Environmental Engineering Reference
In-Depth Information
T
is the transpiration rate (L
3
/T); and
C
is the concentration
of the dissolved-phase contaminant (M/L
3
). Values of the
TSCF
for contaminant compounds range from inefficient
uptake (low
TSCF
) for low-solubility compounds, such as
pentachlorophenol (0.07), to very efficient uptake (high
TSCF
) for compounds with higher solubility, such as toluene
or TCE (0.74). From Eq.
10.1
, the time required to
phytoremediation processes to achieve remedial goals can
be estimated, from first-order degradation kinetics:
10.2.2 Transgenics and Other Obstacles
to Public Acceptance
The proposed use of transgenic plants as part of
phytoremediation plantings is becoming increasingly com-
mon. This field of research, called plant breeding, or plant
biotechnology, is based on the production of plants with
desirable traits that did not have these traits originally,
using recombinant DNA technology. Much confusion
stems from fears of eating food produced by such methods,
and public acceptance has been slow. Ironically, the genetic
mixture of plants with different characteristics to produce a
more desirable outcome occurs naturally, and the frequency
of this interaction increased as man experimented with early
agricultural crops—such benefits reaped from transgenic
foods are not widely known (The Economist 2005).
Successful cultivation, or domestication, of our common
cereal crops that were initially wild plants enabled the
world's population to double 10 times since the last Ice
Age, from 10,000,000 to 6,000,000,000. It started with the
use of wild strains of corn and wheat, for example, and then
the crossing of these varieties through time to produce a
transgenic plant that had very large seeds. Other cereal
plants that had properties found useful by hungry humans
were, therefore, preserved and are the result of natural
genetic mutations—the original plant bears little resem-
blance to the present-day plant. For example, corn kernels
grown today are up to eight-times larger than those produced
by wild corn plants. This process of selection of corn with
larger kernels was started thousands of years ago by Native
North Americans. As a result of such focused crop domesti-
cation, corn grown today cannot reproduce by natural polli-
nation; a consequence is that abandoned cornfields do not
repopulate on their own.
The production of food crops by artificial selection of
natural varieties gave rise to the interest in making artificial
mutations by exposure to processes designed to damage
plant DNA. However, even this is a random process. It was
the need to be more proactive in determining exact outcomes
of these mutations that led to the development of transgenic
processes.
In a similar manner, the pulp-wood and phytoremediation
industries benefit from such manipulations of native plants
to produce hybrids to supply their industry. The controversy
that surrounds the use of transgenic plants not intended for
the food supply is from potential implications that surround
the escape of these transgenes into native plant populations.
The advent of recombinant DNA methods increased the
rate of testing and production of transgenic plants. The
methods used to introduce the desirable trait (the expression
of genes, or genomes, which act as the instruction guide or
recipe for each and every cell) take the gene from one plant
k
¼
U
M
o
(10.2)
=
where
k
is the first-order uptake rate constant (per unit time),
U
is the contaminant uptake rate from Eq.
10.1
(M/T), and
M
o
is the initial mass of contaminant present (M). At any
time
t
during remediation, the mass remaining in the aquifer
can be determined by:
M
o
e
kt
M
¼
(10.3)
where
M
is the mass remaining and
t
is the time. Solving for
time yields
t
¼ð
1
nM
M
o
Þ
=
k
(10.4)
=
where
t
represents the time needed to reach a remedial action
level (T),
M
is the mass allowed at time
t
(M), and
M
o
is the
initial contaminant mass (M).
An example of using this approach to estimate the range
of cleanup times possible was performed at the former
manufactured gas plant site near Charleston, SC, discussed
previously. The criterion assumed for descriptive purposes
was for poplar trees to decrease dissolved benzene
concentrations up to 10% of initially measured
concentrations. Using Eq.
10.1
to Eq.
10.4
different amounts
of dissolved benzene were calculated to be taken up
(Landmeyer 2001). The uptake rate increased with increased
water removal by trees, assuming that all transpirational
demands were being met by groundwater. Proportionally
more groundwater was removed by the older trees, resulting
in a shorter amount of time necessary to reach the clean-up
goal, in this example.
As shown by Matthews et al. (2003), the issue of time to
hydrologic control can be investigated using numerical
simulations run under transient rather than steady-state
conditions. Hydrogeologic characteristics of the site, such
as low specific yields, may require a long time before
groundwater levels are lowered across the site to influence
groundwater flow. If these factors are discovered during site
assessment and characterization, however, it is possible to
increase the size of the planting to reduce the time for
hydrologic control to be reached.
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