Phytotechnology and Photosynthesis - Environmental Biotechnology: Theory and Application - page 151

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 US has shown that the deeply pen-
etrating roots of trees allow 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 quantities of water, transpiring between 200 and 1100 litres daily. In
situations where grassland would normally support a water table at around 1.5m,
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 contaminants in the soil is termed hydraulic containment,
shown schematically in Figure 7.1, and a number of particular applications have
been developed.
Buffer strips are intended to prevent the entry of contaminants into water-
courses and are typically used along the banks of rivers, when they are sometimes
called by the alternative name of 'riparian corridors', or around the perimeter of
affected sites to contain migrating chemicals. Various poplar and willow vari-
eties, for example have shown themselves particularly effective in reducing the
wash-out of nitrates and phosphates making them useful as pollution control mea-
sures to avoid agricultural fertiliser residues contaminating waterways. Part of the
potential of this approach is that it also allows for the simultaneous integration of
other of the phytoremediating processes described into a natural treatment train,
since as previously stated, all plant-based treatments are aspects of the same
fundamental processes and thus part of a cohesive whole.