Phytoremediation of Volatile Organic Compounds in ... - CLU-IN

Discussion
Phytoremediation of Volatile Organic Compounds in Groundwater
The purpose of this report was to assess phytoremediation performance in terms of hydraulic
plume control. It was not possible to collect all data on all relevant projects in the three months
taken to compile this report; however the conclusions below can safely be made.
There is a need for more published information on system performance and lessons learned from
phytoremediation sites, especially those that have been in progress for more than four years. Due
to the inaccessibility of comprehensive data (site hydrogeology, topography, etc.) from the
majority of these sites, it was not possible to compare results between sites in much detail in this
report.
Historical groundwater contaminant concentration data, necessary to assess the degree of
hydraulic control achieved at each site, was a key piece of information lacking at several sites.
Instead, many reports emphasized contaminant mass removal, which is a function of the volume
of groundwater taken up by the trees. Because the goal of plume capture is the elimination of
contaminants in downgradient groundwater, contaminant mass removal estimates should always
be accompanied by groundwater contaminant concentration measurements.
Objectives
While complete plume capture via phytoremediation is theoretically possible, the results to date
show that it is not likely. At Carswell Naval Air Station, for example, the estimated decrease in
volumetric flux of groundwater at maximum transpiration rates for about 660 cottonwoods was
only 20 to 30 percent. This result suggests that reduced transport of contaminants may be a more
realistic goal, especially for areas that are inaccessible to the public.
Although the other case studies presented in this paper are between four and six years old, it was
difficult to identify specific contaminant reduction trends among varying compounds and their
concentrations up and down-gradient of the tree stands. At the Edward Sears Property site, for
example, researchers pointed out a general decrease in TCE and trimethylbenzene, while the
levels of PCE and xylenes remain inexplicably high. At the Kauffman & Minteer site, the
contaminant levels in one-third of the groundwater wells actually increased.
Technology Evaluation
There are still many questions concerning phytoremediation mechanisms, such as rhizodegradation,
hydraulic influence, and the volatilization of contaminants. For example, extensive
research on rhizodegradation was conducted at the Carswell site. However, several key issues still
remain unresolved. It is still not clear whether complete degradation can be achieved via
rhizodegradation, and exactly how the mechanisms involved are enhanced by the presence of
plants. Sites other than Carswell did not seem to be studying rhizodegradation in great detail. Due
to the site-specificity of phytoremediation, rhizodegradation should be explored at several sites to
determine its potential.
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