542R05001
Evaluation of
Phytoremediation for
Management of Chlorinated
Solvents in Soil and
Groundwater
Prepared by:
The Remediation Technologies Development Forum
Phytoremediation of Organics Action Team,
Chlorinated Solvents Workgroup
RTDF
SJH-XS
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Evaluation of Phytoremediation for
Management of Chlorinated Solvents
In Soil and Groundwater
Prepared by:
The Remediation Technologies Development Forum
Phytoremediation of Organics Action Team,
Chlorinated Solvents Workgroup
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Table of Contents
SECTION 1.0 INTRODUCTION.
.1
SECTION 2.0 BACKGROUND 2
2.1 Phytoextraction 2
2.2 Phytovolatilization 3
2.3 Rhizosphere Degradation 4
2.4 Phytodegradation 4
2.5 Hydraulic Control 5
2.6 Summary 6
SECTION 3.0 ASSESSMENT OF APPLICABILITY OF PHYTOREDIATION 7
3.1 Site Conditions 7
3.1.1 Site Layout 7
3.1.2Hydrogeologic Setting 9
3.1.3 Groundwater Capture and Water Balance Modeling 9
3.1.4 Meteorological Monitoring 1°
3.1.5 CVOC Distribution 10
3.1.6 Agronomic Evaluation 1°
3.2 Phytotoxicity J1
3.3 Regulatory Considerations 12
SECTION 4.0 FIELD PILOT TESTS 14
4.1 Plant Selection 14
4.2 Planting Techniques 14
4.3 Fertilization 15
4.4 Soil Amendments ^
4.5 Maintenance Plan 16
SECTION 5.0 MONITORING AND SAMPLING 17
5.1 Soil Sampling 17
5.2 Groundwater Monitoring and Sampling 17
5.3 Plant Monitoring 18
5.4 Plant Sampling 19
5.5 Air Monitoring and Sampling • 20
SECTION 6.0 REPORTING COST AND PERFORMANCE 23
REFERENCES 25
APPENDIX FREQUENTLY ASKED QUESTIONS 32
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Foreword
The Remediation Technologies Development Forum (RTDF) was established in 1992 as a forum
for government, industry, and academia to collaborate on the development of cost-effective
hazardous waste characterization and treatment technologies. The RTDF is a one of a few
government programs designed to foster public-private partnerships for conducting laboratory
and field research to develop, test, and evaluate innovative remediation technologies Through
the unprecedented collaboration of the RTDF, companies, government agencies, and universities
are voluntarily sharing the knowledge, experience, equipment, facilities, and even proprietary
technology to solve mutual remediation problems.
The Phytoremediation of Organics Action Team was established in 1997, as one of a number of
RTDF Action Teams to further the RTDF's goals. The team formed specifically to address the
development and demonstration of phytoremediation technologies.
The RTDF's website can be accessed at http://www.rtdf.org. The Phytoremediation of Organics
Action Team's webpage is at http://www.rtdf.org/public/phvto/default.htm,
Disclaimer
This information represents the views of the participants and has not been subjected to EPA peer
review. Therefore, it does not necessarily reflect the views of the EPA, and no official
endorsement should be inferred. This document is not an U.S. EPA policy, guidance or
regulation. It does not create or impose any legally binding requirements or establish U.S. EPA
policy or guidance. The information is not intended, nor can it be relied upon, to create any
rights enforceable by any party in litigation with the United States or any other party. The
information provided maybe revised periodically without public notice. Use or mention of trade
names does not constitute endorsement or recommendation for use. Standards of Ethical
Conduct do not permit EPA to endorse any private sector product or service.
Acknowledgements
The RTDF Phytoremediation of Organics, CVOCs Workgroup would like to thank those
individuals and organizations who have contributed time, thought and effort into creating this
protocol. Without their efforts, the protocol would not have come to fruition. They include:
Co-Chairs
Robert Tossell, CH2M HILL
Lee Newman, University of South Carolina;
Keith Rose, U.S. Environmental Protection Agency (EPA)
Authors and Reviewers
Frank Beck, U.S. EPA Xiujin Qiu, The Dow Chemical Company
Joel Burken, University of Missouri-Rolla Steve McCutcheon, U.S. EPA
Harry Compton, U.S. EPA Christina Negri, Argonne National Laboratory
Larry Erickson, Kansas State University Valentine Nzengung, University of Georgia
Linda Fiedler, U.S. EPA Steve Rock, U.S. EPA
Milt Gordon, University of Washington Ellen Rubin, U.S. EPA
Greg Harvey, USGS Mike Witt, The Dow Chemical Company
Jim Jordahl, CH2M HILL
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SECTION 1.0 INTRODUCTION _
This document is intended to aid regulators, site owners, consultants, neighbors, and other
stakeholders in understanding the proper application of planted systems to remediate
groundwater contaminated with halogenated solvents. It assumes a familiarity with
environmental and regulatory processes, in general, but little knowledge of plant-based, or
"phytoremediation," technologies. The document is not intended as regulatory guidance, but as
an aid to understanding of the mechanisms of how plants detoxify certain compounds under
certain conditions.
Each field application of a phytotechnology has a unique combination of soil, contaminants, and
climate Therefore, each phytotechnology project must be designed, approved, and installed with
site-specific conditions in mind. This document is intended to create enough understanding of the
science, process, and engineering of phytoremediation systems that site-specific design and
regulation can follow.
Specifically this document is designed to:
• Briefly introduce phytotechnologies;
• Identify potential applications of phytoremediation to control, transform, or manage
chlorinated volatile organic compounds (CVOCs) in soil and groundwater;
• Show how to conduct a preliminary assessment to determine if a particular site is a good
candidate for phytoremediation; and
• Describe monitoring options and show how to assess the effectiveness of
phytoremediation at full-scale field implementation.
The remainder of this document is organized as follows:
Section 2.0 Background: Summarizes the mechanisms of phytoremediation and fate and
transport of CVOCs;
Section 3 0 Assessment of Applicability of Phytoremediation: Presents general methods for
assessing phytoremediation as a remedial technology for CVOCs in soil, surface water, and
groundwater;
Section 4.0 Design and Placement: Explains the design and placement information of pilot- and
full-scale projects;
Section 5.0 Monitoring and Sampling: Presents methods for monitoring, sampling, and
analyzing full-scale phytoremediation;
Section 6.0 Reporting Cost and Performance: Summarizes how to evaluate the cost and
performance of phytoremediating CVOCs;
References: Lists references used in the preparation of this document and helpful web resources
for obtaining further information; and
Appendix: Provides responses to frequently asked questions about phytoremediation.
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SECTION 2.0 BACKGROUND
Phytoremediation is an emerging green technology that uses plants to remove, degrade or
contain toxic chemicals in soils, sediments, groundwater, surface water, and air. It can be used as
a stand-alone remediation alternative or as part of a broader site management alternative
comprising a number of remediation technologies. Plants have grown naturally at contaminated
waste sites and have been planted for aesthetic value or land stabilization. But not until recently
has the use of plants gained attention as a viable remedial technology for site contaminants.
Currently, phytoremediation is used for treating many classes of contaminants, including
petroleum hydrocarbons, pesticides, explosives, heavy metals, and radionuclides, as well as
CVOCs (McCutcheon and Schnoor, 2003).
Phytoremediation of organic contaminants primarily occurs by one or more of the following five
mechanisms:
• Phytoextraction: the uptake and translocation of dissolved-phase contaminants from
groundwater into plant tissue;
• Phytovolatilization: the transfer of the contaminant to air via plant transpiration;
• Rhizosphere degradation: the breakdown of organic contaminants within the microbe-
rich rhizosphere (soil surrounding the root);
• Phytodegradation: the breakdown of organic contaminants within plant tissue.
