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Phytoremediation
has become something of buzzword in the field of landscape architecture, largely
because design with plant materials is the one of the foundations of the field.
This willing acceptance is easily understood in light of the apparent benefits
of the technology. Each of the approaches to phytoremediation will inevitably be
limited to plant material with an ability to perform under conditions that are
often less than optimal. In the selection of hyperaccumulators, for instance,
initial plant choice is often based upon the plants that will simply survive in
soils with high levels of metals. Level of accumulation will be limited to these
plants and their uptake capabilities. In highly contaminated sites,
phytoremediation may be limited by the amount of time it would take for plants
to perform uptake. (Prospects and Limitations by Anthony Randazzo)
The toxicity and bioavailability of biodegradation products is not always known. Degradation by-products may be mobilized in groundwater or bio-accumulated in animals. Additional research is needed to determine the fate of various compounds in the plant metabolic cycle to ensure that plant droppings and products do not contribute toxic or harmful chemicals into the food chain. Scientists need to establish whether contaminants that collect in the leaves and wood of trees are released when the leaves fall in the autumn or when firewood or mulch from the trees is used. Disposal of harvested plants can be a problem if they contain high levels of heavy metals. The depth of the contaminants limits treatment. The treatment zone is determined by plant root depth. In most cases, it is limited to shallow soils, streams, and groundwater. Pumping the water out of the ground and using it to irrigate plantations of trees may treat contaminated groundwater that is too deep to be reached by plant roots. Where practical, deep tilling, to bring heavy metals that may have moved downward in the soil closer to the roots, may be necessary.
Generally, the use of phytoremediation is limited to sites with lower contaminant concentrations and contamination in shallow soils, streams, and groundwater. However, researchers are finding that the use of trees (rather than smaller plants) allows them to treat deeper contamination because tree roots penetrate more deeply into the ground. The success of phytoremediation may be seasonal, depending on location. Other climatic factors will also influence its effectiveness. The success of remediation depends in establishing a selected plant community. Introducing new plant species can have widespread ecological ramifications. It should be studied beforehand and monitored. Additionally, the establishment of the plants may require several seasons of irrigation. It is important to consider extra mobilization of contaminants in the soil and groundwater during this start-up period. If contaminant concentrations are too high, plants may die. Some phytoremediation transfers contamination across media, (e.g., from soil to air). Phytoremediation is not effective for strongly sorbed contaminants such as polychlorinated biphenyls (PCBs). Phytoremediation requires a large surface area of land for remediation. (www.cpeo.org/techtree/ttdescript/phytrem.htm)