Phytoforensics: Using Trees to Find Underground Contamination

The water, air and soil around us can carry contaminants that harm health. Sampling them underground the traditional way — drilling wells or direct-push soil sampling — is costly and slow. Trees, though, are closely tied to the subsurface, and sampling their trunks can reveal pollution below: phytoforensics.

Scientists at the USGS Missouri Water Science Center were among the first to use it to screen sites before traditional sampling, to guide further sampling, and to show its large cost savings (Schumacher and others, 2004, 2005).

Taking a tree core with an incremental borer, forceps, a vial and gloves. Cores are taken about 3 feet above the ground and analyzed at the USGS Missouri Water Science Center or Missouri University of Science and Technology. USGS.

How it works

Contaminants in underground water, air and soil enter tree roots and rise through xylem — small, water-filled tubes in the wood. A tree core from the trunk captures them.

• Quick and cheap: one- or two-person teams can take as many as 100 cores a day; lab results usually come within 2 weeks.
• Non-invasive: it uses existing trees, with no drill rigs or heavy equipment, and little disruption to the landscape.
• Trees heal: coring may scar a trunk locally, but trees repair it within 2 to 3 years.
• Recognized: a 2008 user's guide from USGS and the U.S. Environmental Protection Agency accepted phytoforensics as a viable tool (Vroblesky, 2008).

A tree samples contaminants from a large volume of shallow ground, its effective subsurface sampling volume. USGS.

Why trees are good samplers

• Large volume: roots spread sideways, roughly in proportion to trunk diameter, so one core reflects a large volume of ground.
• Long memory: xylem holds contaminants for a long time, so a core reflects concentrations averaged over time.
• Shallow reach: most roots are near the surface, so trees reveal shallow contamination.
• Direction: roots usually feed xylem on the same side of the tree, so concentrations can differ around the trunk. The side with the highest levels points toward the strongest contamination. Taking several cores around a tree — directional tree coring — shows that direction (Limmer and others, 2013).

Tetrachloroethylene (PCE) in tree cores and the estimated PCE plume in groundwater, on a site by the Missouri River. Modified from Schumacher and others (2004) and the U.S. Environmental Protection Agency (2003). USGS.

Sources

Based on Phytoforensics—Using Trees to Find Contamination, U.S. Geological Survey Fact Sheet 2017–3076 (publication page), Missouri Water Science Center. Figures 1 to 3 are restored from the fact sheet's PDF; its figure 4, adapted from a journal article, is left out. A work of the United States government in the public domain.