Studies are showing that trees and other plants can offer a cost-effective way to clean up polluted soil and groundwater.

Trees and other plants offer a cost-effective way to clean up polluted soil and groundwater, says Steven Rock, an environmental engineer in the U.S. Environmental Protection Agency's (EPA) National Risk Management Research Laboratory in Cincinnati. Since 1994, when EPA first became involved in the work, Rock has been using stands of poplar trees, alfalfa and Indian mustard to cleanse soil and water.

The process takes advantage of the natural abilities some plants have to absorb and remove contaminants from soil and groundwater. These plants are set out in polluted areas and draw water and soil chemicals through their roots, then release the water vapor through their leaves. The process also conditions and anchors the soil, minimizing runoff.

At Aberdeen Proving Ground near Baltimore, a poplar stand planted in 1996 is effectively absorbing toxic solvents from the soil in an old munitions-burning pit.

Research shows very little transpiration of solvents. Each tree removes 7 to 10 gallons of water a day, or 2,000 gallons of water per day in the stand. Plants also are a cost-effective way of cleaning up hazardous waste. The usual approach - digging up the contaminated soil and trucking it to the incinerator or landfill - is expensive. It can cost as much as $6 billion a year. Over a long period of time - such as 50 years - that could stretch to trillions of dollars.

As of January 2001, there were over 40,000 toxic sites, including 1,700 on the Superfund National Priorities List. They are abandoned waste storage or treatment plants, and weapons manufacturing and mining facilities. They pose the greatest threat to public health and the environment. In 1995, nearly 20,000 hazardous waste generators produced 279 million tons of hazardous waste regulated by EPA.

Besides solvents, plants also can clean up metals, pesticides, explosives (such as TNT), crude oil, polyaromatic hydrocarbons and landfill leachates. This new botanical approach is being tested in 200 sites throughout the United States. The method is an aesthetically pleasing, solar-driven, passive technique that is most useful treating sites with shallow, low levels of contamination. Besides having to wait for the plants to grow and work, limitations may include animals eating the toxic-containing plants and the plant roots not being long enough.

Although phytoremediation has been around since the beginning of time, only recently have scientists, engineers and business people come together to define this relatively simple concept: that plants can clean up toxic pollutants in soil and groundwater naturally. Interest in commercial phytoremediation began in the early 1990s. A significant turning point for this technology was its use to decontaminate soil and groundwater at the Chernobyl nuclear site in 1986. After phytoremediation proved successful in this instance, the market for these toxin-eating plants began to grow as new applications were discovered. More recently scientists have discovered an application to clean up a hugely controversial contaminant: arsenic. This discovery may give phytoremediation the publicity it needs to move forward as an effective and reliable technology in the United States.

Although the process of phytoremediation sounds simple, there is a bit more preparation involved than simply planting trees around a toxic waste site and letting them do their thing. A significant amount of preparation and research is involved before an engineer can actually determine which plants to use and how the plants break down the contaminants that are being targeted. For instance, there are occasions where plants may break down a product and produce even more hazardous byproducts.

With this in mind, the implementation and effectiveness of phytoremediation processes is only as good as the research and preparation done before hand. The emphasis on understanding the basics of contaminant break down are crucial for developing phytoremediation solutions. Other concerns when determining which plant to use might be the climate or environment in which the contaminant is being treated. For example, some plants that might flourish in Florida may not grow at all in the colder less humid climates. Additionally, the depth or location of the contaminant must be considered.

According to D. Glass Associates, the overall phytoremediation market will reach revenues between $214 million to $370 million by 2005.