Sustainable Water Management - Sustainable Practices in Geoenvironmental Engineering - page 93

Phytoremediation is the use of plants to remove, contain, or render harmless environ-
mental contaminants. Constructed wetlands use aquatic plants such as water hyacinths
to remove nutrients and contaminants from water. The various mechanisms involved in
phytoremediation include (a) phytoextraction, (b) uptake of contaminants through the
roots and subsequent accumulation in the plants, (c) phytodegradation, (d) metabolism
of contaminants in the leaves, shoots, and roots, (e) release of enzymes and other compo-
nents for stimulation of bacterial activity or biochemical conversion and rhizodegradation,
and (f) mineralization of contaminants in the soil by microbial activity in the rhizosphere.
Phytoremediation is a low cost in situ technology that causes minimal disturbance, and is
aesthetically pleasing. It is acceptable to the public and generates low amounts of waste.
This technology is presently being developed for a treatment of a wide variety of organic
and inorganic contaminants. Better understanding of contaminant uptake by the plants,
development of hyperaccumulators, and increased ield testing are needed.
3.4.1.3 Electrokinetic Applications
Electrokinetics involves the use of electrodes and electrical current to mobilize inorganic
contaminants. It is more effective for treatment of silty soils than for clay soils where
energy requirements can be substantial. Energy levels must be higher than the energy that
binds the contaminants to the soil. This processes involved have been described in detail
by Yong (2001). Electro-osmosis and electrophoretic phenomena are the principal mecha-
nisms in the treatment process. Conditioning luids are required to enhance contaminant
ion movement, and electrode dissolution or fouling is a substantial problem.
3.4.1.4 Natural Attenuation
The attenuation of contaminants, including those caused by assimilative processes of soils,
refers to the reduction of concentrations and/or toxicity of contaminants, including those
contaminants during transport in soils. This process is discussed in detail in Section 10.2
in Chapter 10 where the use of soils as a waste management tool is addressed. For the
present, we will examine some of the phenomena pertinent to the present context of water
and groundwater controls. Reduction in concentrations and toxicity of contaminants in the
groundwater can be accomplished by (a) dilution because of mixing with uncontaminated
groundwater, (b) interactions and reactions between contaminants and soil solids that
can lead to partitioning of the contaminants between the soil solids and porewater, and
(c) transformations that reduce the toxicity threat posed by the original contaminants. Short
of overwhelming dilution with groundwater, it is generally acknowledged that partition-
ing is by far the more signiicant factor in attenuation of contaminants and/or pollutants.
Natural attenuation refers to the situation when attenuation of contaminants results
because of the processes that contribute to the natural assimilative capacity of soil. This
means that contaminant attenuation occurs as a result of the natural processes occurring
in the soil during contaminant-soil interaction. Broadly speaking therefore, natural atten-
uation refers to natural processes occurring in the soil that serve to reduce the toxicity of
the contaminants and/or the concentration of the contaminants. These natural processes
of contaminant attenuation include dilution, partitioning of contaminants, and transfor-
mations. They involve a range of physical actions, chemically and biologically mediated
reactions, and combinations of all of these.
According to the U.S. National Research Council (NRC), the sustainability of natural
attenuation is dependent on the sustainability of the mechanisms for immobilizing or