150 Environmental Biotechnology
volatilise it. The advantages of this approach are clear, given that the current
best available technologies demand extensive dredging or excavation and are
heavily disruptive to the site.
The choice of a poplar species for this application is interesting, since they
have been found useful in similar roles elsewhere. Trichloroethylene (TCE), an
organic compound used in engineering and other industries for degreasing, is a
particularly mobile pollutant, typically forming plumes which move beneath the
soil’s surface. In a number of studies, poplars have been shown to be able to
volatilise around 90% of the TCE they take up. In part this relates to their enor-
mous hydraulic pull, a property which will be discussed again later in this chapter.
Acting as large, solar-powered pumps, they draw water out of the soil, taking up
contaminants with it, which then pass through the plant and out to the air.
The question remains, however, as to whether there is any danger from this
kind of pollutant release into the atmosphere and the essential factor in answering
that must take into account the element of dilution. If the trees are pumping out
mercury, for instance, then the daily output and its dispersion rate must be such
that the atmospheric dilution effect makes the prospect of secondary effects, either
to the environment or to human health, impossible. Careful investigation and risk
analysis is every bit as important for phytoremediation as it is for other forms
of bioremediation.
Using tree species to clean up contamination has begun to receive increasing
interest. Phytoremediation in general tends to be limited to sites where the pollu-
tants are located fairly close to the surface, often in conjunction with a relatively
high water table. Research in Europe and the USA has shown that the deeply
penetrating roots of trees allows deeper contamination to be treated. Once again,
part of the reason for this is the profound effect these plants can have on the
local water relations.
Hydraulic Containment
Large plants can act as living pumps, pulling large amounts of water out of the
ground which can be a useful property for some environmental applications, since
the drawing of water upwards through the soil into the roots and out through the
plant decreases the movement of soluble contaminants downwards, deeper into
the site and into the groundwater. Trees are particularly useful in this respect
because of their enormous transpiration pull and large root mass. Poplars, for
example, once established, have very deep tap roots and they take up large quanti-
ties of water, transpiring between 200–1100 litres daily. In situations where grass-
land would normally support a water table at around 1.5 metres, this action can
lead to it being up to 10 times lower. The aim of applying this to a contamination
scenario is to create a functional water table depression, to which pollutants will
tend to be drawn and from which they may additionally be taken up for treatment.
This use of the water uptake characteristics of plants to control the migration of