Plant Selection, Installation, and Management to Affect Groundwater - Introduction to Phytoremediation of Contaminated Groundwater - page 185

Fig. 7.10 The effect of leaf-area
index, LAI , and vapor pressure
deficit, VPD , (in kilopascals) on
transpiration for two Eucalyptus
species in Australia (Modified
from Rural Industries Research
and Development Center 2000).

hemisphere study site of Australia), the VPD also was high.
This indicates that transpiration would be at a maximum.
This also was during a period of low annual precipitation.
Combined, these factors lead to plant and groundwater inter-
action that can be used for hydrologic control.
Wind also affects the rate of transpiration and, therefore,
plant health by removing air in equilibrium with the leaf
surface and replacing it with air out of equilibrium, thus
continuing the transpiration process. In general, transpiration
is higher on windy days than on calm days when all other
factors are equal. Excessive wind speed can also damage the
entire plant by toppling it during storms. This has a higher
probability of occurring if high winds are preceded by precip-
itation that increased the soil moisture around the plant roots.

returning to it 10 years in the future to realize restored
groundwater may be a reality in the future, but is wishful
thinking for most projects.
As previously discussed, a phytoremediation planting is a
microcosm of forest ecology, silviculture, and agriculture.
What happens in forests happens in controlled
phytoremediation plantings. In forests, the ground usually
is littered with dead branches from lower parts of the tree
that died from a lack of light. If such a branch were left to rot
on the tree, it is possible that the rot would spread to the main
trunk. Limb removal by storms is a form of wind pruning,
after which the tree heals the wound to limit the spread of
any microbial or fungal infestation. In a similar fashion,
plants also undergo air pruning, in which limbs dry out and
die, or new roots that no longer have access to moisture
cease to grow. Some trees, however, such as poplars and
willows, can self prune even during calm weather
(Fig. 7.11 ). This form of pruning is not of dead plant parts
but of live stems, and occurs at abscission zones.

7.3.11 Site Operation and Maintenance,
Pruning, and Fertilization

One of the reasons that phytoremediation is touted as an
inexpensive remedial alternative is the notion that tree
installation is inexpensive. While this may be true as far
as the initial capital costs of trees relative to installing a
pump-and-treat or air-sparge system, annual expenses are
associated with a phytoremediation system that cannot be
ignored. These annual expenses when run out over perhaps
decades of the life cycle of a phytoremediation planting can
result in costs that may approach those of more aggressive
technologies. Even from the standpoint of those who pro-
mote the use of phytotechnologies, the short-term and long-
term costs must be evaluated.
Much like a vineyard can be grown by using low-impact
agricultural techniques but still remains a business that needs
to produce a profit, the installation of a phytoremediation
project also is part of an overall business model for those
individuals doing the work. Costs that should be associated
with a phytoremediation project include operation and main-
tenance costs. Pruning or grounds keeping is a major consid-
eration. The idea of planting a site, walking away, and

Fig. 7.11 Evidence for the self-pruning habit of some willow trees
growing near a pond in Blythewood, South Carolina. The pruned
branches on the ground were not dead but had viable terminal and
lateral buds (Photograph by author).