Chemical and Physical Properties That Affect the Interaction Between Plants and Contaminated Groundwater - Introduction to Phytoremediation of Contaminated Groundwater - page 293

was rendered anoxic because of contaminant release or
because of naturally high levels of BOD, such as in a
swamp or wetland environment.

concentration in the roots above that of the atmosphere, so
CO 2 exits the root zone along a vertical diffusion gradient.
The best examples of this process and its implication for
the phytoremediation of contaminated groundwater are
found in the submerged macrophytes and trees, such as
baldcypress, that transfer oxygen to the root zone as well
as transfer methane, produced in the flooded, anoxic
conditions near the roots, to the atmosphere (Armstrong
and Armstrong 1987). The presence of this process in
woody plants was investigated and reported by Armstrong
(1968). Woody plants are found in swamps, and
phreatophytes have roots that are in the water table at least
some time of the year. In the case of the woody plants
investigated, oxygen from the atmosphere was transported
as a gas into the anoxic, waterlogged root zone, but the
oxygen entered through the bark rather than the leaves. If
the gas openings (lenticels) were covered with grease, then
oxygen diffusion slowed down. In freshwater systems, the
“knees” of baldcypress trees growing where the surface-
water level fluctuates are extensions of lateral roots primar-
ily located in the anoxic, bottom sediments, presumably to
allow gas exchange to occur (Kramer et al. 1952), although
this has not been proven unequivocally. The swamp cypress
Taxodium has a series of protruding pillars of tissue that
arise from the submerged root to above the water line.
These are filled with air channels that are believed to trans-
port oxygen to the other parts of the roots.
In the saline mudflats of Florida, for example, mangroves
( Avicennia ) have roots in anoxic mud but also have roots that
grow off the main stems that project into the air called
pneumatophores (Scholander et al. 1955). In such marsh
conditions, the concentration of oxygen in the pore-water of
the sediments is low. This is because any oxygen that enters
the upper layers of sediment after each tide is rapidly con-
sumed by a thin layer of aerobic organisms, as well as by
abiotic oxygen consumption by reduced mineral oxidation.
Mangroves transport oxygen to the anoxic subsurface in
coastal areas along Florida, for example. The grey mangrove
( Avicennia marina ) found along coastal Australia accomp-
lishes this transport through gas-filled pneumatophores that
store oxygen during periods of low water level (tides) and
transmit oxygen to the roots during periods of high water. In
most cases, the transport of oxygen from the atmosphere into
the pneumatophore through lenticels is a passive process
driven by diffusion. Skelton and Allaway (1996) observed
the time course of oxygen concentrations measured inside
mangrove roots during unflooded and flooded conditions.
They reported that during low water conditions oxygen
entered the exposed roots. Following flooding during high
tide, the oxygen concentration decreased but remained at
levels for 10 h that could support aerobic respiration.
However, plants roots need a continual source of oxygen
in order to live and grow. These limitations also affect

12.3.1 Influence of Oxygen, Carbon Dioxide,
and Methane on Plants and
Contaminated Groundwater

We saw from Chap. 3 that a plant will be adversely affected
if too little water is available. Also, too much water can lead
to the death of a plant where the rate of oxygen uptake by
root respiration exceeds the rate of oxygen diffusion into
water. It has been demonstrated that plants with up to 55% of
their roots flooded by an artificially raised water table
showed decreases in leaf growth within 4 days (Reicosky
et al. 1985). Although this decrease in growth could be
attributed entirely to increases in water potentials associated
with the flooding event, an alternative explanation of the
poor growth observed was the simultaneous decrease in
ambient oxygen content due to the low solubility of oxygen
in water.

12.3.1.1 Oxygen Diffusion into the Root Zone
Land-based plants, in general, have the majority of their
roots located in the subsurface and, therefore, are surrounded
by concentrations of oxygen that are equivalent to that in air
(Conrad 1995). As a consequence of aerobic root respiration
most plants, including phreatophytes, need to have a major
percentage of their roots located in areas of the subsurface
that are above the zone of constant water saturation. As the
water table rises, or during flood conditions that affect ripar-
ian ecosystems, or in wetlands and swamps when the water
level is high and oxygen concentrations lower, plant roots
require oxygen by using other structures to remove this
oxygen limitation. Moreover, the position of the water
table and effect on oxygen conditions was found to reduce
the growth of alfalfa (Bornstein et al. 1984).
Because we are discussing the flow of gases from the
atmosphere to the subsurface though plants, some theory of
gas flow is warranted. If the temperature of a gas is increased
relative to that of the gas through a porous partition, the gas
flow will occur from the warmer gas to the cooler gas; this is
called thermoosmosis. This flow of gas was reported by
Grosse (1989) for both wetland and riparian plants, such as
alder ( Alnus glutinosa ).
The plant cortex works to overcome this oxygen limita-
tion in the root zone as was described in Chap. 3 and is
accomplished by oxygen diffusion from the atmosphere to
the soil. The oxygen concentrations are kept low in the root
zone by root respiration and consequently sustain diffusion-
dominated oxygen transport from the atmosphere to the
roots. Moreover, the production of CO 2 increases the