Phytoremediation--U.S. Air Force
Current
Status of Technology
Phytoremediation refers to the use of plants and plant
systems to remove, transfer, destroy or stabilize contaminants in soils,
sediments and water contaminated with metals and/or organic chemicals. Phytoremediation
has been attempted on a full- or demonstration-scale basis at more than 200
sites nationwide, however discovery of its effectiveness and advances in its
application as an innovative treatment technology at waste sites, including
brownfield sites has been recent.
Description
of Technology
Plants
remove, transfer, destroy or stabilize contaminants in the soil surrounding
plant roots by a variety of mechanisms.
These mechanisms include biodegradation, phyto-accumulation,
phyto-degradation and phyto-stabilization.
Biodegradation by microorganisms in the rhizosphere immediately
surrounding the roots is enhanced by the exudates supplied by the plant
roots. These root exudates, which
contain nutrients, metabolites, and enzymes, contribute to the stimulation of
microbial activity.
Phyto-accumulation refers to the uptake of the contaminants
directly by the plant roots and the transfer of the contaminants to the plant
shoots and leaves. Phyto-accumulation
only involves the relocation of the contaminants in the plant rather than the
degradation of the contaminant.
Phyto-degradation is the direct degradation of the contaminant by the
plant enzymes while phyto-stabilization is the immobilization of the contaminants
in the soil by introduction of chemicals to the soil by the plant roots. The specific mechanisms by which
phytoremediation occurs will be dependant upon the contaminant, plant and site
characteristics. Successful application
of phytoremediation involves characterizing the site and determining the proper
planting strategy to maximize the interception and degradation of organic
contaminants. Monitoring ensures that
the planting strategy is proceeding as planned. Various plants can be utilized based on the depth of the
contamination, the specific contaminant and the specific plant characteristics
such as root depth, water use strategy, and growth rates as well as other
characteristics.
Soil
phytoremediation is best suited for surface soils contaminated with
intermediate levels of organic contaminants, that are relatively nonleachable,
within the reach of plant roots and non toxic to the plants. After the treatment species has been
selected, planted and soil nutrients are added, the plots are intensively
cultivated. Plant shoots are cut during
the growing season to maintain vegetative rather than reproductive growth. Several growing seasons may be required to
meet the site’s remediation goals.
Site
and Contaminants Description
The
U.S. Air Force demonstrated the use of eastern cottonwood trees in remediating
shallow groundwater contaminated with TCE.
The TCE plume lay in a shallow, alluvial aquifer approximately 6 to 11
feet below grade. The eastern cottonwood
trees are expected to act as a “natural pump and treat” system. The groundwater will be remediated by the
trees through one of the following mechanisms:
·
Release
of root exudates and enzymes stimulating microbial activity in the rhizosphere
and enhancing biochemical transformation of contaminants,
·
Metabolism
or mineralization of contaminants within the vegetative tissues: the
contaminated water enters the vegetative tissues by root uptake from the
aquifer,
·
Transpiration
of water by the leaves.
The
overall effectiveness of the system was monitored by examining the TCE
concentrations in the groundwater, in soil from the rhizosphere and in tree
tissues. In addition, microbial
activity was monitored in the rhizosphere, transpiration rates were measured
and concentrations of TCE daughter products were measured.
The
USAF demonstrated this technology on a shallow TCE plume near Air Force Plant 4
at the Naval Air Station Ft. Worth, formerly Carswell Air Force Base in Fort
Worth, Texas. Approximately 660 Eastern
cottonwoods (Populus deltoids) were
planted on 1 acre at the Air Force Base in an effort to contain and remediate a
plume of dissolved TCE located in a shallow alluvial aquifer 6 to 11 feet below
the ground surface.
Two
sizes of trees were planted at the base, whips and five gallon buckets. The whips which were half an inch in
thickness and 18 inches long at planting were planted so that only 2 inches
remained above ground while the five gallon bucket trees were an inch in
diameter and seven feet tall at planting.
