Newsletter and Technical Publications
Freshwater Management Series No. 5
Guidelines for the Integrated Management of
- Phytotechnology and Ecohydrology -
5. Application of phytotechnologies and Freshwater
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.
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 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).
Fig. 5.1. Applications of phytotechnologies
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
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 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 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,
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 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)