Plant Selection, Installation, and Management to Affect Groundwater - Introduction to Phytoremediation of Contaminated Groundwater - page 174

efficiency of plant growth and interaction with groundwater
is important. There are many things that can be done to
ensure that the plant growth at a particular site reaches its
maximum in the shortest time to ensure interaction with
groundwater.
Many environmental factors determine whether or not a
plant simply survives or if it will thrive. The environmental
factors that limit plant growth need to be identified and the
ones that can be controlled removed as a limiting factor.
Common environmental factors include the amount of light
available; the physical and chemical composition of the soil,
such as salinity and pH; the water availability over time,
such as precipitation, soil moisture, surface or groundwater;
and the geochemical composition of the soil moisture or
groundwater. It also must be recognized that the notion, if
a little works, more will be better, does not necessarily hold
true with plants, such as nitrate application, because nitrate
levels are regulated in groundwater. At a minimum, this
approach will lead to wasted resources in terms of capital
and potential environmental liability. Not all limiting factors
can be modified, however. Perhaps the most obvious factor
is light. Also, the genetic capability of plants, for example,
cannot be affected other than during initial plant selection.

along the Bill Williams and lower Colorado Rivers in
Arizona and analyzed the stable H and O composition of
each source. Tissues from trees growing in the flood plain
that had access to surface water, soil water, groundwater,
and precipitation also were sampled for these stable isotopes
and compared to the stable isotope composition of the vari-
ous sources. The types of trees sampled included Populus
fremontii , Salix gooddingii , and Tamarix ramosissima .
Busch et al. (1992) also sampled groundwater in the flood
plain, and adjacent trees were cored with an incremental
borer to obtain xylem samples for stable isotope analysis.
Branches and leaves also were collected at the time of core
collection from the sampled trees. Soil samples also were
collected near the trees.
For the Bill Williams River, the stable isotopes of H
measured in tree cores were more similar to the stable
isotope composition of groundwater than to soil water. One
of the observations made by Busch et al. (1992) was that
there is little difference between stable isotopes run on a
branch sample relative to that of core material collected with
a borer. This indicates that it would be more beneficial to the
trees being investigated to remove branch samples compared
to core samples, and the application of this approach
to understanding groundwater contaminant behavior is
discussed in Chap. 15. The similarity of results between
branch and core samples should be confirmed in the field
for each site, however.

7.3.1 Groundwater-Soil-Plant-Atmosphere
Continuum

As discussed in Chap. 4, water seeks its own level: water at
higher elevations will flow toward lower elevations. The
elevation of groundwater, or head, has a pressure equal to
or greater than the atmosphere. This explains why ground-
water will seep into dug holes or wells (Holzer 2010)
because pressures are greater than 1 atm and, therefore,
greater than the exposed hole in the ground. Water flow
stops when the pressures reach equilibrium. In the unsatu-
rated zone and capillary fringe where water is present under
tension, however, movement is determined by negative pres-
sure gradients, or water (matric) potentials. These concepts
typically have been discussed separately, especially in terms
of the water source to plants—for example, even in this
topic, the information contained in Chaps. 3 and 4 is
separated. For the purposes here, it is more useful to inte-
grate these ideas into one theme of a continuum of
groundwater-soil-plant-atmosphere. This is because it is
important from a phytoremediation perspective to under-
stand the source of the water used by a plant.
For example, differences in the relative uptake of water
from various sources along flood plains in the southwestern
United States, such as surface water, soil water, and ground-
water, was investigated by Busch et al. (1992). They sam-
pled water from all potential sources to flood plain trees

7.3.2 Soil Physical Composition

In general terms, soil is the byproduct of rock weathering.
This production of soil can be caused by abiotic processes,
such as infiltrating or running water, freeze-and-thaw cycles,
earthquakes, or by biological processes, such as microbial-
nutrient acquisition, lichen, and root penetration. These pro-
cesses make the minerals and elements that are contained in
the original rock more bioavailable. Once mobilized by
interaction with water, these nutrients are used by plants
growing in the soil. It is the porosity of weathered rock and
soil that aids in plant establishment by providing stability
and a reservoir to hold water. Soil also is used by plants to
keep the roots protected from the sun's radiant energy and to
keep them from drying out.
Soil formation can take many hundreds of thousands of
years. As plant succession occurs, lichens are replaced by
vascular plants, and the thickness of organic matter on top of
the weathered rock increases. Because of this process, soils
typically are discussed in terms of profiles, or the
characteristics of each vertical layer from the surface down-
ward. The uppermost layer, called the O layer, or horizon, is
high in percent organic matter, such as humus and detritus of