Using certain plants for remediation is an emerging treatment technology.

The most basic definition of phytoremediation is “a technology that uses plants to remediate or stabilize contaminants in soil, ground water or sediments.” An expanded definition is “the use of vegetation to contain, sequester, remove or degrade inorganic and organic contaminants in soils, sediments, surface waters and ground water.” Typical organic contaminants that can be addressed using phytotechnologies include petroleum hydrocarbons, gas condensates, crude oil, chlorinated compounds, pesticides and explosive compounds. The specific phytotechnology mechanisms that could be considered for MTBE remediation in ground water include rhizodegradation (the breakdown of contaminants in the soil through microbial activity that is enhanced by the presence of the rhizosphere), phytovolalitization (chemical removal via transpiration), and possibly phytodegradation (the breakdown of contaminants by plants through metabolic processes within or external to the plant).

Applicability

The applicability of phytoremediation at a specific site depends on certain conditions. It is suggested that phytotechnologies are well suited for sites where the following conditions exist:

• Contamination exists at a depth accessible by the root zone.

• Sufficient area exists for growing vegetation.

• Treatment can be applied over long periods of time.

• Concentrations of contaminants are nontoxic to the plants.

• Other methods of remediation are not cost-effective or practicable.

• Existing systems may be supplemented to achieve remedial goals more rapidly.

• A transition from a primary treatment to a longer-term strategy may be desired.

• Vegetation can be used as a final cap for closing or restoring the site.

Chevron Research and Technology Co. adds that depth to ground water and age of the plants also should be considered in evaluating the potential applicability of phytoremediation at a specific site.

There are several potential applications of phytotechnologies for treating ground water affected by MTBE. These include hydraulic barriers to control/remediate ground water, vegetative stands to reduce infiltration and ground water recharge, constructed wetlands to treat surface water runoff and near-surface ground water seeps, and hydroponic systems as the treatment portion of a pump-and-treat system.

Performance

Data have been reviewed for several sites where phytoremediation is being evaluated. One review included two sites that have ground water affected by MTBE. Performance data collected at one of the sites, a service station in northern California, suggested that pine trees at the site might be inhibiting the off-site migration of MTBE and TBA. Data supporting this theory includes concentrations of MTBE and TBA in transpirate samples from the trees and reduced ground water concentrations of MTBE and TBA in wells located within or downgradient of the tree stand. The second site, a former service station at Vandenberg Air Force Base, was reviewed but the data collected there were not as conclusive.

Also reviewed was data collected at several sites with ground water affected by MTBE, including a phytoremediation system installed at a Shell-Equilon site in Houston, to provide hydraulic control for a ground water plume containing MTBE. It was installed to replace a pump-and-treat system installed during a previous remedial phase. Based on data review, it is estimated that the phytoremediation system had removed approximately 441,000 gallons of water during the growing season.

Another site studied was a service station in South Carolina. Analytical data from xylem core samples of mature trees collected at this site revealed detectable concentrations of MTBE; this was interpreted as suggesting that the trees were taking up MTBE-impacted ground water.

Phytoremediation Costs

As phytoremediation is an emerging technology/ application, there is little cost information regarding its use at MTBE-affected sites. Several studies suggest the use of phytoremediation could result in significant cost savings over the life of a project; however, none of those studies dealt with MTBE-impacted ground water. It has been estimated that phytotechnologies could be at least 40 percent less costly than other in situ remedial approaches, and ex situ technologies could be 90 percent less costly compared to alternatives.

Advantages and Limitations

Several factors need to be considered in determining whether a site is suitable for the implementation of phytoremediation technologies, including growth habit of the planted system, root penetration of the selected plant(s) and the amount of land available for planting. If it is determined that phytoremediation is applicable for a site, it can present several advantages over conventional treatment alternatives:

• low-maintenance, passive, in situ, self-regulating, solar-driven system

• potentially applicable in remote locations without utility access

• decreased air and water emissions as well as secondary wastes

• control of soil erosion, surface water runoff, infiltration, and fugitive dust emissions

• applicable to simultaneously remediate sites with multiple or mixed contaminants

• habitat creation or restoration provides land reclamation upon completion

• favorable public perception, increased aesthetics and reduced noise

• increasing regulatory approval and standardization viability

• carbon dioxide and greenhouse gas sequestration

Phytoremediation technologies also have limitations and, therefore, are not applicable at every site. Some potential limitations of applying phytoremediation at MTBE-impacted sites include the following:

• slow plant growth rates

• dependence on climate and growth season

• susceptible to infestation and diseases

• may be difficult to establish/maintain vegetation

• slow/shallow root penetration

• limited contaminant mass transfer into root zone

• phytotoxicity of contaminants

• limited database and performance data available

• potential transfer to secondary media

• by-products may be more toxic

• mechanisms not completely understood

• fate, transport and efficacy not well documented

• unfamiliarity by public/regulatory communities

Many of the limitations can be addressed using field-engineering techniques (e.g., planting methods, lateral root growth inhibitors, down-hole hardware, irrigation systems, maintenance practices and fertilizers); mixed/diverse plant communities; and bench, laboratory or greenhouse studies.
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