Site Assessment and Characterization - Introduction to Phytoremediation of Contaminated Groundwater - page 138

than 97% of the biomass on earth (Campbell et al. 2006). In
fact, observations by researchers of plant growth at
contaminated sites, such as made by Cunningham and Ow
(1996), Fletcher (1991), and Chaney et al. (1997) indicated
the possibility of using plants to restore contaminated sites—
the genesis of phytoremediation. Two central questions aris-
ing from these observations were “What is the source(s) of
water to the plants?” and “How do plants survive in a
contaminated area?”
At a minimum, plant growth at sites where subsurface and
groundwater contamination have been detected is a general
indication that the necessary requirements to support plant
growth are present. Growth indicates that plants are able to
find sufficient amounts of nutrients, micronutrients, and sup-
port to withstand erosion or toppling by winds. It also
indicates that water is available from precipitation, soil
moisture, or even groundwater.
Much useful, preliminary information relative to the
establishment of phytoremediation can be gained by an
inspection of the plants at contaminated sites during a site
visit. Previously existing site-assessment reports not focused
on phytoremediation often indicate the presence of plants on
a site map but in generic terms, such as “wooded area”. A
site visit would enable defining the distribution and types of
such mapped vegetation, such as deciduous or conifer, at the
site. This observation is important in the initial stage of a
phytoremediation evaluation, to help in answering the
questions “Is the site conducive to support plant growth?”
and “Will extensive soil amendments be required?” and
“Can existing vegetation be used as part of the overall
phytoremediation strategy?” During a site visit, it is impor-
tant to realize that some plants may not be indigenous to the
area. For example, a site undergoing construction or revital-
ization may have had the native plants removed and replaced
with landscape plants.

table. Plants in these locations use predominantly either
surface-water runoff or groundwater. Conversely, in upland
areas, the depth to water table typically is greater, and the
plants typically rely solely on precipitation and soil mois-
ture. Between such extremes, plants can rely on water from
all sources, including groundwater.

6.4.2 Plant-Nutrient Availability

Plants at a contaminated site indicate not only water avail-
ability but the presence of other environmental factors nec-
essary for plant establishment, growth, and survival. These
factors include the availability of essential and trace
nutrients, the status of plant health with respect to infestation
of insects or other pathogenic organisms, and perhaps the
presence or absence of subsurface contamination.
A few simple methods can be used to assess this plant-
nutrient availability at a contaminated site as part of a site
visit. Soil samples can be collected from the surface by using
a variety of methods, and the samples can be analyzed for
geochemical properties important to plant growth. This
includes assessment of the concentrations of nitrogen, phos-
phorus, and potassium, and percent organic content. A geo-
logic classification, such as particle-size analysis, can
provide information about the relative grain-size distribution
of the soils and the percentages of sands, silts, and clays
which are directly linked to water movement and bioavail-
ability as described in Chap. 4. In addition, samples can be
analyzed for cation-exchange capacity, pH, and alkalinity.
The important concept here is that some of these soil
characteristics that affect plants can be changed by the
addition of appropriate amendments. This is in contrast to
the many site-specific variables that cannot be changed, such
as precipitation amounts, solar radiation intensity, and local
weather patterns.
The geochemical condition of groundwater also can be
assessed during a site visit to determine if plants installed for
phytoremediation can be sustained on the ambient water
quality. The salinity of the groundwater, for example, as
defined by measurement of total dissolved solids, needs to
be assessed, because levels that are too high may not warrant
the installation of even salt-tolerant species. The dissolved-
oxygen (DO) content of groundwater also can be assessed,
because roots require oxygen to support cellular respiration,
and lack of oxygen in groundwater can often lead to death of
the entire plant. The DO content generally is higher in
moisture in the unsaturated zone relative to groundwater
because the exchange of soil air with the atmosphere occurs
at a more rapid rate than the interaction of the groundwater
and the atmosphere through precipitation events.
A release of reduced organic matter to groundwater,
however, will result

6.4.1 Potential Water Sources and Plant
Distribution

A visit to a contaminated site provides a general view of the
plant and water relations. Healthy plants indicate that water
must not be a limiting factor. This indication is at least valid
for the type of plants present under the prevailing weather
conditions and conditions of the subsurface. The issue of
natural water availability is important, because even if native
vegetation is not part of a future phytoremediation design, its
presence can indicate whether a supplemental water supply
is needed.
Close inspection of the types of plants encountered during
a site visit can provide an idea of the source of water being
used by the plants. At one extreme, the presence of swamps
or wetlands near the site indicates shallow depth to the water in lowered concentrations of DO.