• Hydraulic control: the use of trees to intercept and transpire large quantities of
groundwater or surface water in order to contain or control the migration of contaminants.
One of the most important yet least understood topics regarding phytoremediation mechanisms is
the fate and transport of contaminants within vegetation and its rhizosphere. Many experiments
evaluating the fate and transport of environmental contaminants in vegetation and the
rhizosphere are conducted in a laboratory or greenhouse. Due to the use of artificial sunlight,
artificial air, and immature vegetation, such experiments are conducted under very different'
conditions than encountered in the field. Therefore, while laboratory experiments can provide
useful data, their results cannot always be replicated in field settings. Other variables yielding
different laboratory and field results can include seasons, field settings, vegetation types,
growing methods, exposure methods and times, and contaminant concentrations.
The early results of using plants to mitigate the risk of CVOC-contaminated soils and
groundwater look promising. However, there is still much work to be done regarding the
mechanisms of CVOCs phytoremediation. The following sections further look at the current
understanding of the mechanisms of phytoremediation and their effect on the fate and transport
of CVOCs.
2.1 Phytoextraction
Phytoextraction is the uptake and translocation of contaminants from groundwater into plant
tissue as the plant takes in water and micronutrients from soil through its root system. Plant
uptake of chlorinated solvents is influenced by many factors including soil pH, clay content,
water content, and organic matter content, as well as the properties of the chlorinated solvent
(Ryan et al., 1988). Briggs et al. (1982) quantified plant uptake of a chemical by its octanol-
water coefficient (K<,w), a measure of the chemical's hydrophobicity. Burken and Schnoor (1998)
developed a new relationship-also based on K<,w- for organic contaminants and hybrid poplar
trees, that demonstrates that trichloroethene (TCE) is readily taken up by hybrid poplar trees.
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This relationship, called the "transpiration stream concentration factor" (or TSCF), which
represents the translocation of groundwater contaminants to the Pl"^™^0^ f^JL
from 0 02 to 0 75 for TCE (Burken and Schnoor 1998; Davis et al. 1999; Orchard et al., 2000,
and Ma and Burken 2002). The TSCF studies were based on hybrid poplar cuttings used in
hydroponic experiments. The wide range of measured TSCF values may be due to several
variables such as the initial contaminant concentration, type of vegetation, measurement
techniques, and experimental design. The range in values also suggests that there may be other
mechanisms in the uptake and transport of CVOCs within plants. Analysis of plant tissue for
chlorinated solvents and their degradation products is an important step in determining fate and
transport of these chemicals in phytoremediation systems.
CVOCs and their degradation products are usually found in vegetation in contact.with soil or
groundwater contaminated with CVOCs. It appears that the concentration of CVOCs in the plant
tissue is proportional to the level of exposure, although field data are still being collected. Uptake
from the atmosphere, via passive binding, should also be considered as plant matter makes up the
majority of both surfaces and organic mass in the atmosphere. It is important to factor in plant
uptake of airborne solvents as "background" in determining uptake from soil and roots.
2.2 Phvtovolatilization . . „
Phvtovolatilization is the transfer of a contaminant to air via plant transpiration. Plants normally
transpire water as vapor, but volatile compounds can be transpired as well. Phytovolatilization
occurs via diffusion from the tree's xylem (a tissue that begins at the root of the tree and
continues through the tree to the upper side of the leaf (Kozlowski and Pallardy, 1997)) through
its bark or leaves.
Much more research has been conducted on phytovolatilization of contaminants from tree leaves
than on the newer concept of volatilization from tree bark. Early hydropomc laboratory
experiments involved enclosing the entire subaerial portion of a tree. The TCE measured in the
enclosure was presumably transpired through the leaves (Newman et al., 1997 Burken and
Schnoor 1999 Davis et al., 1998), although this was not confirmed in the field (Newman et al.,
1999 and Compton et al., 1998). Furthermore, the reported evidence that TCE is taken up and
volatilized from the tree, and biodegradation did not play a significant role in TCE reduction.
Orchard et al. (2000), who conducted hybrid poplar uptake studies with TCE, detected
phytovolatilization in only 12 of the 96 sampling events.
Vroblesky et al. (1999) analyzed tree core samples with the intent of correlating the samples to
groundwater plumes. The analysis revealed that TCE concentrations dropped considerably with
increasing trunk height. However, the mechanism causing the drop was unexplained. Some of
the possible mechanisms include volatilization through the bark of the tree and degradation
within the tree. Degradation, however, has been shown to be rather low, and no significant
accumulation of metabolites have been detected that could explain the losses observed.
Laboratory research later showed that diffusion from the trunk tissues was indeed a major
mechanism for removing CVOCs from the plant following uptake (Ma and Burken, 2003at and
Hu et al 1998) Ma and Burken showed that diffusion from the xylem was linearly related to the
concentration of aqueous solution in hydroponics studies. They hypothesized that the difference
in experimental arrangements could explain the variable volatilization from leaves and
subsequent TSCF calculations (Burken and Schnoor, 1998; Davis et al., 1998; and Orchard et al.,
2000).
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In other laboratory experiments, partitioning coefficients were determined in order to estimate
onnoT ™° concentration in transpiration stream within the xylem (Ma and Burken
2002). The measurement of aqueous concentrations in the xylem also exhibited a concentration
gradient between the interior xylem tissue, the outer tissue, and the atmosphere, providing direct
theoretical support for diffusion to the atmosphere (Ma and Burken, 2002 and 2003a).
Volatilization of CVOCs from plant tissues to the atmosphere is a major pathway for CVOCs in
phytoremediation applications. Although transpiration of chlorinated solvents has been
confirmed in studies, researchers predict that transpiration from vegetation will not result in
unacceptable levels of airborne CVOCs in the surrounding area (Davis et al., 1998; Narayanan et
al., 1999, and McCutcheon and Schnoor, 2003). This hypothesis is supported by earlier studies
that could not detect VOCs in the middle of the phytoremediation test plots. Furthermore
calculations show that during the slightest of wind velocities, the flux of VOCs to the atmosphere
from a phytoremediation application leads to trivial concentrations in the atmosphere.
2.3 Rhizosphere Degradation
Rhizosphere degradation is the breakdown of organic contaminants within the rhizosphere- a
zone of increased microbial activity and biomass at the root-soil interface. Plant roots secrete and
slough substances such as carbohydrates, enzymes, and amino acids that microbes can utilize as
a substrate. Contaminant degradation in the rhizosphere may also result from the additional
oxygen transferred from the root system into the soil causing enhanced aerobic mineralization of
orgamcs and stimulation of co-metabolic transformation of chemicals (Anderson et al., 1993).
The fate of TCE was investigated in laboratory settings (Walton and Anderson 1990) by
comparing degradation of TCE in both rhizosphere soil and non-vegetated soil'collected from a
TCE-contammated site. The results showed that TCE degrades faster in rhizosphere soils
Anderson and Walton (1995) also reported that TCE mineralization was greater in soil rooted
with the Chinese lespedeza, loblolly pine, and soybeans than in non-vegetated soil.
