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image:filtered light in the forest

Toxics Cleanup Program

Fern “Phytoremediation” Study: Can plants clean up soil?

Chinese brake fern
Chinese brake fern

Photo: Sampling
Sampling the soil for arsenic before harvesting the fern fronds.

Photo: Fern plot on Vashon Island
Fern plot on Vashon Island

Fern fronds were about 34 inches tall. This plot grew over 18 pounds of fronds.

Some plants readily take up arsenic and other heavy metals from the soil.  The Chinese brake fern (Pteris vittata) can take up arsenic into its fronds at a very high rate.  From 2005 to 2007, Ecology did a study to see if the Chinese brake fern could clean up Tacoma Smelter Plume contamination.

Findings: Ferns are not a good cleanup tool for the Tacoma Smelter Plume.

Ecology is not using ferns to clean up soils within the Tacoma Smelter Plume for some of the following reasons:

  • Climate: Brake ferns did not grow well in the winter, in shady or cool areas, or when the soil became dry in the summer.
  • Risk: Arsenic levels in the fern fronds were so high that they could easily poison a child.  The plots had to be fenced and locked at all times.
  • Disposal: The high arsenic levels in the fronds meant that Ecology had to dispose of them at a dangerous waste landfill.
  • Metal uptake: The ferns did not take up lead, which is the other main Tacoma Smelter Plume contaminant.
  • Cost: Replacing plants each year, the only way to effectively clean up soil would be too expensive.

Basic Study Design

The study tracked ferns planted on seven small plots, five on Vashon Island and two at Point Defiance Park Tacoma.  Ecology sampled soils before, during, and after the study to see if ferns were reducing metal levels.  Ecology also harvested and analyzed fern fronds to see what they were taking up from the soil.

For more details, view the slideshow.

Ecology thanks all of the agencies and volunteers involved!

Health department staff from King and Pierce counties did much of the soil sampling and collecting fern fronds.  The following agencies provided access to land for the test plots:

  • Vashon Island School District
  • King County Parks and Recreation Division
  • Tacoma Metro Parks

Almost 20 community volunteers from Vashon and Maury Islands and the Vashon Island Fire Department helped plant and water fern plots during the study.