ABSTRACT
Phytoremediation does ef”ciently and effectively transform or remove many contaminants from the soil. Depending upon the depth of the contamination (often associated with the depth to the water table), and the type of plant used, phytoremediation will remove contaminants down to a depth of up to about 5 to 10 m (sometimes deeper) below ground surface. This is a root zone phenomenon of the type of plants selected and the depth of their root system. One would not expect grasses to be effective in remediating a site that contained contaminants more than a meter deep; similarly, one would not expect a tree to be able to reach down and remediate contaminants that are 20 m or more below the ground surface.†
Phytoremediation is de”ned as the “use of vegetation and its associated microorganisms, enzymes and water consumption to contain, extract or degrade contaminants from soil and groundwater.” Its mechanisms include
• Phytoextraction-removal and storage of contaminants from media into the plant tissue;
• Rhizodegradation-degradation of contaminants by microorganisms in the soil zone that is surrounded or in£uenced by plants, also known as the rhizosphere;
• Phytodegradation-degradation of contaminants within the plant tissue; • Phytostabilization-isolation and containment of contaminants with the
soil through the prevention of erosion and leaching; • Phytovolitization-uptake and transpiration of contaminants from the
media through the plant tissue into the atmosphere; and • Hydraulic control-containment of contaminants within a site by limiting
the spread of a contaminant plume through plant evapotranspiration.‡
Complete site and chemical and physical contaminant characterization (including site hydraulics, soil types, seasonal water tables, and aquatic and chemical transportation across the site) is important in the contemplation of phytoremediation. One also has to pay attention to the time frame for accomplishment of the remediation as well as the costs. Phytoremediation will clean up the soils if correctly applied, but there is still the challenge as to what transformations are being made by the plants. If, as in the case of metals, the plants may absorb the metals, one has to evaluate the receptor sites-leaves, bark, and so forth-and determine how to harvest the plant, and where and how to dispose of the harvested material to prevent its release and re-contamination of the affected sites.