Additional research on CVOC fate in the rhizosphere has shown varying results Chlorinated
pesticides were shown to have enhanced degradation in the rhizosphere (Shann, 1995), and a loss
T ^ 1>1'1-trichloroethane (TCA) was observed in the rhizosphere of alfalfa (Narayanan
et al., 1995). Higher numbers of methanotrophic bacteria, which have been shown to degrade
TCE, were detected in rhizosphere soils and on roots of Lespedeza cuneata and Pinus taeda than
in unvegetated soils (Brigmon et al., 1999). Orchard et al. (2000) detected TCE metabolites in
the roots of hybrid poplar saplings suggesting rhizosphere degradation and concluded that the
greatest degradation of TCE occurred in the rhizosphere.
However, Newman et al., (1999) observed no degradation of TCE in the rhizosphere of hybrid
poplars. Similarly, Schnabel et al. (1997) observed no degradation of TCE in the rhizosphere of
edible garden plants.
Recently, studies have indicated that wetland vegetation and rhizosphere microbial communities
can effectively treat chlorinated compounds (Dhanker et al., 1999; Bankston et al 2002-
Nzengung et al., 1999; and Kassenga, 2003).
2.4 Phytodegradation
Phytodegradation is the breakdown of organic contaminants within plant tissue. Although data
are limited, it appears that both the plants and the associated microbial communities play a
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significant role in attenuating chlorinated compounds. Plants produce a large number of
enzymes, of which one or more may transform PCE and TCE into daughter products. Although
not completely understood, dehalogenase, cytochrome p-450, glutathione-S transferase, methane
mono-oxygenase, and monochloroacetic acid are all thought to play a role in chlorinated solvent
transformation. Intermediate stable metabolites of these chlorinated compounds include 2,2,2-
trichlorethanol, 2,2,2-trichloroacetic acid (TCAA) and 2,2-dichloroacetic acid (DCAA), and have
been reportedly found in hybrid poplar (Gordon, 1998; Newman et al., 1997; and Compton et al.,
1998), oak, castor bean, and saw palmetto (Doucette et al., 1998).
Some researchers believe that chlorinated solvents are being metabolized within vegetation;
however, the exact mechanism has not been determined yet. Bench-scale laboratory TCE uptake
tests with poplar cuttings grown in soil were reported to have measurable amounts of TCE
transpired to the air (Newman et al., 1997). A three-year study commencing with rooted poplar
cuttings in aperies of constructed, lined, artificial aquifers evaluated the fate and transport of
TCE in the poplar tree. The mature trees were able to remove 99% of the TCE from the
groundwater, and less than 9% of the TCE was transpired to the air in the first two years. After
two years, TCE was not detected in the air stream. Researchers believe that the mature hybrid
poplar tree was dechlorinating the TCE and inferred that degradation in the rhizosphere was not
contributing to the loss of TCE (Newman et al., 1999).
In addition, Gordon et al., (1998) detected TCE metabolites in hybrid poplar cutting experiments
and suggested that TCE is oxidized as it moves through the cutting. When grown hydroponically
in a laboratory, the tropical leguminous tree, Leuceana leucocephala, was shown to metabolize
TCE as indicated by the formation of one of its degradation products, trichloroethanol (Doty et
al., 2003).
An alternate theory about the fate of TCE in poplar trees is that TCE is taken up by suspension
cell cultures and is incorporated as a nonvolatile, nonextractable residue (Shang and Gordon,
2002). Another investigation of the fate and transport of TCE in carrot, spinach, and tomato
plants showed that TCE was taken up, transformed, and bound to plant tissue (Schnabel et al.,
1997). This binding, or "sorption" of organic compounds, has been linked to plant lipid content
and tissue chemistry. Mackay and Gschwend (2000) have studied the sorption of chemicals to
wood and developed wood-water partitioning equations. Partitioning onto wood was determined
to depend predominantly on the water-lignin partitioning of a compound. Lignin is the chief
noncarbonhydrate constituent of wood, which binds to cellulose fibers and strengthens the cell
walls. Lignin is hydrophobic and shows strong affinity to hydrophobic organic compounds.
Ma and Burken (2002) measured the wood-water partitioning coefficient values for CVOCs
binding to poplar tissues. The results ranged from 20.7-59.3 mL/g for the tested compounds
(tetrachloromethane, TCE, and 1,1,2,2-tetrachloroethane), which is in agreement with the range
of literature values. The fraction of lignin in poplar trees was assumed to be 20%. The lignin
content of hardwood is about 18-25%, and the content of softwood is a little higher at 25-30%
(Haygreen and Bowyer, 1982). A linear relationship was observed between the partitioning
coefficients and vapor pressure and Henry's law constants.
2.5 Hydraulic Control
A great deal of research has focused on the use of trees—poplar trees, in particular—to intercept
shallow groundwater plumes (Wang et al., 1999; Jones et al., 1999; Thomas and Krueger, 1999;
Tossell et al., 1998; Gordon, 1997; Newman et al., 1999; Compton et al., 1998; and Quinn et al.,
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2001). Most of these studies have shown that trees can extract large enough quantities of
groundwater to depress the water table, locally inducing flow toward the trees. This depression
can be sufficient to create a hydraulic barrier or hydraulic control. Hydraulic control mitigates
potential risks by controlling offsite transport of CVOCs and providing more opportunity for the
other four mechanisms of phytoremediation to remediate the CVOCs. Proper hydraulic control
involves the selection and planting of vegetation to intercept and transpire large quantities of
groundwater or surface water.
2.6 Summary
In summary, some researchers believe CVOCs are degraded in the rhizosphere, while others
believe that the CVOCs are taken up by plants and phytovolatilized through the leaves or bark.
Yet others believe that when CVOCs are taken up by the plant, they are either degraded within
the plant or sorbed to its tissues. All five mechanisms have been shown to occur, and research is
continuing to further understand how and under what conditions they occur. The varying
occurrence of each mechanism may be due to site or laboratory conditions, meteorological
conditions, measuring techniques, etc. Through a better understanding of the role of plants,
researchers, engineers, and site managers can better manage sites that have been impacted by a
broad range of CVOCs. Appropriate field tests and site conceptual models are needed as
discussed in the Section 3.
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SECTION 3.0 ASSESSMENT OF APPLICABILITY OF PHYTOREDIATION
Screening level assessments are vital to the final remedial selection and often result in a "go" or
"no go" decision for a given remedy. This section summarizes the factors to consider when
assessing the applicability of phytoremediation for a contaminated site. These factors include site
conditions, the phytotoxicity of the contaminants, and regulatory requirements. Figure 1 is an
adaptation of the Interstate Technology Regulatory Council's and the Center for Waste
Reduction Technologies' decision flow chart (developed in 1999), to help decide whether to use
phytoremediation.
3.1 Site Conditions
A screening level assessment of phytoremediation as a treatment technology and the ultimate
decision to use the technology is partly based on site-specific conditions (e.g., layout,
hydrogeologic setting, and distribution of contaminants). The initial assessment begins with a
fundamental understanding of site conditions in order to develop of a site conceptual model. The
site conceptual model is important since it will form the basis for evaluating the effectiveness of
phytoremediation for meeting all or part of the site management objectives. Developing a site
conceptual model will include the following steps:
• Review background site data (consistent with project objectives), including but not
limited to the project files, including historical documents, the Remedial
Investigation/Feasibility Study (RI/FS), draft record of decision (ROD), applicable or
relevant and appropriate requirements (ARARs), etc.;
• Develop a geologic cross section to help determine if phytoremediation can achieve
hydraulic capture or CVOC sequestering goals;
• Perform basic hydraulic modeling;
• PloUhe distribution of CVOCs in the subsurface in both plan view and in cross section;
• Identify and develop approaches to best achieve remedial action objectives for the site,
including information on cost and ARARs compliance;
• Review site risks and consider applicability of phytoremediation; and
• Prepare a site-specific phytoremediation conceptual design that meets the site
management remedial goals and objectives.
