Phytoremediation is the process by which plants are used to treat or stabilize contaminated soils and groundwater (USEPA, 2000). The technology is complex and is only introduced here (Lasat, 2002). The technology has emerged as a response to the clean-up efforts for sites contaminated with toxic and hazardous wastes. Contaminants that have been successfully remediated with plants include petroleum hydrocarbons, chlorinated solvents, metals, radionuclides, and nutrients such as nitrogen and phosphorus. Glass (1999) estimated that in 1998 at least 200 field remediations or demonstrations had been completed or were in progress around the world; however, the "remediation" technology as currently used is not new but rather draws on the basic ecosystem responses and reactions documented in this and other chapters in this book. The most common applications depend on the plants to draw contaminated soil water to the root zone, where either microbial activity or plant uptake of the contaminants provides the desired removal. Evapotranspiration during the growing season provides for movement and elimination of the contaminated groundwater. Once taken up by the plant, the contaminants are either sequestered in plant biomass or possibly degraded and metabolized to a volatile form and transpired. In some cases, the plant roots can also secrete enzymes, which contribute to degradation of the contaminants in the soil.

Obviously, food crops and similar vegetation, which might become part of the human food chain, are not used on these remediation sites. Grasses and a number of tree species are the most common choices. Hybrid poplar trees have emerged as the most widely used species. These trees grow faster than other northern temperate zone trees, they have high rates of water and nutrient uptake, they are easy to propagate and establish from stem cuttings, and the large number of species varieties permits successful use at a variety of different site conditions. Cottonwood, willow, tulip, eucalyptus, and fir trees have also been used. Wang et al. (1999), for example, have demonstrated the successful removal by hybrid poplar trees (H11-11) of carbon tetrachloride (15 mg/L in solution). The plant degrades and dechlorinates the carbon tetrachloride and releases the chloride ions to the soil and carbon dioxide to the atmosphere. Indian mustard and maize have been studied for the removal of metals from contaminated soils (Lombi et al., 2001). Alfalfa has been used to remediate a fertilizer spill (Russelle et al., 2001).

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