Phytotechnology and Photosynthesis 151Figure 7.1 Schematic hydraulic containment
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.
Another approach sometimes encountered is the production of vegetative caps,
which has found favour as a means of finishing off some American landfill sites.
The principle involves planting to preventing the downward percolation of rain-
water into the landfill and thus minimising leachate production while at the same
time reducing erosion from the surface. The method seems to be successful as a
living alternative to an impermeable clay or geopolymer barrier. The vegetative
cap has also been promoted for its abilities to enhance the biological breakdown
of the underlying refuse. In this respect, it may be seen as an applied form of rhi-
zodegradation or even, arguably, of phytodegradation. How effective it is likely
to be in this role, however, given the great depths involved in most landfills and
the functionally anoxic conditions within them, appears uncertain.
To understand the overall phytoremediation effect of hydraulic containment, it
is important to realise that contaminating organics are actually taken up by the