The goal of the site data review, as described below, is to develop a thorough understanding of
site conditions, hydrogeological conditions, groundwater capture, contaminant distribution,
agronomic conditions, and meteorological conditions.
3.1.1 Site Layout
A review of the site layout could include elements such as property boundary, surrounding
features, infrastructure, buried utilities, and other obstacles that would prohibit planting or would
have to be removed or altered for planting to occur. Also, the review should determine if there is
sufficient land available to plant the amount of vegetation required for a successful remediation
project.
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Review available
site data.
Has site been
characterized
adequately?
No
Conduct additional
site characterization.
Develop site conceptual
phytoremediation
model.
Questions to help with the
decision making!
Q] - Climate is favorable for selected plants?
Q2 - Plants can meet depth requirements?
Q3 -Results of water balance modeling is promising?
Q4 - Space or area is available?
Q5 - Time requirements are sufficient?
If the answer is yes to all questions, proceed with
phytoremediation evaluation.
// Does phytoremediation
v technologies?
Select alternative
technology.
Questions to help with the
decision making!
Q6 - Will COC degrade in root zone?
Q7 - Will COCs degrade in plants?
Q8 - Are hazardous or toxic intermediates produced?
Q9 - Can the phytotechnology design be implemented
easily?
Q10 -Does phytoremediationcompare well with other
technologies on effectiveness, cost and environmental
protection?
Qll -Are anywastes produced, and can they be
effectively disposed?
Integrate with
other strategies.
1
Preps
concep
design
regulat
docum
Yes
ire
tual
ory
snts.
Prepare the
final design.
Construct the
system.
Perform
long-term
operations,
maintenance,
and monitoring.
Assess the performance
of the system.
Is the phytoremediation
system achieving
the objectives?
No
Re-evaluate other strategies.
Adapted from CWRT 1999 and ITRC 1999.
Yes
Phytoremediation can be 1
effective at the site. |
Figure 1
Phytoremediation
Decision Making
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3.1.2 Hydrogeologic Setting
An understanding of the hydrogeologic setting, such as depth to the water table, he geologic
makeup and extent of aquifers and aquitards, groundwater recharge rate, flow velocity and
direction, soil porosity, hydraulic conductivity, and seasonal variations is essential.
3.1.3 Groundwater Capture and Water Balance Modeling
Computer modeling can be used to better understand and define the site water balance and to
provide a preliminary indication of phytotechnology performance. The purpose of predictive
modeling is to assess whether a phytoremediation approach can be used to extract groundwater at
rates sufficient to create a water table depression that can alter or contain the migration of
CVOCs As phytoremediation requires time to implement, the retardation of groundwater flow
will provide greater opportunity to capture CVOCs. Models also can be useful for evaluating
potential risks to human and ecological receptors at the site. Therefore, dual-approach modeling
using a plot of trees is suggested. This approach requires:
• Analysis of plant water extraction and transpiration to estimate potential water removal
by vegetation; and
. Groundwater capture zone analysis to determine the required water extraction rates to
maintain hydraulic control.
Spreadsheet models can be used to simulate plant transpiration using evapotranspiration as the
basis for water extraction and flow estimation. Evapotranspiration can be estimated by both
meteorological methods and by sap flow measurements. Reference or potential ^
evapotranspiration (ET0) can be estimated using the Food and Agriculture Organization s (FAO)
Penman-Monteith method rhttp://www.fao.org). This parameterization of the Penman-Monteith
equation is the new worldwide standard estimating equation as recommended by the FAO the
International Commission for Irrigation and Drainage, and the World Meteorological
Organization.
Several computer models, HELP, EPIC, UNSAT-H, and HYDRUS-2D can be used to estimate
evapotranspiration and rainfall infiltration to groundwater. A model validation study indicated
that the UNSAT-H and HYDRUS-2D models, which are based upon complex equations for
flow, were superior to the HELP and EPIC models, which are based upon simple tracking of the
amount of water (Wilson et al., 2001).
MODFLOW™ is traditionally used for capture zone analysis of conventional mechanical
groundwater extraction systems, but can be adapted for phytotechnology applications. An
alternative groundwater capture zone model is WinFlow (Version 1.07, Copyright 1995,
Environmental Simulations, Inc.). For applications involving groundwater remediation a
capture-zone calculation (Domenico and Schwartz, 1990) can be used to estimate whether
phytoremediation can effectively capture the plume of contaminants. UNSAT-H, LEACHM and
PRZM are other available models that can be useful for simulating chemical transport in soil,
soil water soil gas, and groundwater in a dynamic, time-variable fashion. When analyzing the
site water balance, the growing season and movement of water during no growth season are
important to consider. Also, the proportion of groundwater vs. surface water uptake should be
considered (Clinton et al., 2004).
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3.1.4 Meteorological Monitoring
To help assess the evapotranspiration rates used in groundwater capture and water balance
modeling, information on the following meteorological conditions should be obtained-
precipitation, ambient air temperature, soil temperature, wind, solar radiation and relative
humidity. These conditions influence the rates of water uptake from by plants. If a weather
monitoring station is not set up onsite, local weather stations and NOAA can provide
meteorological data. Although meteorological data from local weather stations can provide a
mete°r°logical data' the data are not necessarily representative, especially
3.1.5 CVOC Distribution
Management of CVOCs requires a fundamental understanding of those chemical and physical
processes that ultimately affect their fate. Once released to the environment CVOCs are affected
by a number of processes including:
• dissolution;
• adsorption;
• advection and dispersion;
• volatilization; and
• biological transformation (microbial attenuation).
These processes will have some role in affect on the nature and distribution of the CVOCs in the
subsurface. The relative impact of each process will depend on the nature of the CVOC and site
conditions. The geologic and hydrogeologic data should be used to evaluate the rates directions
and pathways of migration. Evaluation of spatial and temporal trends will help to define the fate'
and transport of a CVOC and estimate its mobility.
3.1.6 Agronomic Evaluation
The performance of phytoremediation systems is contingent on soil quality, which depends on
the physical, chemical, and biological parameters of the soil. Physical parameters are very
important and include compactness (bulk density), texture, and permeability. Equally important
are chemical parameters, which include fertility, salinity, and presence ofphytotoxic elements or
compounds (See Section 3.2). Biological factors, including plant and chemical interactions with
bacteria, fungi, insects, and burrowing animals, need to be assessed for an effective
phytoremediation project.
A list of soil parameters to measure would include:
• available nutrients, including nitrogen, phosphorus, potassium, calcium, sulfur etc •
• particle size distribution;
• bulk density;
• salinity;
• oxidation/reduction (redox) potential;
• microorganism(s) present for degradation;
• cation exchange capacity;
• pH; and
• organic matter content.
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The nature and properties of the soil will govern the type of plant suitable for phytoremediation.
