeowner, less intrusively, using this very green technology," said Gary Schilling, the Corps of Engineers' Spring Valley program manager.

Some in the community have reacted to the ferns with cautious optimism. Alma Gates, chairman of Advisory Neighborhood Commission 3D, which includes Spring Valley, said she was eager to see the results of the study. "I approve of it because it is a much less intrusive method," she said.

Others are more skeptical. Kent Slowinski, a member of the Restoration Advisory Board, a community group that advises the Corps on the cleanup project, questioned the ferns' effectiveness in Spring Valley. "I am concerned that it will only address surface contamination," said Slowinski, a landscape architect. "It will not address elevated arsenic two to three feet below the surface."

An official with Edenspace Systems Corp., a Dulles-based contractor that supplies the ferns, acknowledged that the plants are most effective at removing arsenic in the top foot of soil; anything much deeper would require another method. Corps officials said the plants are capable of reaching the contamination zone at the three test sites.

Most of the ferns can be found on a fenced-in strip of grass on Van Ness Street NW, between 44th and 45th streets. There, beneath a black shade cloth in between the curb and the fence of the federally owned Van Ness Reservoir, are hundreds of ferns of two varieties, Pteris multifida and Pteris cretica. The rest of the ferns were planted at two residences.

Schilling said that crews were scheduled to remove the soil in the Van Ness area, including several large oak trees that line the street, but neighbors asked them to save the trees and look for an alternative to digging.

Spring Valley "has a large number of beautiful, mature trees that add to the grace and beauty of the neighborhood," said Dorothy Zolandz, co-chairman of the Restoration advisory board. "So in cases where the remediation can be done properly and trees can be saved, I think most people would favor that approach if it proves to be viable."

The ferns have skinny, light-green leaves and do not resemble the typical household fern. Edenspace officials said their plants, dubbed edenferns, absorb the arsenic through their roots and store the substance in their leaves, or fronds. The fronds are then clipped and disposed of.

"Most plants don't take up very much arsenic at all," said Michael Blaylock, director of technology for Edenspace. "These plants have an unusual ability to take up very high concentrations of arsenic."

Why? The answer is somewhat of a mystery, though one theory is that the ferns developed this ability to deter insects. "Why these plants do it, why they have this capability, we don't really understand that yet," Blaylock said.

Blaylock said the use of edenferns can decrease the soil concentration of arsenic by 10 to 20 parts per million each year. The Corps is cleaning up soil in which arsenic levels are at or above 20 parts per million.

Earlier this year, the Corps of Engineers planted the ferns in soil from Spring Valley at a greenhouse in a research center in Vicksburg, Miss. Two kinds of ferns -- both of which have been planted in Spring Valley -- were proven to take arsenic out of the soil, said research physical scientist Cindy Teeter, who oversaw the study for the Corps of Engineers.

Teeter said the Corps of Engineers does not believe that phytoremediation -- the use of plants to remove contaminants from soil or water -- would prevent further digging and soil-removal in Spring Valley. Rather, the agency sees the process as being "just a tool in our toolbox," she said.

The agency will analyze the ferns and the soil at the end of September to determine how successful the ferns were at extracting arsenic. In the meantime, the ferns in Spring Valley are not hazardous to people or animals, but Edenspace suggests that they should not be eaten.

Edenspace staff monitor the soil moisture, make sure there has been no damage and look for weeds or signs of disease on the ferns. So far, preliminary tests show that the plants in Spring Valley are absorbing arsenic.

Scientist Michael Blaylock examines some of the ferns. "These plants have an unusual ability to take up very high concentrations of arsenic," he said. The fern fronds are clipped after arsenic is absorbed by the roots.