Environmental Engineering Reference
In-Depth Information
compounds, the loss of contaminant mass over time caused
by phytoremediation can be monitored using various passive
methods that trap the contaminant vapors, or soil gas, for
subsequent analysis. One simple soil-gas method is a passive
soil-gas
aggregates, accomplish the same goal. These tests also can be
used to understand chemical toxicity and plants.
15.4.2 Laboratory Approaches
sampler
that
consists of various
adsorbents
(W.L. Gore
and Associates, Inc.). The sampler permits soil-gas vapors
to enter but excludes water and other liquids. The samplers
can be installed in the soil or in a well and are retrieved for
subsequent analysis.
This method was used at a phytoremediation site installed
near Elizabeth City, North Carolina; the installation of this
site is discussed in Chap. 7 and additional information is
contained in Cook et al. (2010). Soil-gas samplers were
installed in a grid pattern on 100-ft-centers in 2006 prior to
plant installation. Since plant installation, soil-gas samplers
are installed and retrieved once each in the winter and
summer. To date, the soil-gas sampling indicates that soil-
gas masses of TPH, BTEX, and naphthalene have decreased
following plant installation (Shaw et al. 2010).
surrounded by a thin tube of GORE-TEX
®
Adam and Duncan (1999) investigated the effect of diesel
fuel hydrocarbons on the growth of plants, considering that
the level of diesel contaminants could be toxic to plants,
especially if introduced to a site where seedlings are present.
These authors reported that, for a wide range of grasses
useful at sites for cleanup, the germination rate of seeds
was inhibited at diesel fuel concentrations near 50 g/kg.
If seedlings are to be used, they will most likely be for
grasses, as most deciduous trees installed at phyto-
remediation sites will be installed as cuttings or whips.
Although the authors observed that the germinated plants
had roots completely around diesel-contaminated sediments
if uncontaminated sediment also was available, the roots
would grow through diesel-contaminated sediment if no
clean soil was present. Whether or not this colonization of
diesel-contaminated soil was the result of rhizospheric
microbes is unclear.
15.4
Toxicity Testing
Phytoremediation involves putting plants in contaminated
environments in an effort to restore these areas to preconta-
mination conditions. Because many of the contaminants are
carcinogenic, there are concerns that these chemicals will
impart toxic effects on the plants. There is a relation between
the type and concentration of chemicals and degree of plant
toxicity—an extreme example is the use of herbicides to
specifically induce death in plants. Some of the various
tests and their usefulness to understand chemical toxicity
affects on plants are discussed in Chap. 13.
15.5
Summary
At this time, it is not sufficient to simply plant a
phytoremediation system and then walk away. Most remedi-
ation efforts, including phytoremediation, require monitor-
ing of the groundwater or remediation system to verify that,
indeed, remediation is occurring and to document its perfor-
mance over time such that human health and the environ-
ment are protected.
Why is this information important to the
phytoremediation of contaminated groundwater?
The
basic approaches outlined in this chapter can be used to
meet this need for the long-term evaluation of
phytoremediation. The use of both plant- and hydrology-
based approaches leads to a decrease in the uncertainty
inherent to each method and provides a higher degree of
confidence that phytoremediation is helping to achieve
remedial goals at a site.
15.4.1 Axenic and Nodule Analogs
Axenic cultures are sterilized cell cultures that do not contain
bacteria. Axenic cultures are the conventional way to observe
the function of plant cells without the interference of bacterial
cells. Nodule cell cultures, or spherical photosynthetic cell
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