Phytotechnology and Photosynthesis - Environmental Biotechnology: Theory and Application - page 153

However, it must also be remembered in this context that, should the pollutants
not themselves actually be taken up by the plants, then the effect of establishing
a hydraulic containment regime will be to increase their soil concentration due
to transpiro-evaporative concentration. Thus, the mass of affected water in the
contaminant plume reduces, as does the consequent level of dilution it offers and
hence, increased localised concentration can result.
The transpiration pull of plants, and particularly tree species, has also
sometimes been harnessed to overcome localised water-logging, particularly
on land used for agricultural or amenity purposes. To enhance the effect at
the point worst affected, the planting regime may involve the establishment of
close grouping, which then function as single elevated withdrawal points. The
noted ability of poplars to act as solar-powered hydraulic pumps makes them of
great potential benefit to this kind of phytotechnological application. Although
other plant-based processes could be taking place at the same time to remediate
land alongside this to clean up contaminated soils, this particular technique is
not itself a type of phytoremediation. Instead, it is an example of the broader
bio-engineering possibilities which are offered by the appropriate use of flora
species to wider environmental nuisances, which, for some sites, may be the
only economic or practicable solution. This may be of particular relevance to
heavy soils with poor natural inter-particulate spacing, since laying adequate
artificial drainage systems can often be expensive to do in the first place and
are frequently prone to collapse once installed.
Another similar example of the use of phytotechnology to overcome nuisance
is the bio-bund, which consists of densely planted trees, often willows, on an
engineered earthwork embankment. This system has been used successfully to
reduce noise pollution from roads, railways and noisy industrial sites, the inter-
locking branches acting as a physical barrier to deaden the sound as well as
having a secondary role in trapping wind-blown particulates. Depending on the
individual site, the bio-bund can be constructed in such a way that it can also
act as a buffer strip to control migrating chemical pollution, if required.
Plant Selection
It should be obvious that the major criteria for plant selection are the particular
requirements for the method to be employed and the nature of the contaminants
involved. For example, in the case of organic phytotransformation this means
species of vegetation which are hardy and fast growing, easy to maintain, have
a high transpiration pull and transform the pollutants present to non-toxic or less
toxic products. In addition, for many such applications, deep rooting plants are
particularly valuable.
On some sites, the planting of grass varieties in conjunction with trees, often in
between rows of trees to stabilise and protect the soil, may be the best route since
they generate a tremendous amount of fine roots near to the surface. This par-
ticularly suits them to transforming hydrophobic contaminants such as benzene,