Can trees replace machines?

  • Published
  • By 60th Civil Engineer Squadron
  • 60th Civil Engineer Squadron
TRAVIS AIR FORCE BASE, Calif. --  There is a special grove of trees on the western side of the base that may become part of a large groundwater cleanup effort. 

In 1998, the base began a long-term study that involved the planting of 480 red iron-bark eucalyptus trees over solvent contaminated groundwater. The source of the solvents was a battery acid neutralization dump adjacent to the base battery shop. 

Known as a phytoremediation study, these trees offered a lot of promise, but there was insufficient information to prove whether they could successfully clean up groundwater or at least stop the contaminated groundwater from moving any further. 

After ten years of growth, it is time to find out if the trees can be a part of a groundwater treatment strategy. Last year, the Air Force Center for Engineering and the Environment funded a series of field visits to answer this question. Parsons Inc., an environmental and engineering consulting firm, and a team from Utah State University are carrying out the field work. 

"There are several ways that trees can contribute to groundwater cleanup," says Mark Smith, Travis Remedial Program manager. "They can absorb solvents, they can release enzymes around the roots that can destroy contaminants, or they can move dissolved solvents through the sap and out of their branches or leaves. We hope to find out which one is the most dominant process and whether it is enough to get the job done." 

In early June, the Parsons/USU team conducted their first set of tests. They extracted and analyzed core samples from the trunks of the largest trees to demonstrate whether contaminants can be absorbed through the roots and moved with the flow of sap to other parts of the tree. 

They also collected 'surface flux samples' around and in the center of the tree stand. Surface flux sampling are a way of collecting air samples directly from top soil to measure the amount of contaminants that is released directly from the ground. In theory, trees with well established roots will break up the subsurface soil and create paths for solvent vapors to move from groundwater to the soil surface. Although not expected to be significant, surface flux measurements will improve our understanding of the trees' impact on groundwater contamination. 

Toward the end of June, the team returned to Travis to carry out a sequence of transpiration tests. Transpiration is the way that plants 'breathe.' They absorb carbon dioxide and release oxygen. They can also release solvents slowly into the atmosphere, where they will eventually break down into harmless compounds. 

Using a specialized gas collection apparatus designed at USU, transpired air is run through filters that collect both solvents and water vapor. The filters are processed in the laboratory, and the results are used to estimate the concentration of solvents in the water vapor and the amount of solvents that the entire grove of trees releases into the atmosphere each day. 

Parsons plans to repeat this sample collection and analysis when the soil is drier and the water table is lower. They will then report their findings and provide Travis AFB with a recommendation to either use this cleanup strategy as part of an overall groundwater remedy or discontinue further evaluations. 

"Obviously, we hope that the trees can help the base to reach groundwater cleanup standards at this site, because they offer a number of advantages over pump systems," said Mr. Smith. "Trees require no electricity and almost no maintenance, their root systems grow and improve their ability to extract solvents from the subsurface, and they generate no hazardous waste." 

Before phytoremediation can be selected as part of the final remedy at this site, it must receive both regulatory and community acceptance. For the next two years, Travis will be working with both federal and State of California environmental regulatory agencies and the public to select the most technically appropriate and cost-effective remedies for all contaminated groundwater sites. If the study supports the use of trees for groundwater cleanup, then phytoremediation will be included in the decision-making process. 

"Otherwise, we may have a lot of cheap tree mulch to give away in the near future," Mr. Smith joked.