Chemical and Physical Properties That Affect the Interaction Between Plants and Contaminated Groundwater - Introduction to Phytoremediation of Contaminated Groundwater - page 295

source of carbon for reduction. The partial pressure of CO 2
in the subsurface unsaturated zone (soil pores) will be higher
than that in the leaves as it is removed there to drive photo-
synthesis. In fact, Levy et al. (1999) reported partial
pressures, as pCO 2 , from 3,000 to 9,000 Pa within the
woody stems of trees. They also reported that this transport
of CO 2 in stems to leaves was equivalent to less than 7% of
leaf fixation rates.
Wium-Anderson (1971) showed that CO 2 from sediment
sources (as free CO 2 rather than HCO 3 - , the form used
by higher plants) increased carbon-fixation up to five times
more than CO 2 from the water in the hydrophyte Lobelia
dortmanna . The absorbed CO 2 , presumably from anaerobic
microbial processes in the sediment, is transported inside the
plant to the leaves, where it is fixed. In turn, the authors stated
that a zone of oxidized iron in the bed sediments exists around
the roots to a depth of 20 cm. This iron oxidation must not
have detrimental impacts on iron uptake, since reduced iron
is not likely to be limited in reduced sediments. Moreover,
the presence of a large root surface area to above-sediment
biomass in submerged aquatic macrophytes suggests the
ability to take up dissolved CO 2 from the sediments.
An excellent example of this increased below-ground
growth and CO 2 root absorption is expressed by, of all
things, a terrestrial plant, Stylites andicola (Keeley et al.
1984). S. andicola is found in Peru in fens in higher ground
adjacent to bogs in organic-rich peat areas. Two-thirds of the
total biomass is underground. They lack stomata in the
leaves, even though they produce an evergreen rosette.
They have the ability to undergo crassulacean acid metabo-
lism (CAM), with CO 2 uptake and conversion to organic
acids at night, and with the reformation of CO 2 during the
day for fixation as carbohydrate. This is similar to desert
plants that during that day must tightly close their stomata.
Under these conditions, CO 2 is taken in during the night, and
fixed into simple organic acids, using CAM. The acids are
stored in vacuoles until daylight, when the acids are
decarboxylated into CO 2 .
In one way, the acquisition of CO 2 by plants from sub-
surface sediment sources rather than atmospheric sources
makes sense. The atmospheric concentration of CO 2 is
low, no more than 0.030%. Conversely, it can be very high
in anoxic sediments, where the degradation of organic mat-
ter leads to CO 2 production. Roots are present in this CO 2 -
rich media (Raven et al. 1987). This process is similar to the
recycling of root-respired CO 2 back into organic carbon
within the plant.
As stated previously, CO 2 is necessary for photosynthe-
sis; terrestrial plants take up CO 2 from the atmosphere, and
aquatic photosynthetic organisms take it from aqueous solu-
tion. Willows represent a phreatophyte that can be planted at
a contaminated site where groundwater contains low DO and
high CO 2 . In these plants, even though the majority of the

CO 2 is derived from the atmosphere, between 1% and 2% of
the carbon taken in by leaves is taken in by the roots
(Vuorinen et al. 1989). Essentially, the CO 2 is transported
to the plants parts in a manner similar to that of other
dissolved substances. An investigation by Brix (1990) also
stated, using a 14 C-radiotracer study, that less than 1% of
the total plant carbon-fixation, in Phragmites australis , was
derived from sediment pore-water CO 2 in the root zone.
These plants grow in anoxic mud and have extensive root
systems. The internal concentration of CO 2 in such plants
can exceed 8% of the total volume of air space.
Too much CO 2 in the soil gas of the unsaturated zone can
be lethal to plants, however. At Mammoth Mountain, CA,
volcanic activity has sent large plumes of CO 2 gas upward,
and it has collected in the soil gas of the unsaturated zone.
This was discovered when scientists were investigating the
death of trees in an area of about 100 acres around the
mountain (Sorey et al. 2000). Concentrations of CO 2 were
present in the soil gas at concentrations greater than 20-95%
of the soil-gas volume. Such high concentrations of gas also
may be found in the unsaturated zone above groundwater
plumes. The presence of CO 2 can be monitored with a soil-
gas probe lowered into holes dug above the water table. This
and other measurements are discussed in Chap. 15.

12.3.1.3 Plants, Gases, and Groundwater
Contamination
What do these gas-transport processes in situ have to do with
the phytoremediation of contaminated groundwater? The
interest in gas transport in plants that remove groundwater
from contaminated aquifers is twofold. First, most aquifers
contaminated by petroleum hydrocarbons contain little
dissolved oxygen. This is detrimental to plant root survival,
as well as the efficiency of degradation of contaminants under
aerobic processes. Thus, plant mechanisms to supply roots in
anoxic environments with sufficient quantities of oxygen to
support respiration may have positive consequences for con-
taminant degradation. Also, contaminated groundwater can
become methanogenic, and the release of methane to the
atmosphere can occur, and perhaps hydrogen sulfide, which
can be toxic to plants at low concentrations. For aquifers
contaminated by oxidized contaminants like PCE or TCE,
oxygen limitations might not occur. In this case, the presence
of plants and their release of root exudates could potentially
render the groundwater anoxic and support reductive dechlo-
rination (Eberts et al. 2005).
An elegant study by Wießner et al. (2002), having
implications for phytoremediation of contaminated ground-
water, was to determine the release of oxygen from the roots
into the rhizosphere. Wießner et al. (2002) were able to vary
the redox status of vials of plants growing in hydroponic
solution by differential titration with titanium (III) citrate.
Redox measurements in the hydroponic solution were