Agronomist Cristina Negri collects poplar samples to measure the pollutants sucked from the earth.
George Joch/Argonne National Laboratory
Argonne, Illinois—A legacy of the Argonne National Laboratory’s early foray into atomic energy lies buried here on its campus, about 25 miles southwest of Chicago. Although solid wastes from all sorts of experiments have been sealed in a landfill, certain liquids, mostly chlorinated solvents, still taint the water that runs under the site. The ongoing attempt to remove these contaminants occupies an enormous experimental facility that covers four acres and looks like a forest.
“I like to brag that I have the biggest lab at Argonne,” says agronomist Cristina Negri, indicating an expanse of 900 poplars and willows growing in rows. The trees stand about 30 feet high. More important, their roots extend 30 feet down, where they tap the contaminated aquifer and literally pull pollutants out of the ground.
“The purpose here is not to grow the most beautiful trees,” Negri says as she walks among them. “It’s to make them work.”
Under normal circumstances, tree roots prefer to sip water from sources as close as possible to the surface. But these trees, planted in 1999, don’t have that option. They are impressed laborers, set into plastic-lined pits that force their roots to tunnel deep for drink. The roots lift the contaminated water into the tree trunks, where transport tissues conduct it on up to the branches and leaves. From there, as droplets transpire through leaf pores, the water evaporates and sunlight breaks down the dissolved solvent molecules, rendering them harmless. As Negri explains the process, the smell of poplar seems to excite her like catnip.
Before the willows and poplars took over the job of wastewater management, Argonne was using extraction wells to pump contaminated water to a treatment plant. But the static mechanical pumps could not chase after groundwater that continually changed course through the complex terrain. The natural pump of a willow or poplar, on the other hand, is not only self-sustaining but so water-loving that it will snake down or fan out as far as need be to reach moisture.
Negri’s trees each pump as much as 26 gallons of water per day at their summer peak. She measures the daily flow through the trunks by inserting probes that transmit data to solar-powered recorders mounted on tripods nearby. Periodically, laboratory workers roam the forest to sample bark, small branches, and leaves to assess the trees’ success in pollution extraction.