The larger trees were expected to have higher evapotranspiration rates
since they were estimated to have about twice as much leaf mass as the
whips. Two plots, one containing the
whips and one the larger trees were planted perpendicular to the ground water
flow direction. The whips section was
placed “upstream” in terms of the groundwater flow with respect to the larger
trees, so that the contaminated plume moved first through the whip plot and
then the larger tree plot. The site
already contained a mature, 70 foot tall cotton wood tree, which upon sampling
of the groundwater in its vicinity appeared to be degrading TCE. This observation is speculative and is based
simply on the levels of TCE in the vicinity of the tree versus the levels
outside the zone of influence of the roots.
Figure
1 below shows a photo of a hybrid poplar field used for phytoremediation of TCE
at Edward Sears Property
Performance
of Technology and DRE
Field Performance Data
Unpublished
monitoring data during the first 3 years after planting indicate that TCE
concentrations remained nearly constant due to a continuous influx of
contaminated ground water. However, the
mass flux of TCE out of the site decreased by 11 percent due to the
transpiration of contaminated water directly from the aquifer. Therefore there was an overall decrease in
the amount of contaminated groundwater flowing from the site. By the end of the fifth growing season, in
situ biodegradation of the TCE had occurred locally beneath and immediately
down gradient of the trees. Full-scale
implementation of phytoremediation at the site with planting of additional
trees is anticipated by the USAF during 2003.
Treatment Effectiveness
Research
has shown the plants can potentially treat a wide variety of contaminants or
families of contaminants such as organics and metals.
System Reliability
The
reliability of any phytoremedation system will be variable based on a variety
of environmental factors.
Ease of Operation
Periodic
maintenance of the site is required to maintain the health of the plants used
in the phytoremediation system.
Energy Consumption
There is no energy
consumed with this technology except that used for site monitoring.
Space Requirements
Phytoremediation
systems generally require a very large space when compared to alternative
technologies.
Worker Health and Safety Issues
Since
workers do not come into contact with the contaminated soil there is little
chance for workers to be come exposed to the hazardous contaminants. However evapotranspiration and
volatilization of contaminants into the air surrounding the plants provides a
potential inhalation route for workers.
Risk and Consequence of Catastrophic
Failure
Not
applicable.
Noise/Aesthetics, etc.
Phytoremediation
often has a “positive” effect on the aesthetic character of a site. There is no noise associated with this
remediation alternative.
Capital
and Operating Costs
General
costs of phytoremediation are limited to the initial site preparation, planting
and occasional maintenance activities such as irrigation. Innovative treatment technologies will
typically have elevated costs due to the significant monitoring take place at
the site to evaluate the remediation effectiveness. An additional $200,000 in extensive site monitoring was not
included in the final cost analysis at this site. Table I below presents the costs associated with this
demonstration.
|
Table
I. Costs associated with the Carswell Air Force Base phytoremediation
demonstration |
|
|
Activity |
Estimated
Cost |
|
Wholesale
cost of trees (not including delivery or installation) |
$8/tree
for five gallon bucket tree $0.20/tree
for whips |
|
29
wells (including surveying, drilling and testing) |
$200,000 |
|
Subsurface
fine biomass |
$60,000 |
|
Source:
Chappell, J., Photoremediation of TCE using Populus, 1997, http://www.clu-in.org/download/remed/phytotce.pdf |
|
Vendor
EPA
Project Manager:
Steven
Rock
U.S.
EPA
National
Risk Management Research Laboratory
26
West Martin Luther King Drive
Cincinnati,
OH 45268
513-569-7105
Air
Force Project Manager:
Gregory
Harvey
U.S.
Air Force
Mail
Stop ASC-EMR
1801
10th Street, Building 8, Suite 200
Area
B
Wright
Patterson Air Force Base, OH 45433
513-255-7716,
ext. 302
Fax:
513-255-4155
SITE,
1999, U.S. Air Force (Phytoremediation of TCE-Contaminated Shallow
Groundwater), SITE Technology Profile, Demonstration Program, EPA/540/R-97/502,
February.
EPA,
2001, Brownfields Technology Primer: Selecting and Using Phytoremediation for
Site Cleanup, Environmental Protection Agency, Office of Solid Waste and
Emergency Response, EPA/542/R-01/006, July.
HSIA, 2002, Update: Halogenated Sovelnts Industry Alliance, Inc., May/June, http://www.hsia.org/