If the soil is unsuitable for plant growth, it may require extensive soil amendments to make it
useable.
of contaminants can be poisonous to plants or "phytotoxic." Therefore,
laboratory testing may be needed to investigate whether site soil and groundwater can be
phvtotoxic to plants used in phytoremediation. Phytotoxicity studies are recommended when
CVOCs and other constituents (e.g., metals, dissolved solids, or other organics) are present in
soil or groundwater at concentrations close to published toxicity indices. If there is little concern
regarding phytotoxocity, a laboratory test may not be necessary. However, even if contaminant
concentrations are below these indices, a properly designed laboratory study is useful for
providing data on the performance of the phytoremediation system design. Phytotoxicity studies
designed to mimic site conditions can more readily address concerns about potential cumulative
effects of multiple chemicals, as most phytotoxicity data available in the literature are for a
single chemical or condition. Because the phytotoxicity levels for some compounds can be
species-specific, depending on a plant's inherent enzyme types and levels, the plant species
intended for use at the site, can also be tested in the laboratory using site soil and groundwater.
And even if the levels of CVOCs are not phytotoxic to a plant, the level dissolved solids or
salinity may be phytotoxic.
Laboratory tests should focus on screening species for their ability to tolerate suspected
phvtotoxic compounds present in site soil and groundwater. The test can be conducted onsite in a
greenhouse or at a university or commercial laboratory with experience in conducting treatability
studies using site soil. Listed below are some basic guidelines for phytotoxicity screening (ISO
1 1269-2, 1995; ASTM, 2002; and OECD, 2004):
• A completely randomized block design can reduce environmental bias.
. Toxicity treatment variables (CVOCs or constituents in soil and groundwater that may be
toxic) should be tested in triplicate with at least three other different concentrations.
. Controls (uncontaminated soil and water) can be used to assess the baseline performance
of plants unaffected by the CVOCs and toxic constituents.
• Controls should be treated the same as toxicity treatments and replicated three to four
times.
U1I1C&.
Plants need to be propagated in a sufficiently sized container with site soil.
• Soil should be kept at the same moisture level as at the site.
• Controls on CVOC emissions from soils and plant must be made for health and safety
purposes.
. Qualitative and quantitative monitoring can be conducted over a period of approximately
two months following an acclimation period of one month.
• Site soil should be obtained from both the surface and from a depth 3-5 feet below
ground surface. (Even if the surface soil is not contaminated, soil closer to the
groundwater may be highly contaminated.)
Qualitative monitoring for phytotoxicity includes visual observation of indicators of plant health
such as the percent germination (if plants are grown from seed), leaf yellowing or browning
percent dead plants, and average height of plants per pot (measured from mam stem to tip of
leader).
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Quantitative monitoring for phytotoxicity involves soil and plant tissue analyses. These analyses
should be ^ducted by an accredited analytical laboratory for a range of parameters, including
(but not limited to) chloride and nutrients (nitrogen, phosphorous, and potassium) Toxic
constituents (CVOCs and other constituents in soil and groundwater that may be toxic) and
selected other parameters (e.g., pH and metals) that may affect plant growth may be analyzed
periodically, depending on the composition of the soil.
Ferro et al., (1999) assessed the toxicity of certain volatile organic compounds (VOCs)-a
mixture of aromatic compounds, chlorinated aliphatics, and alcohols-in poplar trees The studv
observed no phytotoxic effects from three different concentrations (42 mg/L, 85 mg/L and 169
mg/L). Dietz and Schnoor (2001) found hybrid poplar cuttings could be grown hydrop'onically in
concentrations of chlorinated aliphatic compounds ranging from O.lmM to 5 mM The cuttings
were more susceptible to chlorinated ethenes than to chlorinated ethanes, and growth was
restricted mostly by highly chlorinated solvents.
Research has also revealed that hybrid poplars are tolerant to high levels of chlorinated solvents
Ma and Burken (2004) reported that no acute phytotoxicity-in terms of decreased water
transpiration or chlorosis (the absence of green pigments)-was observed throughout a short-term
experiment at 440 ppm TCE. In an earlier study, no acute phytotoxicity was observed when
similar reactors were dosed with 550 ppm TCE (Ma and Burken, 2004). However after 24 davs
acute phytotoxicity signs, such as wilting leaves and decreasing water uptake, were reported for
one reactor dosed with 820 ppm TCE; one plant died after 36 days.
3.3 Regulatory Considerations
Regulatory consideration for phytoremediation is consistent and equal in evaluation to any other
remediation technology. Regulations (40 Code of Federal Regulations [CFR] 300 430) require
that a remediation technology be "protective of human health and the environment maintain
protection over time, and minimize untreated waste".
There are many questions and issues that can be raised by regulators, potentially responsible
parties, and the public. One such question is - What is the regulatory driver for the cleanup (e g
£™SyA°i T*' ComPrehensive Environmental Response, Compensation, and Liability Act
LCERCLA], Resource Conservation and Recovery Act [RCRA], underground storage tank, etc.)?
A variety of permits may be required when using phytoremediation at a site:
• If surface water is treated, a National Pollutant Discharge Elimination System (NPDES)
permit may be required.
• If phytovolatilization is the operating mechanism, an air permit may be required
• If groundwater will be pumped to the ground surface to irrigate the plants or if
contaminated soil is excavated or moved from one area to another, a RCRA permit may
be required.
Cities and states may have additional restrictions about using invasive plant species.
City ordinances may apply.
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Other steps to consider when proposing phytoremediation as a remedial technology at a site are:
• Determine whether the anticipated time frame of phytoremediation can fit the regulatory
status, the site strategic development plan, and the current or potential risks associated
with the groundwater.
• Identify the contaminants of concern, media affected, and cleanup levels. Provide
laboratory or field data to the regulator that are consistent with the plants and
contaminants of concern at the site.
• Customize monitoring requirements to the site and plants selected. Monitoring the
efficacy of an innovative treatment such as phytoremediation may be more extensive than
would be required for a more accepted technology.
• Define the fate and transport of the contaminants of concern, including the effects on the
food chain and volatilization to the surrounding air. If a contaminant is shown to be
bioavailable, an ecological risk assessment should be performed.
• Determine whether the plant material may become hazardous with uptake of
contaminants and how it will be disposed.
• Create a contingency plan in the case that phytoremediation does not perform as
expected.
• Define the criteria that will determine when remediation is complete and the site closed.
• Assess if there is sufficient containment of contaminated soils, groundwater, and
sediments until the phytoremediation plants have established themselves at the site.
Since there are many regulatory issues to be tackled, involvement of regulators and the public is
recommended early in the project. Owners and contractors should provide as much data as
possible to support the feasibility of phytoremediation at their site. Such data should include
laboratory and field studies with the contaminant of concern, the proposed plant species, and
similar geologic and climatic conditions. Regulators may accept laboratory studies as evidence
that this technology will be appropriate for a site. However, some regulators may require field
studies before allowing full-scale phytoremediation to be the chosen remedial technology.
Performance standards should be developed so the effectiveness of phytoremediation can be
measured against an agreed upon set of criteria. Performance standards may include hydraulic
measures, contaminant movement, or contaminant concentration reductions.
Since phytoremediation is a newer technology, it does not have the track record that some other
remedial technologies have obtained. However, this is changing. More and more research has
been conducted, both in the laboratory and in the field. The research completed in the last decade
has been promising and more regulators are open to phytoremediation than in the past.
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SECTION 4.0 FIELD PILOT TESTS
._
A field pilot test can document phytoremediation under real site conditions and if successful
may generate design and cost data for full-scale application. Pilot tertii^^^^^Ur
^^^^-^^^^^^^^i
^r^r^^^
• a description of the soil investigation, analytical sampling results, a borehole/well
location map, and soil logs, as appropriate;
• field design and layout;
• maps and figures showing details of the proposed field test design-
• the number and type of trees/plants to be used in the pilot (assume some will die)-
• the desired spacing of planting to optimize plant water use (400 to 800 trees per acre)-
• maintenance requirements including monitoring for diseases;
• institutional controls (e.g., keeping animals and people out of the area)-
• the recommended fertilization program; and
• a description of the analytical monitoring program.
