Environmental Engineering Reference
In-Depth Information
For example, the effect of toluene on representative
plants was studied and results presented by Reporter et al.
(1991). The study used plant cell cultures, rather than whole
plants. In this way, the effect of toluene on plant cell growth
could be observed readily. Although a wide range of plant
species was studied, the results for alfalfa (
Medicago sativa
L. cv. Vernal) and black locust (
Robinia pseudoacacia
L.)
only are summarized here because of their importance as a
herbaceous and woody phreatophyte, respectively, and
potential use at phytoremediation sites. Alfalfa and black
locust cells were grown in the presence of 500 ppm toluene
for 12 days, and controls did not contain toluene. At the end
of 12 days, cell growth (as grams fresh weight) had increased
in both the control and treatment.
One of the most obvious plant processes to be used to
assess the toxicity of a particular chemical is the process of
shoot growth and its related transpiration. Measurements of
transpiration can be readily made. This is similar to lab tests
with microbes in which respiration is measured. Although
the gross characteristic of a dying or dead plant is easily
detected, it is subjective and the results are not easily com-
parable among sites and different labs. A toxicity test devoid
of this subjectivity was described by Trapp et al. (2000).
Using their method, cuttings can be weighed and then placed
in flasks filled with a nutrient solution. A stopper can be used
to seal the space around the cutting and the flask neck, and
the gaps sealed with silicone caulk. The flasks are then
wrapped with various materials to prevent light from enter-
ing. The flask-cuttings can be placed in artificial or natural
light to acclimate to conditions prior to addition of the test
toxicant (no addition for the control). The cutting-solution-
flask needs to be weighed every day; the loss of weight is a
surrogate for transpiration. Transpiration of the treatment
flasks is normalized by the control flask transpiration. This
hydroponic test also can be accomplished using various solid
media.
The results of various toxicity tests on plants suggest that
it is prudent to assess the levels of soil and groundwater
concentrations that the plants might encounter prior to
implementing a phytoremediation planting. Contaminant
delineation also is needed to determine
a priori
if the levels
are too high and contraindicate plant survival.
contaminants as an energy and/or growth source if oxygen
is present but cannot derive the same benefit from a highly
oxidized compound such as PCE. Conversely, anaerobic
bacteria will be able to reduce PCE and will only be able
to transform reduced organic contaminants if alternative
electron acceptors are available.
A similar approach can be used to assess if
phytoremediation of groundwater contaminated by specific
types of xenobiotics will occur. Essentially, knowledge of
the biotransformation potential of different plants can be
used to match the plant to the type of groundwater contami-
nant present, even if the goal is hydrologic control, rather
than contaminant transformation, as discussed in Chap. 8.
However, even this interaction between plants and
contaminated groundwater will lead to contaminant trans-
formation, which can be enhanced if the correct plants are
used. Contaminant transformation can be enhanced by com-
paring the fate of groundwater contaminants with various
plants, using the metric of transformation rate or transforma-
tion extent. Not all plants, however, possess the correct
enzymes to carry out Phase I, II, and III detoxification
reactions. Most plants, however, can offer an important
microbial component to remediation through the establish-
ment of root-zone microbes in the rhizosphere.
Some of the plant traits necessary for the efficient inter-
action with contaminated groundwater include a deep root
system, a documented interaction with the water table
or capillary fringe, the ability to interact with common
groundwater contaminants at environmentally relevant
concentrations, the ability to process these contaminants
through
in planta
detoxification reaction, and robustness in
the face of contaminants and low intensity agricultural
practices (Table
13.4
).
As can be seen, members of the family
Salicae
, such as
willows and poplars, are useful for a range of contaminants
that can be found in groundwater. Similar results were
shown by Zalesny et al. (2005) for plant interaction with
soils contaminated by petroleum hydrocarbons. They
planted a wide range of hybrid poplars and willows at
sites characterized by petroleum hydrocarbon contamination
of soil and groundwater and monitored plant-growth
parameters over time. The greatest survival rate was for the
hybrid poplars relative to the willows, at 97% versus 56%,
respectively. The larger cuttings of poplars had higher sur-
vival rates than did smaller cuttings. The smaller, less expen-
sive cuttings experienced more growth in length and height,
however, versus the larger cuttings. As such, sites that
require a larger area to be planted or sites with a smaller
budget need not be placed at a disadvantage. Moreover, their
study confirmed that the widespread application of poplar
and willow clones is not just due to their availability from
the paper or nursery industries but also to their survival at
contaminated sites.
13.8
Plant Selection for Specific
Groundwater Contaminants
To implement the bioremediation of groundwater contami-
nation, knowledge of the ecological niche of soil and
groundwater microorganisms and metabolic pathways was
shown to be a useful approach to determine the type of
contaminants that can be transformed (Chapelle 1993).
For instance, aerobic bacteria will use reduced organic
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