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United Nations Environment Programme
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Newsletter and Technical Publications
Freshwater Management Series No. 5

Guidelines for the Integrated Management of the Watershed
- Phytotechnology and Ecohydrology -


5. Application of phytotechnologies and Freshwater ecosystem management

The economic activities of humans, wasteful exploitation of natural resources - including water resources, and modification of the natural environment and landscape have caused significant changes in the quality and quantity of water resources, as well as a lessening of the "self-purification" capabilities of water bodies. The natural role of vegetation in modifying the quality of surface and ground waters has become greatly diminished. There have been clear increases in the extent and amount of pollution. Destructive environmental transformations connected with the mutual relationship between vegetation and water quality comprise:

  • Undesirable landscape modifications, especially the enlargement of agricultural operations at the expense of forests, bush, meadows, and marshes, with the concomitant loss of natural filtering within their ecotones.
  • Accelerated water and nutrient cycling in river basins as a consequence of river regulation, construction of hydrotechnical devices such as dikes and canals, and the development of drainage systems restricting numbers of natural ponds and swamps. To a large extent, while these constructs may have enhanced economic growth and the advance of civilisation, these constructs have been destructive within the natural environment. One result is a reduction in the natural retention capacity of river catchments, causing accelerated drainage of spring runoff and precipitation, a worsening of the water balance within the basin, and a speeding up of nutrient recycling and transport. Nutrients (nitrogen, phosphorus, and potassium) are preferentially transferred from the land surface, where they can be productively used for crop production, into rivers and standing waters.
  • Increased amounts of pollution, especially non-point source pollution, resulting from agricultural practices and increased use of mineral fertilisers. Equally, increased air pollution has led to an increase in the amount of chemical compounds carried onto the land surface by both dry and wet precipitation.

In contrast with point source pollution, non-point source pollution is much more difficult to control, although plants may play a key role in limiting the transport of these forms of pollution in surface runoff and by ground waters.

A. Phytotechnologies*

Phytotechnology is the application of plants in scientific and engineering approaches to resolve environmental problems. Phytotechnologies are well-suited for environmental restoration and remediation, as shown in Figure 5.1.

Phytoremediation

Phytoremediation is defined as the in situ use of plants to stabilise, remediate, reduce, or restore contaminated sites, including terrestrial and aquatic sites. The process can be applied to both surface and ground waters, and includes all of the biological, ecological, chemical, and physical processes involving vegetation that affect remediation of contaminated substrates. Applied phytoremediation UNEP (UNEP-IETC 2002).


*Sub-chapters A, B and O, P elaborated using U.S. EPA documents.

Fig. 5.1. Applications of phytotechnologies (lager image)

Phytoextraction and phytoaccumulation

Phytoextraction is the uptake of contaminants through plant roots and the translocation of the contaminants within the plants. Certain plants, called hyperaccumulators, absorb unusually large amounts of metals or other contaminants in comparison to other plants. One, or a combination, of these plants is selected and planted at a site based upon the types of metals present and other site condition. This procedure may be repeated as necessary to bring soil contaminant levels down to allowable limits. Contaminants are generally removed by harvesting the plants, which also allows the possible utilisation of the harvested biomass. Nevertheless, care must be taken to limit the environmental exposure of harvested plant materials, as some heavy metals and other contaminants can bioaccumulate if the plant material is used as cattle feed or left to decompose without measures being taken to capture the resultant leachate.

Phytodegradation and phytotransformation

Phytodegradation or phytotransformation is the breakdown of contaminants taken up by plants by metabolic processes within the plant. Alternatively, the term is also applied to the breakdown of contaminant, external to the plant, by compounds or enzymes produced by, and released from, the plant. In both cases, the primary treatment mechanism is plant uptake and metabolism. Degradation caused by micro-organisms associated with the plant root is considered rhizodegradation (see below).

Phytostabilisation

Phytostabilisation is defined as (i) the immobilisation of a contaminant within the soils through adsorption and accumulation by roots, adsorption onto roots, or precipitation within the root zone of plants, and (ii) the use of plants and plant roots to prevent contaminant migration via wind and water erosion, leaching, and/or soil dispersion.

Phytovolatilisation

Phytovolatilisation is the uptake and transpiration of contaminant by a plant. This process results in the release of the contaminant from the plant, in a modified form of the contaminant to the atmosphere from the plant. Key elements of this process include contaminant uptake, metabolic transformation, and transpiration.

Rhizodegradation

Rhizodegradation is the breakdown of an organic contaminant within the soil through microbial activity that is enhanced by the presence of the root zone of a plant. Rhizodegradation is utilised in a process known as "plant-assisted bioremediation", "plant-aided in situ biodegradation" or "enhanced rhizosphere biodegradation".

Rizofiltration

Rizofiltration is the process of adsorption or precipitation onto plant roots, or absorption into the roots, of contaminants that are in solution in the soil environment surrounding the root zone. The process may be either abiotic or biotic in nature. If the process is biotic in character, uptake by the plant, concentration, and translocation within the plant structure may occur, depending on the particular contaminant. If the process is external to the plant, exudates from the plant roots may cause precipitation of some metals. Rizofiltration results in contaminant containment, wherein contaminants are immobilised by accumulation on or within the plant. Contaminants can then removed by physically removing the whole plant, including the root system (Figure 5.2).

Fig. 5.2. Mechanisms of phytoremediation (U.S. EPA 2000, changed)

 

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