Brief summaries of design elements, such as selection of plants, planting techniques
fertilization, soil amendments, and maintenance, are presented in the following sections.
4.1 Plant Selection
• climatological requirements;
• translocation and uptake capabilities;
• tolerance levels with respect to chemicals known to exist at the site;
• tolerance to drought-prone or poorly-drained conditions;
• tolerance to pH and salinity of the soil and groundwater;'
• depth of the root zone;
• growth rate;
• transpiration rate or water use;
• whether it is deciduous or evergreen (affects the period of effectiveness);
• maintenance requirements;
• native vs. non-native species; and
• commercial availability.
Some nurseries or plant propagation companies will propagate some species of plants that are not
normally commercially available. This can be very valuable if native species are desirable or
even required by other agencies. cwiaoic or
4.2 Planting Techniques
The number of trees to plant depends on the size of the space available and the desired plant
density. Widely spaced planting has the potential benefit of cost savings if the trees are being
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installed by augering boreholes, and minimizes or eliminates the cost of thinning. However,
greater plant densities will achieve canopy closure more rapidly; therefore, water uptake will also
reach a maximum rate more quickly. Greater densities are also appropriate for tree varieties that
are more columnar in shape. Very dense plantings should be thinned when trees are about 3-
years-old to improve tree vigor and performance. Row spacing is should be at least 10-ft wide to
allow mowing equipment between rows during establishment. Digging holes for plantings may
be accomplished effectively using techniques such as: 1) augering borehole to the water table or
3 feet below ground surface; 2) trenching rows to the water table or to 3 feet below ground
surface; or 3) using a ripper shank-type planter to furrow 1.5 to 2 feet below ground. With each
technique, the trees are planted in a mixture of soil and soil amendments, which is backfilled into
the holes.
Herbaceous plants used for ground cover and erosion control can be planted using standard
agronomic planting techniques such as a seed drill. More information of planting techniques can
be found in "Phytoremediation of Groundwater at Air Force Plant 4 - Carswell, Texas" (U.S.
EPA, 2003).
4.3 Fertilization
Soil samples can be sent to a laboratory for agriculture analysis to evaluate them for nutrients,
grain size and cation exchange capacity. It is most effective to collect soil samples from the
entire rooting depth required to meet remedial goals. Sampling can be stratified based on major
changes in soil texture, structure, color, and rooting patterns of existing vegetation.
Nitrogen is likely to be the most limiting nutrient in a soil. Nitrogen application rates are not
readily measured from soil tests, and are determined based on estimates of plant uptake and loss
mechanisms, such as denitrification. Rapidly growing hybrid poplar plantations may require 200
Ib of nitrogen (N) or more per acre per year, although 50 to 150 Ib N/acre/year is typically
applied (Heilman et al., 1995). Many phytoremediation sites would benefit from at least 100 Ib
N/acre/year The actual rates applied can be adjusted based on plant growth, leaf tissue nitrogen
(need for fertilization indicated if leaf N less than about 2.7%), and enhanced microbial growth.
This is vital for sites impacted by petroleum hydrocarbons or at risk of nitrate groundwater
contamination. Nitrogen may be deficient if leaves are small or light green to yellow in color,
especially if lower leaves yellow first. Interveinal chlorosis (yellowing of leaves between veins),
if present, is due to other nutrient deficiencies or toxicities.
Phosphorus and potassium fertilization also is likely to be required on many sites. In addition,
hybrid poplars may respond well to addition of zinc, especially in calcareous soils. Use of zinc in
a chelated form is recommended to ensure that the zinc remains mobile and available for plant
uptake in the presence of CaCO3. However, caution should be used when using zinc fertilizers
since they can be phototoxic at low levels, especially under low pH conditions. Fertilizers
applied in slow-release form are recommended.
Required application rates and methods are somewhat site-specific, and should be developed in
consultation with a qualified soil scientist or agronomist.
4.4 Soil Amendments
Restoration of soil quality is a prerequisite for successful phytoremediation implementation. In
most cases, soils will not require any soil amendments. In cases where soils are of very poor
quality or the site has a substantial amount of fill material, soil amendments may help with
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establishing and maintaining healthy vegetation. It is helpful to contact a local agriculture agency
for the amount of amendments needed for the site soil. A soil pH within a range of 6 to 8.5 is
best for vegetation. In low pH soils, lime can be used to adjust soil pH, as needed, based on
baseline monitoring data. Aged compost or peat moss can be blended with site soil during
planting to accomplish the following goals:
• as bulking agent to decrease soil density;
• to obtain a consistent soil structure;
• to improve soil aeration;
• to increase plant nutrient holding capacity and reduce the need for fertilization;
• to increase the moisture holding capacity of the soil; and
• to enhance rhizodegradation of the CVOCs.
4.5 Maintenance Plan
Maintenance of the phytoremediation plot includes routine inspection of groundwater,
meterological, and plant monitoring equipment and the plants themselves. Equipment
maintenance will be conducted based upon manufacturers' recommendations regarding the level
of use. Maintenance of plants and cover vegetation will include:
• mowing ground cover vegetation on a regular basis can greatly enhance growth of trees
(Mowing can be difficult with 400 - 800 trees per acre. It may only be recommended
when herbaceous plant competition becomes detrimental to trees);
• fertilizing the area based on the soil monitoring results;
• following an animal control plan to keep out deer, voles, beavers, etc.; and
• pruning and replacing trees.
A well thought out maintenance plan can often make the difference between success and failure.
Disease or competition can affect a large number of phytoremediation plants especially if the
plants are dealing with less than ideal conditions.
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SECTION 5.0 MONITORING AND SAMPLING
Monitoring the fate and transport of CVOCs in a phytoremediation application should
encompass those mechanisms responsible for solvent attenuation or loss. It is recommended that
monitoring include the sampling of soil, groundwater, plants, and air. The approach to sampling
these media to track the progress of phytoremediation is discussed in the following sections.
5.1 Soil Sampling
Soil samples should be collected before starting phytoremediation to serve as a baseline tor
comparison to the mid-point and final sampling results. Sampling locations can be selected based
on the site conceptual model. If the hydraulics of the site are well known, the site hydrogeologist
can select points that will allow the site manager to be confident that the remediation effects can
be documented. The initial soil samples can be collected and analyzed when monitoring wells
are installed for groundwater sampling. Soil from all of the monitoring well locations can be
sampled, unless there is an overriding reason this cannot be done. Sufficient data must be
collected, however. When sampling:
• collect a sufficient number of cores across enough depths so that soil concentrations
within the plume can be documented;
• leave space for equipment when the remediation plots are installed so subsequent cores
can be collected; and
• when the cores are collected, examine them to be sure that the soil in that area is
homogeneous; If the soils are not homogeneous, the number of cores collected will
probably have to be increased.
5.2 Groundwater Monitoring and Sampling
Groundwater monitoring should be conducted within the phytoremediation study location and, if
possible, both upgradient and downgradient of the test location. Monitoring wells should be
installed where groundwater samples will be collected for chemical analysis. Piezometers can be
installed in locations where the only data needed are water level measurements for water table
contouring and aquifer tests for estimating hydraulic conductivity. A large number of
piezometers may be needed to measure small changes in groundwater levels. Use of in-well
continuous monitors will give more accurate water level data than periodic measurements by
hand because many factors can affect the plant water uptake at a given point in time. Factors
such as precipitation, recharge rate, and root locations should also be considered when analyzing
water level data.
