Plant Interactions with Biogeochemical Environments - Introduction to Phytoremediation of Contaminated Groundwater - page 269

That the growth of the above-ground parts of plants is
related to temperature is obvious. Below ground, however,
temperature changes are not as drastic. Root elongation also
is related to temperature, with maximum rates of growth
near 30 C that decrease with additional increases in temper-
ature. There are exceptions of course for plants in desert
climates, where growth can proceed at temperatures above
60 C. One of the influences of temperature is that even
though above-ground air temperatures may decrease and
depress shoot growth, below-ground temperatures are more
stable. Roots can elongate even during winter months when
trees appear to be senescent. If roots are growing, respiration
is occurring, and this requires the uptake of water. This issue
of dormancy and its effect on phytoremediation is discussed
in Chap. 16.

installed at contaminated groundwater sites interact with the
unseen water table some distance beneath the land surface.
In general terms, a wetland is a zone of transition of plant-
water availability that ranges from aquatic constraints to
terrestrial conditions. In legal terms, the U.S. Army Corps
of Engineers, the Federal agency charged with wetland reg-
ulation, defines a wetland as having shallow water at least
some length of time, such as during the growing season,
having anoxic sediments, and vegetation. In the 1989 Fed-
eral Wetland Delineation Manual, a wetland was defined as
land where the soil is saturated within 18 in. of the surface
for at least seven consecutive days during the growing sea-
son. This was changed in 1991 to reflect that the land must
completely be under water for 15 consecutive days or
saturated to the surface for 21 consecutive days.
In biological terms, wetlands are ectones—areas that
are not always wet or always dry—and are found between
predominantly dry and predominantly wet land. With respect
to groundwater, wetlands are areas were uplands interface
with lowlands and the water table is near or at the land
surface. Wetlands are transitional places between aquatic
and terrestrial ecosystems. But they also are only a temporary
part of the continuum from exposed land to wetland to swamp
and back to terrestrial land as the wetlands fill in.
The water level is not the only variable that can be used to
describe wetlands. Vegetation also can be used. Wetlands
contain a variety of plants. As we have seen, many of the
plants that possess characteristics of phreatophytes, such as
poplars and willows, are found in wetland or near-wetland
environments. Submerged aquatic macrophytes also are
present. Wetland plants tend to be herbaceous.
Other types of wetlands exist. Swamps are wetlands
characterized by having more woody plants. Marshes tend
to have emergent macrophytes, and bogs and fens have
grasses, mosses, and some trees. Some states along the
eastern coast of the United States are characterized by ellip-
tical-shaped landforms of unknown origin called Carolina
Bays that often contain wetlands. Other wetlands are called
sloughs and bogs. Wetlands also can be classified by their
water quality. Some wetlands are dominated by freshwater
and some by saltwater. For saltwater wetlands found along
the coasts, fresh surface water draining the uplands mixes
with saline water from the oceans in estuaries. Also along
rivers, wetlands can be found as oxbow lakes and in periodi-
cally inundated flood plains.
The science behind the use of modern-day, westernized
wastewater treatment can be traced back to the use of natural
wetlands to “treat” waste. For example, wetlands in Europe
have long been referred to as “wastes” (Horne 2000).
Using such natural systems to cleanse wastes probably can
be considered the beginning of bioremediation, monitored
natural attenuation, and even, by connection with the rhizo-
sphere, phytoremediation. Wetlands naturally improve water

11.4 Plant Interactions with Contaminated

Soil and Water

We have seen how plants have been used as indicators of
environmental contamination and for toxicity assessment.
Plants also have been used as a means to decrease contami-
nant levels in the soil and aquatic environment. This has
been achieved mainly through constructed wetland systems
or landfarming approaches, both practices which have a long
history in the United States and even longer in Europe. In
fact, the application of plants for phytoremediation of
contaminated groundwater can trace its inception to these
earlier plant-based contaminant or waste-reduction
practices. This scenario is strikingly similar to the linkage
between the application of in situ subsurface microbes for
monitored natural attenuation and the long history of using
microbes as the main part of municipal wastewater treat-
ment. Essentially, plants have been and are being used to
provide cleaner water.

11.4.1 Natural and Constructed Wetlands

Wetlands are more complex than simply shallow land areas
that contain water and plants most of the year. To be defined
by water managers as possessing characteristics of a true
wetland, the land has to be inundated with water a specific
amount of time each year. But how deep does the water have
to be? Does it have to be flowing or stagnant? Is the water
source groundwater or surface water? What types of herba-
ceous and woody plants are to be found? The answers vary,
but the point is that most wetland systems provide a unique
physical location where groundwater is essentially exposed
at land surface and provides the opportunity to evaluate the
interaction between plants and water. Moreover, these phe-
nomena can be used to understand how phreatophytes