Chemical and Physical Properties That Affect the Interaction Between Plants and Contaminated Groundwater - Introduction to Phytoremediation of Contaminated Groundwater - page 296

correlated to dissolved oxygen concentrations in the solution
by creating a calibration curve of different titanium (III) citrate
concentrations relative to redox status measured as oxidation-
reduction potential (ORP). The oxidation of titanium (III)
citrate to titanium (IV) citrate was determined by adding
different amounts of oxygen-saturated water to the media.
They couldn't measure the oxygen zone directly but could
do so indirectly by the measurement of titanium (III) citrate
oxidation. Measurement of this was facilitated by mechanical
stirring in the hydroponic solution, which displaced the oxy-
gen produced near the roots into the bulk media.
Wießner et al. (2002) reported that under all initial redox
conditions, oxygen was released by all the Typha latifolia
plants studied. From initial reduced conditions of Eh near

that are reduced can be oxidized using these various alterna-
tive electron acceptors, although the rates of mineralization
are slower than if oxygen was used. Contaminants that are
already in the oxidized state, such as chlorinated solvents,
also can be used as electron acceptors in a manner similar to
those above after oxygen and alternative electron acceptors
have been depleted.

12.3.3 Plant-Induced Redox Changes
at Contaminated Groundwater Sites

The interaction of plants and the redox reactions that occur
in surface- and groundwater is perhaps best shown by an
example. Take for instance dissolved iron. The NSDWS
MCL for iron is 2,000 mg/L in surface-water systems. At a
site near Cecil Field, FL, concentrations of iron were near
the MCL in surface water fed by iron-rich groundwater. The
presumed source of the observed high iron concentrations
was the reduced conditions in the ambient shallow ground-
water that discharged to the surface water, as the spring was
downgradient of a landfill. A natural forest of oak trees,
however, also was located between the landfill and the
iron-contaminated spring. The trees probably contributed
to the high concentrations of iron in the groundwater as a
result of the release by the roots of high concentrations of
organic matter that leached to the water table. The resultant
high natural source of high BOD caused iron-reducing
conditions in the shallow groundwater, the release of mobile
ferrous iron, and transport to the spring. As such, the source
of the iron in the surface water was from the natural input of
organic matter from plants, rather than the landfill,

400 millivolts (mV), within 6 h all plants studied had
released enough oxygen to increase the Eh to millivolts
above 0, or final near +300 mV, and dissolved oxygen
concentrations as determined from titanium (III) citrate oxi-
dation went from 0 to 0.9 mg/L in one plant, and to 0.5 and
0.3 mg/L in the other two plants. Under conditions of differ-
ent initial redox status (from highly reduced to less reduced),
the production of oxygen was higher when the redox was
strongest (lowest Eh) and lowest when redox was lower
(higher Eh). At the end of 24 h oxygen production ranged
from 1 to near 8 mg/L. Moreover, the release of oxygen was
continuous, even after oxygenated conditions had been
established.
Because the zone of oxygenation near roots is so small,
the impact on driving the aerobic microbial oxidation of
reduced organic contaminants will also be small, although
not insignificant. The main advantage for phytoremediation
projects from the root-zone release of oxygen is the initial
establishment of the young roots of plants often added to
contaminated aquifer sediments that have no source of oxy-
gen other than recharge. However, too much oxygen can
increase the rate of all oxidative processes and, therefore,
can lead to the depletion of cellular oxidative enzymes.
Many of these processes of diffusion gas transport have
not been examined in poplar trees, however, and such studies
would shed light on how poplar trees can use groundwater
rendered anoxic due to high levels of contamination (Eberts
et al. 2005).

the

suspected anthropogenic source.
A similar process of plants rendering a shallow oxic
aquifer to anoxic conditions, and its effect on TCE degrada-
tion, was reported by Eberts et al. (2005). They report that
where cottonwood trees were planted over a shallow, aerobic
aquifer contaminated with TCE, after 6 years of tree growth,
the dissolved organic carbon (DOC) concentration in the
shallow aquifer increased—this resulted in a lowering of
dissolved oxygen concentrations and anoxic conditions.
The anoxic conditions led to the reductive dechlorination
of TCE following the establishment of iron- and sulfate-
reducing conditions, and finally methanogenesis (Eberts
et al. 2005).
Eberts et al. (2005) also report that as the DO content of
the shallow aquifer decreased, the ratio of the mean
concentrations of TCE/ cis -DCE also decreased. Because
cis -DCE is derived from the reductive dechlorination of
TCE, a decreasing TCE/ cis -DCE ratio indicates the conver-
sion of TCE to cis -DCE. Increased bacterial numbers, deter-
mined by the most probable number (MPN) method, were
observed for methanogenic bacteria capable of degrading

12.3.2 Naturally Anoxic Aquatic Environments
and Contaminant Fate

In the subsurface or in aquatic sediments, heterotrophic
microbes can reduce electron acceptors such as nitrate,
iron, manganese, sulfate, or CO 2 even after oxygen is
depleted. These processes were facilitated by plants as they
produced oxygen in the early earth and forced microbes deep
into the sediments to avoid oxygen toxicity. Contaminants