Groundwater samples collected from monitoring wells should be analyzed for of CVOCs and
their microbial daughter product compounds, nutrients; and field parameters such as temperature,
oxidation-reduction potential, pH, and specific conductance (which can be measured in a flow-
through cell, when practical).
Sampling design and frequency will be based on study objectives; however, it is recommended
that groundwater sampling be conducted at least twice per year. Groundwater sampling within
the test location and in a control area can provide data for rhizosphere effects and hydraulic
control. These data can be used to evaluate whether or not trees are creating a depression in the
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water table. A single transect may be used for this purpose; however, two or three transects are
recommended, to fully evaluate water levels within the planting area and to collect groundwater
samples to evaluate the fate of CVOCs.
If the project schedule provides insufficient time for trees to grow large enough to extract
groundwater, then sap flow monitoring can be used as an alternative to or in addition to
groundwater elevation monitoring. Plant transpiration can be measured with thermal dissipation
sap flow sensors that are attached to the trunk of the trees (Dynamax, 1997). A continuous
recording datalogger will record sap flow on time weighted basis. Using these data, diurnal
fluctuations and daily cumulative values of plant transpiration will be assessed and estimates of
total transpiration can be made. These data can be matched against estimates of expected
groundwater extraction rates required for hydraulic control.
Over the past ten years, studies have been conducted by the United States Geological Survey
(USGS) and United States Air Force (USAF) to quantify the effects of trees on shallow
groundwater geochemistry. These studies have shown that cottonwood plantations (5-6 years
old) can induce groundwater geochemical changes year round that can initiate in-situ
dechlorination of TCE. TCE biodegradation rates at this Texas site were noted to increase and
this increase in natural attenuation capacity was associated with a potential decrease in plume-
stabilization distance (Harvey, 2004, personal communication).
5.3 Plant Monitoring
Recommended plant monitoring includes qualitative monitoring (i.e., plant health observations),
quantitative monitoring (i.e., growth as indicated by leaf area index, girth, and height), and plant
tissue sampling for analysis of CVOCs. Randomly selected trees should be used as bench marks
for plant monitoring throughout the duration of the phytoremediation project. Monthly
qualitative plant monitoring is recommended for a newly planted site, and tissue sample
collection is recommended quarterly to semi-annually.
The qualitative health observations are used to assess the overall vigor of the phytotechnology
system. Through this process, nutrient deficiencies can be identified and remedied, and diseases
can be identified early before they become epidemic and threaten the entire phytotechnology
approach. The qualitative assessment will involve observing leaf condition related to nutrient
deficiencies, diseases, and pest infestations. Aerial infrared pictures can help reveal health effects
such as infestation.
The following quantitative measurements can be conducted throughout the study and at its
completion:
• Evapotranspiration can be measured by both meteorological methods (e.g., the Penman-
Monteith equation) and sap flow measurement sensors that are attached to the trunk of the
trees. These sensors can be linked to a continuous recording datalogger that records sap
flow. Using this monitoring approach, diurnal fluctuations and daily cumulative values of
plant transpiration can be assessed.
• Leaf area index (LAI) can be assessed in the field using a handheld lux meter that measures
light intensity within the canopy relative to light intensity above the canopy. These
measurements should be taken monthly or bi-monthly during the growing season to
monitor tree canopy development and to correlate plant transpiration measurements with
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the LAI. The LAI will also be used to assist with estimation of evaporation and
transpiration components of the calculated ET0.
• Monitoring of overall canopy height is recommended at the onset of the study and at the
end of the growing season each year. Average canopy height is estimated by measuring the
height of several benchmark trees or plants. This is done by vertical distance from the soil
surface to the highest point of the vegetation.
• Tree girth (i.e., circumference) of the main stem of the tree is measured at the study onset,
and at the end of the growing season each year. Girth can be measured using a soft
measuring tape or calipers to determine diameter at breast height and converting to a
standard girth.
• Depending on soil characteristics and groundwater depth, it may be worthwhile to evaluate
whether the roots have reached the groundwater in the first two to four years of the project
by excavating a hole adjacent to a tree.
• Collecting plant and core tissue samples (leaves and stems) from selected benchmark trees
or plants and analyzing them to detect chemical uptake is recommended.
5.4 Plant Sampling
A measure of plant interaction with the CVOC in soil or groundwater can be determined by
analyzing the presence of the parent compound and metabolites within the plant tissues. Perhaps
the'ideal sampling approach is to look for the parent compound in tree trunks. The trunk of the
tree has been shown to contain the highest concentrations of the original contaminant, before it
has a substantial chance of being volatilized out of the bark and leaves. However, the trees must
be large enough to be cored without significant harm to the tree, which will limit the approach's
usefulness in young plantations. If the trees are large enough, coring results can help determine if
the plant roots have reached the contaminated water. The method for tree coring has been
described in detail in Vroblesky et al. (1999) and Ma and Burken (2002). In brief, a core is
removed from the tree at breast height, placed into a pre-weighed glass vial with a Teflon®
stopper, and stored on ice. The vial is allowed to come to room temperature for two days in the
laboratory, and the headspace gases sampled for the presence of the parent compound.
An alternative approach to analyzing plant tissue is to sample the tree branches. This approach
does not have the same age or size limitations on the trees that trunk coring has, nor the concerns
about harming tree health. Because branch sampling does not harm tree health, it allows for
collection of replicate samples. Contaminant concentrations may vary with branch size, however,
so the utmost attention needs to be given to optimize sample uniformity during each sampling
event. Branches that are closest to the ground may be the best candidates for sampling, as they
intercept sap before substantial diffusion losses occur (Negri et al., 2004).
Sampling for the presence of volatile compounds in plants can be complicated by the very nature
of the compounds. Volatile compounds are unstable and can dissipate from plant tissues before
analysis can be performed. Therefore, it is necessary to collect and ship the samples under
conditions that will allow the retention of the compound in the sample. Sample collection and
holding also will depend on the method being used for sample analysis. If samples will be
examined by head space analysis, samples should be collected in the field in the containers in
which analysis will be performed. If samples need to be ground in preparation for analysis, the
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best method for analyte retention is to grind the samples in a stainless steel bowl under liquid
nitrogen, transfer the samples to the analysis containers, and seal the containers as soon as the
samples stop off-gassing the liquid nitrogen and are still frozen. Samples can be transported the
laboratory on regular ice.
If the samples will be extracted for analysis, additional sample handling protocol is needed.
Extraction can be done to detect the parent compound and is necessary to see many of the
metabolites that may not be volatile enough to be analyzed for directly from the plant tissues.
Many methods are available for extraction and metabolite derivatization, including those
presented by Newman et al. (1997 and 1999). For this type of analysis, samples must be frozen
immediately on collection-preferably in liquid nitrogen-and stored on dry ice until arrival back
in the laboratory. Samples should be stored at -80° until analysis.
Analysis of plant transpiration gases is another way of determining if the plants have reached the
contaminant. This analysis may also be required by regulatory agencies to determine the extent
of contaminant release into the atmosphere. Transpiration gas samples should be collected from
both the leaves and the trunk of the trees. Trunk samples can be collected non-destructively by
the methods described by Schumacher et al. (2004). Transpiration gases collected from leaves
are more problematic as the volume of water transpired by the leaves is much larger than the
volume of contaminant. Since water vapor can foul gas collection systems, care must be taken to
avoid fouling or laboratory instruments can be damaged and samples can be invalidated.
