Phytoremediation is the use of certain plants to clean up soil, sediment, and water contaminated with metals and/or organic contaminants such as crude oil, solvents, and polyaromatic hydrocarbons (PAHs). It is a name for the expansion of an old process that occurs naturally in ecosystems as both inorganic and organic constituents cycle through plants. Plant physiology, agronomy, microbiology, hydrogeology, and engineering are combined to select the proper plant and conditions for a specific site. Phytoremediation is an aesthetically pleasing mechanism that can reduce remedial costs, restore habitat, and clean up contamination in place rather than entombing it in place or transporting the problem to another site.

Phytoremediation can be used to clean up contamination in several ways:

• Degradation by plants. Organic contaminants are absorbed inside the plant and metabolized (broken down) to non-toxic molecules by natural chemical processes within the plant.
• Extraction. Plant roots can remove metals from contaminated sites and transport them to leaves and stems for harvesting and disposal or metal recovery through smelting processes.
• Microorganism stimulation. Plants excrete and provide enzymes and organic substances from their roots that stimulate growth of microorganisms such as fungi and bacteria. The microorganisms in the root zone then metabolize the organic contaminants.
• Volatilization. Plants take up water and organic contaminants through the roots, transport them to the leaves, and release the contaminants as a non-toxic gas (called volatilization) into the atmosphere.
• Stabilization. Plants prevent contaminants from migrating by reducing runoff, surface erosion, and ground-water flow rates. "Hydraulic pumping" can occur when tree roots reach ground water, take up large amounts of water, control the hydraulic gradient, and prevent lateral migration of contaminants within a ground water zone.

Phytoremediation can be used in combination with other traditional and innovative remediation technologies. Cleanup can be accomplished to depths within the reach of plants' roots. Sites need to be maintained (watered, fertilized, and monitored) and results are slower (3+ years) than mechanical excavation methods. "Attractive nuisance" and food chain issues must be considered at each site and care taken to avoid unwanted exposure of wildlife. Cost savings compared to traditional remediation can range from 20 to 80 percent.

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