Letting Nature Clean Up Contaminated Sites
Environmental article by Harold Hough   

            There was a time when the only way to clean up contaminated soil was to dig it up and send it to a toxic waste dump, where it was isolated from the groundwater with geomembranes. Today, however, more and more contaminated soil is being treated on site with a technique called Phytoremediation.  Phytoremediation uses plants to clean up contaminated soil.  Not only is it cheaper and less intrusive, environmentalists like it because its aesthetically pleasing and preserves the environment in a more natural state.
            Phytoremediation uses a type of plant called hyperaccumulators.  These plants naturally accumulate high levels of toxic materials found in the soil as they grow.  In nature, this process protects the plant by killing insects, fungi, and molds that threaten them.  It also discourages larger, plant eating animals by making them sick.  As a result, the animals give them a wide berth. 
            Today environmental experts use this natural mechanism to treat contaminated sites.  Tobacco plants are being used to degrade organic explosives like TNT and yellow poplar saplings can collect mercury compounds at old gold mining sites that used mercury for amalgamation.
            However, phytoremediation takes more than growing plants on a piece of ground.  A remediation and reclamation program must look at the problem and the specific needs of the site.  If the problem is metal contamination, experts use a process called phytoextraction.  Reclamation experts pick one or more types of plants that are known for collecting the specific metals that contaminate the site.  As they grow, the plants absorb the metals through their root system and move it to the stalk, stems, and leaves.  After the plants have been allowed to grow for some time, they are harvested and incinerated or composted to recycle the metals.  The procedure can be repeated as necessary to bring the soil contamination down to safe limits.  Although the harvested plants are considered hazardous, they take up less space than the contaminated soil at the toxic waste dump.  And, depending on the type of metals at the site, it is possible to recover and recycle them.
            If contaminated groundwater is the problem, the process is slightly different.  These plants are grown in greenhouses, where their roots are kept in water rather than soil.  Once the plant is picked to go to a specific site, contaminated water from the waste site is brought to the greenhouse and substituted for the clean water in order to acclimate the plants.  The plants are then planted in the contaminated area, where the roots take up the water and the contaminants along with it.  As the roots become saturated with the contaminants, they are harvested.  One successful example was the use of sunflowers to remove radioactive contaminants from pond water it Chernobyl.
            Plants can also be used as natural water pumps to draw contaminated water to itself so it doesn’t contaminate the water table.  Poplar trees, for instance, can transpire 50 to 300 gallons of water a day from the ground.  Since these trees reach down towards the water table and establish a thick root mass, surface contaminants are less likely to migrate down into the table to contaminate it.  These plants can be planted in belts to protect streams that flow near a contaminated area.
            If the contamination is so bad the normal vegetation can’t grow on it, metal resistant plants can be planted, not to extract the metal, but to stabilize the soil.  That prevents erosion and limits the contamination of nearby ground or streams.  This is a popular method for mine tailings.
            In some cases, the plants don’t just store the contaminants; they either chemically break them down or “exhale” them.  In this method, plants pick up the contaminants through its root system and then with the help of emzymes break it down into simple molecules that can be incorporated into the plant tissue.  This method was used by the Army to clean up a site contaminated with TNT.  Poplar trees can pick up trichloroethylene (TCE) and exhale it into the atmosphere.
            Despite the attractiveness of phytoremediation, it does have its limitations.  It is not a fast process and takes a commitment of many seasons before contamination levels are reduced to a safe level.  The process is also limited to the ability of the roots to reach the contamination.  However, for those places where it works, it is definitely a more attractive alternative to traditional striping and storing of contaminated soil.