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Freshwater Management Series No. 2

Phytoremediation: An Environmentally Sound Technology for
Pollution Prevention, Control and Redmediation

An Introductory Guide To Decision-Makers

Does Phytoremediation Work at Every Site?

Phytoremediation technologies have been used to clean up metals, pesticides, solvents, explosives, crude oil, polyaromatic hydro-carbons, and landfill leachates. Phyto-remediation can be used in combination with other cleanup approaches as a ‘finishing’ or ‘polishing’ step. Some phytoremediation applications are slower than mechanical and chemical methods and are limited to the depths that are within the reach of the plant roots.

Generally, the use of phytoremediation is limited to sites with low to medium contaminant concentrations, and conta-mination in shallow soils where phytotoxicity does not occur and the roots of plants can easily access the contaminant. Plants can also be used to clean up contaminants in streams and groundwater. Researchers are finding that the use of trees (rather than smaller plants) allows for the treatment of contamination at greater depths, as tree roots penetrate more deeply into the ground. Very deep contaminated ground-water may be treated by first pumping the water out of the ground and then using plants to treat the contamination.

Further research is needed to study the effects of bioaccumulation and biomagnification in the food chain that could occur if insects and small rodents eat the plants that are collecting contaminants and are then eaten by larger mammals. In addition, scientists need to establish whether contaminants can collect in the leaves and wood of trees used for phytoremediation and be released when the leaves fall in the autumn or when firewood or mulch from the trees is used.

Photo 6: Test and selection of plants for phytoremediation purposes
Photo 6: Test and selection of plants for phytoremediation purposes. (Photo: Environment Canada)

Where Has Phytoremediation Been Used?

Phytoremediation has been successfully tested in many locations, but full-scale applications are still limited. At a U.S. Army testing facility in Maryland, hybrid poplar trees have been planted in a one-acre site over a shallow groundwater plume contaminated with organics from several toxic disposal pits. The poplar trees act as hydraulic pumps to prevent the spread of contaminants to a nearby marsh. In addition to hydraulic control, researchers have determined that phytovolatilisation and rhizofiltration are other mechanisms by which the system is treating the contaminated plume. After the second growing season, the trees successfully contained the contaminated groundwater plume with rates estimated at 2-10 gallons of water per day for each tree. Results have also indicated that the poplar trees are degrading the contaminants in the groundwater plume.

Constructed Wetlands
In Canada, Environment Canada is working with the company that operates the world’s largest smelter to treat landfill leachate using constructed wetlands. Also, at the Milan Ammunition Plant in Tennessee, U.S.A., a constructed wetland design deployed submersed and emergent plants in surface and subsurface flow systems to degrade TNT, RDX and other explosives contaminants found in groundwater onsite. By testing ten species of aquatic plants in growth chambers and hydroponic solutions, researchers were able to select a number of species for the lagoon (surface flow) unit of the wetland system. The species recommended were elodea (Elodea Michx), sago pondweed (Stuckenia pectinatus) and water grass (Luziola fluitans).

Vegetative Caps
The use of plants and soil as a vegetative cap for landfill sites is another effective phytoremediation technique. Vegetative covers serve as a form of hydraulic control by maximising the available storage capacity of soil, as well as the evaporation rates and transpiration processes of plants, thereby minimising water infiltration. They can also assist in the degradation of wastes and contaminants contained within the landfill or on land. In Slovenia, one landfill is covered with poplar trees to prevent creation of landfill leachate that could harm groundwater and surface water.

Photo 7: Construction for phytoremediation test site in Canada
Photo 7: Construction for a phytoremediation test site in Canada. (Photo: Environment Canada)

In Kazakhstan, a program has begun to remediate pesticide contamination through the use of planted systems. International oil companies have also begun using phyto-degradation at many sites, with over sixty sites planted or in the planning stages.

Soil Conditioning
In the United Arab Emirates, a study has been carried out examining the growth characteristics and performance of mangroves, halophytes and other plants in soil irrigated with saline water. Salinity is a serious threat for crop production in arid regions, where the high rate of evaporation, combined with the demand for scarce water reserves, causes salt build-up during the cultivation process. The study showed that some plants have the necessary physiological mechanisms and capacities to accumulate significant concentrations of iron, manganese and magnesium, calcium, sodium and chloride ions, thereby reducing the overall salinity of the soil system and potential related effects on crop physiological and growth performance. Based on this work, it has been suggested that by introducing highly salt-tolerant species (i.e., conocarpus erectus, Atriplex lentiformis, etc.) that can be irrigated with saline water, higher plant and agricultural production levels in arid regions can be achieved.

Protection of Riparina Corridors
One of the promising aspects of phytotechnologies is the possibility of deriving additional benefit from the planted system during or after the prevention or clean-up of pollution. Trees planted as a riparian corridor that protect streams from agricultural pollution may be managed to provide forest products like nuts, fruits, lumber or fibre. Trees can be selected that do not move toxins from roots to above ground, thus allowing for a safe harvest while roots clean up groundwater. In addition, contaminated urban sites may be planted to clean up soil and groundwater while at the same time providing recreation and park use.

What Are the Challenges Faced by Decision-Makers?

Environmentally sound technologies (ESTs) encompass technologies that have significantly improved environmental performance relative to other technologies. ESTs protect the envi-ronment, are less polluting, use resources in a sustainable manner, recycle more of their wastes and products, and handle all residual wastes in a more environmentally acceptable way than the technologies for which they are substitutes.

Photo 8:  Phytoremediation test site showing the effect of toxical plants
Photo 8: Phytoremediation test site showing the effect of toxical plants. (Photo: Environment Canada)

The development and application of phytoremediation as an environmentally sound technology involves a number of challenges, including the development of local capacity to understand and apply phytoremediation technologies, and the establishment of an effective regulatory framework. In some countries, there is a lack of experience in the use of phytoremediation. This is often coupled with a lack of data, performance standards and cost-benefit analysis regarding phytoremediation technologies. Hence, there is a need for:

  •  appropriate phytoremediation technologies and techniques applicable to different geographic regions with varied weather conditions
  •  site characterization, clean-up and technology selection criteria
  •  assessment and evaluation methods that can be applied to determine the applicability and appropriateness of various phytoremediation techniques
  •  local training for environmental reme-diation practitioners on the planning and implementation of phyto-remediation schemes.

There are also a number of areas where research is required. For example, the rate of biodegradation and mineralisation during phytoremediation is usually affected by the nature and concentrations of contaminants present, as well as surrounding oxygen levels, soil/air moisture, pH, temperature, soil elemental contents and their bioavailability, and the supporting microbial media. Plant physiological and root growth expansion studies are needed to optimise plant uptake of contaminants and to maximise process output performance. Research is needed to determine the best density of plants per unit area to achieve the maximum utilisation of resources (too may plants can negatively adverse the processes, weaken the plants and be costly during disposal processes), and to determine the proper timing for irrigation and harvesting in order to control the amount of biomass produced and removed at harvest. Other areas requiring further research include the effect of fertilisers and conditioners on the soil characteristics and the fate of these compounds.

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