The objective of this project was to evaluate the use of trees as a low-cost alternative to traditional pump-and-treat systems for remediating groundwater contaminated with chlorinated hydrocarbons. This technology, called phytoremediation, is based on phreatophytic plants (i.e., plants that send a root to groundwater) being able to reduce the mass flow of groundwater by utilizing it during evapotranspiration. In the process, chlorinated hydrocarbons in groundwater may be degraded or assimilated by the plants, thereby reducing aqueous contaminant concentrations.
The hybrid poplar trees used in this study validate an inexpensive alternative method to process and "clean" chlorinated hydrocarbon-contaminated soil and groundwater. Young poplar trees ranging in size from saplings to those with 4-inch diameter trunks were planted and maintained for three years in varying densities over a dissolved trichloroethylene (TCE) plume in a shallow aquifer at Air Force Plant 4 near Fort Worth, Texas. Changes in groundwater flow and TCE concentrations were measured at regular intervals to estimate the amount of groundwater used by the trees as well as changes in TCE groundwater concentrations.
After three growing seasons, there was an 8 percent decrease in groundwater mass flux and an 11 percent decrease in TCE mass flux attributed to the presence of trees. However, in spite of groundwater and TCE mass flux reductions, the objectives of 30 percent reduction of TCE mass flux during the second growing season and 50 percent reduction during the third growing season were not met, primarily due to the fact that the trees had not yet matured. The results indicate that using phreatophytic trees to reduce groundwater and TCE mass flux is applicable; however, further time may be needed for tree maturity in order to achieve the best results. TCE uptake into the trees was observed over the course of the project although reductive dechlorination of TCE was not significant by the end of the third growing season.
It has been estimated that as many as 1,000 Department of Defense (DoD) cleanup sites worldwide could utilize this technology, saving hundreds of millions of dollars. Current pump-and-treat technologies are costly and not always completely effective, and phytoremediation may provide a low-cost, low-maintenance technology for long-term treatment of contaminated sites. In addition, it is becoming more evident that communities where hazardous material releases have occurred favor remediation via vegetation for its aesthetic effects. The total estimated cost for groundwater TCE remediation over a ten-year period is $500 to $660 thousand; however, the efficiency has not been verified over this time period in the current field demonstration. The predominant costs include analytical services, labor, and a supplemental irrigation system for the first two years. Start-up, material, and maintenance costs are low compared to other remediation technologies.
The results indicate that phreatophytic trees can be used, under appropriate conditions, to reduce the mass fluxes of groundwater and TCE flowing beneath a tree plantation. These conditions include a depth to groundwater that tree roots can reach and a water balance that favors the trees' use of groundwater to supply some of their evapotranspiration demands. Properties of the contaminant are also important. While the trees used in this study are relatively fast-growing, it may take two to three growing seasons for roots to reach groundwater and up to ten years for canopy closure and optimum efficiency to occur. Supplemental irrigation may be required during the first one to two years to supply adequate water before their roots reach groundwater. (Project Completed - 2004)