Located about 20 miles south of Aiken, SC, the Site encompasses about 310 square miles of South Carolina Coastal Plain-a flat, scrubby landscape of mixed pine and hardwood forests. The Federal Government bought the Site in the early 1950s to produce materials for nuclear weapons. Though the five reactors built on the Site were shut down by 1988, numerous defense activities continue, and a priority mission is the cleanup of legacy wastes. The Site's mission as a research park enables cooperative studies of a broad range of environmental restoration and remediation projects involving numerous university, Federal, and State cooperators.

Site managers discovered that rainwater was running through an old radioactive burial ground where tritium-contaminated waste was stored, forming a plume of contaminated ground water. A radioactive element first discovered in 1934, tritium reacts readily with water to form tritiated water. At the Site, tritiated water was leaching into the ground water-and then into nearby Fourmile Branch, a tributary of the Savannah River.

Not a small leak either, more like millions of gallons of tritiated water a year. It was the single greatest source of tritium in the Savannah River. Something had to be done.

At the time, cleaning up a site like this involved complex and expensive engineering systems for treating waste onsite-often by separating the contaminant from the soil or vegetation and then burying the residue in a special basin. It's an incredibly expensive process. Site managers were looking for a simpler way to reduce the possible risk from tritium to as close to zero as possible. The situation presented the opportunity to try out phytoremediation, a relatively new approach that seemed tailor made for a forested research park in the sunny South.

Phytoremediation uses plants (the prefix phyto refers to things botanical) to remediate (set right or restore) sites by breaking contaminants down in the soil, sequestering them in plant tissues, or releasing them into the atmosphere. More specifically, phytoremediation harnesses transpiration-the unique power of plants to move water from the soil through their tissues and out through their leaves. Because trees can move so much water (as much as a ton a day in some places), they're often the plant of choice for phytoremediation.

Long-Term Data Proves its Mettle

Just up the hill from the leaking burial site is a small forest. The team tasked with the cleanup project came up with the idea of pumping the contaminated water up to the forest, letting the trees disperse tritiated water to the atmosphere. But before they started the project, they needed some assurance that a tree-based system could actually work.

"The Site's team naturally turned to the Forest Service, which came in to reforest and manage natural resources on the Site in the 1950s," says John Blake, Forest Service Savannah River assistant manager for research. "They had some basic questions. Could tree transpiration really control how much contaminated water leached into ground water and nearby streams? If so, what effect would time of year, site vegetation, soils, and root systems have on how much water the trees could move?"

For answers, they turned to the long-term research set up decades before at Coweeta by Wayne Swank to study how evapotranspiration (evaporation from tree leaves and the soil) from forest stands affects stream discharge at different landscape levels. They added regional data and results from water uptake experiments on the Crossett Experimental Forest in Arkansas. "Based solely on the reliability of estimates provided by long-term Forest Service research, the Site invested several million dollars in an experiment to manage the tritium discharge," says Blake.

In fall 2000, the Site cleanup team worked with SRS and university partners to install and test the two components of what became an experimental-eventually, fully operational-phytoirrigation system. As a first line of defense, they built a dam and a 2.6-million-gallon retention pond at the base of the old burial pit to prevent contaminated ground water from flowing into Fourmile Branch. Evaporation from the pond surface was also expected to contribute to remediation. For the second component, the cooperators designed a system that pumps water from the pond up the hill into a 22-acre forest of mostly loblolly and slash pine, where a sprinkler system adds another opportunity for evaporation. When the contaminated water reaches the ground, it is taken up and transpired from the trees, the tritium released to the atmosphere.

"Every part of the system was designed to take advantage of evaporation," says Blake. "But the assumption was that the greatest amount of tritium removed from the system would be that transpired into the atmosphere from tree leaves."

In April 2001, cooperators installed a monitoring system to validate the estimates used to start the project, and to provide the data needed to develop a method to regulate irrigation on both a daily and seasonal basis. At the end of 3 years, cooperators-including former SRS forest hydrologist Chris Barton-published their findings in a special issue of the journal Environmental Geosciences. As of March 2004, 35.2 million gallons of water had been irrigated, and tritium activity in Fourmile Branch reduced by almost 70 percent as a result of both pond storage and evapotranspiration from the forest.

"The project validated the reliability of the estimates used to start it, while saving the Site the hundreds of millions of dollars alternative cleanup technologies would have cost," says Blake. "As an experiment, it also provided new knowledge such as water uptake in winter, information useful for developing future ground-water management and remediation strategies at the Site and other facilities located in similar landscapes."

Future studies include expanding the phytoirrigation treatment to young plantations using the short woody crop rotation studies Mark Coleman (SRS biological scientist located at the Site) has established for research on intensive management of southern pines for biomass.

Savannah River Site: www.srs.gov/general/srs-home.html

For more on Mark Coleman’s work on short rotation woody crops, see Locally Grown Power starting on page 8 of Issue 1 of Compass, available online at www.srs.fs.usda.gov/compass/winter2005/index_winter2005.htm.

For more information: John Blake at 803–725–8721or jblake@fs.fed.us.