Conceptual Frameworks for the Phytoremediation of Groundwater Contamination - Introduction to Phytoremediation of Contaminated Groundwater - page 345

atmosphere through foliar uptake. Importantly from the per-
spective of phytoremediation, this model accounts for
changes in the contaminant concentrations within the plant
caused by the plant metabolism processes introduced in
Chap. 11.

water table” or as “elevation or altitude of water table above
a common datum”. The difference in water-table elevation,
or head, L , between two wells,

will provide the head gradient, i , that will cause groundwater
to flow in response to gravity. The effective porosity can be
determined by laboratory tests or from reference tables.
Movement of solute by diffusion can be estimated using
different approaches. In the field, tracer tests can be
performed using a conservative solute such as bromide or
chloride. The diffusion coefficient can also be selected from
reference tables.
Samples of aquifer material are collected and examined
for sorption using standard methods. The absorption coeffi-
cient for a particular compound for site sediments can
be determined in the laboratory by adding various
concentrations of a compound to vials containing the sedi-
ment and determining the fraction that remains in the added
solution. Once the samples are collected, a known amount,
between 5 and 30 g, can be added to vials and then amended
with a solution of the contaminant(s) of interest. This is
allowed to equilibrate for a period of time on a shaker
table. Samples of the liquid phase are then analyzed to
determine the fraction of the contaminants that remain in
the solution; the difference will represent the degree of
sorption. A linear response can then be plotted as a function
of different initial contaminant concentrations. The slope of
this line can provide the partition coefficient, K d , for that
compound in that sediment. Contaminant volatilization can
be determined using a field or laboratory approach, such as
that described by Lahvis et al. (1999).
The microbial degradation of contaminant compounds is
a special case of the microbial metabolism of substrates. For
most field situations, this metabolism can be considered to
follow first-order kinetics, such that the degradation of a
substrate is not limited by the availability of the appropriate
enzymes, or

h , divided by the distance,

D D

14.3.2 CTSPAC

Lindstrom et al. (1990) produced a one-dimensional analyti-
cal model called CTSPAC to simulate the transport of a
contaminant from the soil to plant to atmosphere from a
source in the vadose zone. Movement of water in the vadose
zone is simulated by the Richards equation (see Tindall and
Kunkel 1999 for equation derivation), and movement of
solutes by convection, diffusion, dispersion, sorption, degra-
dation, and plant uptake. The model simulates all aspects of
plant physiology, including separate xylem and phloem
compartments. This model was calibrated by Ouyang
(2002) using the contaminant 1,4-dioxane as a model xeno-
biotic for which experimental properties that needed to be
entered into the model were known experimentally after
research by Aitchison et al. (2000).

14.4 Site-Characteristic Data Needed

to Support the Framework That
Accounts for Solute Transport
and Plant Processes

Although the solute-transport equation in Eq. 14.7 contains
multiple parameters, each is a physical process that can be
measured at a site. If all the parameters in the equation are
quantified using a combination of field and laboratory
approaches, then the change in solute concentration over
space and time can be solved, within the degree of parameter
variability at the site. This section summarizes each param-
eter in the solute-transport equation.
Because advection is related to the rate of movement of
groundwater and therefore the aquifer hydraulic conductiv-
ity, K , Darcy's Law can be used to determine the general
extent of solute transport by advection. The hydraulic con-
ductivity can be determined from single-well rising- or fall-
ing-head slug tests, a pumping test with some wells used to
observe the time-dependent water-level drawdown, labora-
tory permeability tests done on aquifer material removed
from the site, or look-up tables in reference topics. More
information on these field tests is presented in Chap. 4.
In order to calculate a Darcy velocity, the hydraulic
gradient can be calculated from a water-table map prepared
using synoptic groundwater-level measurements. Each
depth to groundwater is then plotted on a map that shows
the well distribution; the data can be recorded as “depth to

V ¼ kS (14.10)

where V is the rate of substrate uptake (moles per time per g
of cells), k is the rate constant (per time), and S is the
substrate concentration.
This dependence of rate on substrate concentration is
described in the Michaelis-Menten equation,

V ¼ ½ v max = ð K s þ S Þ ðÞ SB (14.11)

where v max is the maximum rate of substrate uptake, K s is the
substrate concentration at which v

½ v max , S is the sub-
strate concentration (moles per liter), and B is the amount of
cells (g). If concentrations at a site are high, first-order
kinetics may not apply.
Field data also can be used to determine biodegradation
rates, and many examples of this approach exist in the

¼