152 Environmental Biotechnology
plant at lower concentration than they are foundin situ, in part due to membrane
barriers at the root hairs. In order to include this in a predictive mathematical
model, the idea of a transpiration stream concentration factor (TSCF) for given
contaminants has been developed, defined as TSCF= 0 .75 exp{−[(logKow−
2. 50 ) 2 / 2 .4]}, (Burken and Schnoor 1998) whereKowis the octanol–water par-
tition coefficients. These latter are a measure of the hydrophobia or hydrophilia
of a given organic chemical; a logKowbelow 1 characterises the fairly soluble,
while above 3.5 indicates highly hydrophobic substances.
Thus the uptake rate (Uin mg/day) is given by the following equation:
U=(TSCF)T C
Where:
TSCF=transpiration stream concentration factor, as defined
T=transpiration rate of vegetation, l/day
C=concentration in site water, mg/lHowever, 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 some-
times 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 groupings,
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 bioengineering
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 interparticulate 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