Economic and Regulatory Factors That Affect the Phytoremediation of Contaminated Groundwater - Introduction to Phytoremediation of Contaminated Groundwater - page 372

commercial crops. Even after more than 27 years since the
first genetically modified plant was created in 1983, no more
than 69% of the cotton planted in the United States was
genetically modified, as well as up to 26% of corn planted,
and this is not for human consumption but for fodder. It may
take as long for transgenes to become commonly used when
designing a phytoremediation plan. As precise as the tech-
nology may be, there still is the potential for unintended
consequences, unpredictability, and unanticipated outcomes.

There are at least two considerations in terms of
bioaccumulation that must be discussed when plants are
exposed to contaminant compounds at phytoremediation
sites. First, because plants essentially remove gaseous CO 2
from the atmosphere and convert it into solid biomass, it
follows that this biomass itself acts as a sink for organic
compounds. Chemicals that have lower solubility in water
would tend to partition into the plant structure. However, the
location of this sequestration would most likely be below
ground in the root zone if the contaminant source is assumed
to be groundwater. This distribution would change, however,
if an atmospheric source were also present. The degree of
bioaccumulation from this process is likely to be small,
because even though most xylem cells from the previous
year's growth are dead, some contaminant metabolism will
occur in the living tissues in the cambium layer just below
the bark. The cortex cells will permit the rapid volatilization
of certain chemicals before concentrations are increased to
high levels.
The second concern would be accumulation of
contaminants in those parts of a plant that most likely
might be a route of exposure, such as to humans or wildlife.
Plants are primary producers, and at the base of most trophic
levels. Chemicals can enter the plant from air, soil, or water
pathways. Once in the plant, the chemical could stay
localized at the point of entry, be translocated within certain
parts of the plant, or be translocated unmediated to exit the
plant by transpiration. In plants, the chemicals would reside
in the fertilized ovaries of a female plant, the fruit, or in the
sap of commercial plants, like the sugar maple. However,
even though the xylem is located near the phloem,
concentrations will not exceed those of the xylem on account
that no physical process has yet been identified that shows
that this occurs; see Marschner (1995) for a dissenting opin-
ion. The implication for phytoremediation is that a sink for
phloem-transported sugars such as fruits, will not be a sink
for contaminants as long as they do not cross phloem
membranes.

16.3 Bioaccumulation Potential

at Phytoremediation Sites

Plants are the basis of most food chains on earth. If plants
become contaminated, the potential exists for the contami-
nant to move through the food chain. Even at contaminated
sites where native vegetation exists, however, the plants
present have not typically been assessed for contaminant
levels to the extent that soil, water, and groundwater are
required to be monitored by state and federal regulators.
Because of the interaction between plants and
contaminants at contaminated sites, there exists the potential
for bioaccumulation. Bioaccumulation, or bioconcentration,
refers to the uptake and retention of a particular compound
once it enters a living organism. As such, contaminant gains
are greater than losses. The result is that an organism has a
higher internal concentration of the compound relative to the
concentration external to the organism (Schwarz and Jones
1997).
As was introduced in Chap. 12, a bioconcentration factor,
or BCF , can be expressed as

BCF C org =

¼ C env (16.1)

Where C org is the concentration of the chemical in the
organism, such as a plant, and the C env is the concentration in
the environment, be it soil, water, or air.
The above equation can be enhanced after inclusion of a
coefficient of bioconcentration, K bc . This parameter reflects
the bioavailability of the particular contaminant, as log K ow ,
of the compound. Therefore,

16.4 Technical Impracticability and the Role

of Phytoremediation

The needs of consultants or responsible parties are often
understandably at odds with the needs of regulators to
carry out their jobs of environmental and human health
protection. There is one area, however, that often both
sides can agree on—the issue of technical impracticability
(TI). This is a waiver for remediation at sites that are
characterized by contaminated media that are not available
for conventional restoration (U.S. Environmental Protection
Agency 1993). A TI waiver does not mean that nothing is
required to be done by the responsible party. At a minimum,

C org ¼ K bc C env (16.2)

There are forces within the plant that can act to decrease
the potential for bioaccumulation to occur. These detoxifica-
tion processes were discussed in Chap. 12 and include the
uptake, oxidation, conjugation, and sequestration of
metabolites into less bioavailable parts of the plant. The
assumption is that these processes lead to a less toxic

endpoint.