Chemical and Physical Properties That Affect the Interaction Between Plants and Contaminated Groundwater - Introduction to Phytoremediation of Contaminated Groundwater - page 288

however, will be satisfied once solubility limits are reached,
and no further uptake will occur, and excess “free-phase”
chemical will remain. No chemical energy is spent by a
plant during passive uptake. Basically, the plant cannot con-
trol the entry of such compounds into its structure.
Computationally simple models exist to describe the
fate of groundwater contaminants and plants (Schnoor
1997; Burken and Schnoor 1998). However, the simplifying
assumptions necessary to use these models, such as constant
contaminant concentrations, steady-state distribution, and
no microbial biodegradation, can constrain their use to
one of site conceptualization. Essentially, the uptake of
organic contaminants dissolved in water by plants can be
described by

upon the type of plant and its location. Fibrous root systems
typical of grasses will have a greater percentage of lateral
roots, whereas taproot systems typical of woody plants will
have a greater percentage of deeper roots. In sandy soils with
naturally low amounts of soil organic matter, the mere pres-
ence of a root mass will increase the content of organic
matter. As a plant grows and extends its below-ground
space, relatively organic-poor soils become enriched by the
root system over time. This enrichment is both a conse-
quence of the presence of the root tissue itself as well as
root exudates released by the roots as they grow and by the
death of roots. For example, the root tip produces mucigel to
aid its penetration into unrooted soil. Root cells slough off as
roots elongate. Jordahl et al. (1997) reported that hybrid
poplars grown in a lab incubator in sand released less than
0.30% of the total biomass as soluble exudates.
As plant roots move through soil, they tend to follow the
path of least resistance, too, through the pore spaces that
are most interconnected. This explains the relation between
root cross-section and pore size. It is not surprising, then,
that root-soil contact exceeds 60% (Kooistra et al. 1992). If
water is a limiting factor and the water potential of that in
roots decreases, then roots will shrink in diameter, and the
extent of soil and root contact will also decrease.
The term “rhizosphere” reflects the confluence of the
research in the nineteenth and twentieth centuries that
described the interactions between microbes and the soil
zone, as was introduced in Chap. 3. The extension of
research into the increased numbers of microbes on plant
roots in soil gave rise to the term rhizosphere by Lorenz
Hiltner in 1904. Rhizospheric communities in roots are not
limited to terrestrial plants. Aquatic plants have been shown
to have increased numbers of microbial communities
(Federle and Ventullo 1990). This increased number also is
associated with the increased degradation by the microbes
associated with cattail ( Typha latifolia ) roots relative to root-
free sediments.
As we will see later this chapter, one of the selective
reasons for the interaction of plant roots with bacteria and
fungi is that their presence renders a greater degree of
protection to the whole organism than by each alone. This
dependency is similar to one in which the intestinal flora of
animals can render imbibed toxicants harmless, to an
amount that is equivalent to or greater than the amount the
liver can process. This is because these microflora have
similar enzyme systems as the mammalian liver.
Because contaminants also can be present in the root
zone, degradation in the rhizosphere where contaminants
are in the soils or unsaturated zone, has important implica-
tion to groundwater contamination, because groundwater
is often contaminated by surface or subsurface releases
to the soils above the water table. These soils become
contaminated, and if not removed, this material becomes

U ¼ ð TSCF Þð T Þð C Þ (12.50)

where U is the uptake rate of the contaminant (M/T), TSCF
is the transpiration stream concentration factor, 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
range from inefficient uptake (low TSCF ) for low-solubility
compounds, such as pentachlorophenol (0.07), to efficient
uptake (high TSCF ) for high-solubility compounds, such as
TCE (0.74).
The time required to reach remedial concentrations can
be estimated, from first-order degradation kinetics, as

k ¼ U = M o (12.51)

where k is the first-order uptake rate constant (per time, t ), U
is the contaminant uptake rate (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 deter-
mined by

M o e kt M ¼ (12.52)

where M is the mass remaining (M) and t is the time (T).
Solving for time yields

t ¼ ð ln M = M o Þ= k (12.53)

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).

12.2 Plant Rhizosphere Processes

and Contaminant Fate

Water must often pass through the root zone as part of the
hydrologic cycle. The dimensional extent of the root zone, as
was discussed in Chap. 4, will vary considerably depending