The most common way to collect transpiration gases is to enclose a leaf or small group of leaves
in a Teflon® bag connected to a vacuum pump via a sample collection tube (carbon or Tenex).
The sample collection tube should be wrapped with heat tape to keep the internal temperature
above the dew point of water in order to prevent water vapor from fouling the collection
material. Air is pulled through the bag at a set flow rate for a set period of time. At the end of this
time period, the surface area of the leaf or leaves is measured onsite, or the leaf (leaves) is taken
to the laboratory to be scanned for surface area. The surface area used to calculate the amount of
compound transpired per unit leaf area per unit time (Newman et al., 1999).
All of these methods have been described in detail in various articles. These sample collection
methods have been optimized for conditions encountered by the groups that used them.
Therefore, some experimentation will be needed to devise the best methods for a given site.
Contacting some of the authors of these studies may be the best way to proceed, as they can
provide valuable information about methods that have not succeeded or improved methods they
have been developed but not yet published.
5.5 Air Monitoring and Sampling
Monitoring of plant-transpired CVOCs can be conducted using modified, flexible, Tedlar® bags,
which are made of non-sorbent plastic. The Tedlar® bags are placed over individual leaves,
leafed branches, or around the tree shoot. Samples can be analyzed using sorbent tubes, summa
canisters, or solid-phase microextractors (SPMEs). Evaluation of the sample results must account
for the CVOC background concentrations in air to determine the levels due to transpiration.
Gaseous emissions from soil surfaces can be characterized using flux chambers. Flux chamber
monitoring is particularly useful for distinguishing between the VOCs volatilized from soil and
groundwater versus VOC emissions due translocation, transpiration, rhizosphere degradation,
sorption, etc. The flux chamber is designed to isolate those emissions that emanate from the soil
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without augmentation or suppression of the natural flux emission rate. The frequency of
measuring soil mass emissions will be a function of cost, regulatory mandate, and technology
design (monitoring may assist with planting location). Quite often, this monitoring effort is used
during the first year of planting to establish a baseline level of soil emissions, with subsequent
monitoring once the tree's root system is established, such as year three.
The method employed to measure phytovolatilization at Aberdeen Proving Ground in Maryland
fhttp://web.ead.anl.gov/ifield) entailed the use of 100-L, clear, Tedlar® bags. Each bag was split
open to envelop a well-leafed branch, temporarily sealed, and allowed to equilibrate anywhere
from two to four hours, depending upon incident solar radiation intensity. However, it is
recommended to allow air to flow through the bag at all times to prevent the plant from shutting
down transpiration due to localized humidity. The best reading is obtained during the first hour
of measurement. Transpiration results can be affected by solar radiation intensity (which varies
due to sun, shade, and cloud cover), humidity, recent rainfall, time of day, wind speed, and
temperature. Temperature and humidity instruments can be incorporated in the Tedlar® bag
apparatus for continuous monitoring.
In addition to leaf sampling, research efforts have targeted measurement of CVOCs that diffuse
from xylem tissues during transpiration (Ma and Burken, 2002). Activities at the Aberdeen
Proving Ground's J-Field site have been documented in detail (Compton et al., 1998 and
Schneider et al., 2000), where phytoremediation has been in place since 1996, when 183 hybrid
poplars were planted over a contaminant plume. To capture transpired VOCs, an 8-L, Tedlar®,
gas-sampling bag was cut open along three seams to yield a Teflon® sheet, measuring roughly
35 cm x 70 cm. The bag was wrapped around the trunk of selected hybrid poplars (P. deltoides x
trichocarpa) on the site. An activated carbon trap was attached to the valve/hose barb
arrangement on the Tedlar® bag. The Teflon® sheet was attached to the tree using adhesive
tape, with three 2-inch-long, '/4-inch inner diameter, Teflon® tubes, protruding from the
Teflon®-tree connection on the opposite side of the tree. The Teflon® tubes allowed influent air
to enter the wrapped section of the tree. The activated carbon trap was connected to a personal
air-sampling pump. At the start of a sampling period, the pumps were turned on and adjusted to
deliver a constant 1 L/min flow through the wrapped section and the activated carbon trap.
Background activated carbon traps were placed roughly within the center of the
phytoremediation plot. When using carbon tubes, it is important to heat them to drive off
transpired water before analysis.
Upon completion of the last sampling period of the day, the sampling valves in the Tedlar® bag
were closed. The next morning, SPME field samplers were inserted in the bags. The SPME
fibers were exposed to the air in the wrapped section of the trunk for 20 minutes. The fibers were
retracted into the sample holder. Duplicate samples were collected, and trip blanks and
background samples also were collected with SPMEs. Trip blanks serve to monitor background
contamination picked up in the laboratory or during transport. At the J-field site, samples were
taken in the ambient air in the center of the phytoremediation plot. Personnel at Argonne
National Laboratory analyzed the SPMEs on a gas chromatograph with an electron capture
detector and mass spectrometer. Use of SPME analysis for environmental samples has proven to
be accurate and compares favorably with EPA-validated methods for VOC analysis (Llompart et
al., 1998). Quantification was not accomplished with the SPME samples, however. Due to the
non-linear response curve (Chai and Pawliszyn, 1995), and the non-uniform conditions (e.g.,
humidity, volume, and geometry) in the sampled media, only qualitative data was obtained.
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Similar methods have been undertaken using thermal desorption sampling methods, with similar
success.
Samples can be collected every 15-20 minutes throughout the study, or they may be collected at
the onset once the apparatus is established for a baseline, then at the end of the study. There is no
threshold concentration for action. These are measures of fate and transport of parent VOCs that
indicate trees are planted appropriately with respect to plume capture for both area and depth.
These measures provide answers for assessing a mass balance.
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SECTION 6.0 REPORTING COST AND PERFORMANCE
Accurate and thorough documentation of the cost and performance of phytoremediation projects
is important to evaluate its cost efficiency at sites where it is being implemented or considered.
The FRTR, has published the Guide to Documenting and Managing Cost and Performance
Information for Remediation Projects (FRTR, 1998). The purpose of the guide is to help
encourage, streamline, and standardize data collection and reporting efforts for remediation
technologies. The guide describes a standard set of parameters for reporting cost and
performance information about treatment technologies. This section summarizes the suggested
cost and performance reporting parameters for cleanup treatment technologies, in general, and
phytoremediation, in particular.
Performance often is characterized only in terms of a removal percentage or concentration level
attained. However, that information alone, in the absence of information about other parameters,
may not be adequate to assess the overall performance of a technology. Table 1 summarizes the
suggested site and operating parameters to report for applications of groundwater
phytoremediation. The parameters to be documented for demonstration-scale projects are similar
to those for full-scale projects. Some additional information applicable to demonstrations
includes commercialization issues and competing technologies. In addition to the parameters
listed in Table 1, documentation should contain all demonstration or cleanup objectives, and
whether or not the objectives were met.
Reporting costs of cleanup applications is helpful to others assessing the feasibility of using
similar approaches at other sites. The FRTR guide recommends a simplified cost reporting
format that is consistent with various ongoing federal programs under which costs are collected.
The format is based on conventional capital and operation and maintenance (O&M) components
for reporting costs of specific cleanup technologies (i.e., technology-specific costs). In addition
to these, most agencies also account for the overall costs of remediation efforts (total project
costs), which include such items as management and support activities, site work such as
security, access, and utilities; permitting; monitoring; and preparation of various plans.
Table 2 summarizes the recommended technology-specific cost elements. The guide provides
more details on each and a sample cost work sheet. The recommended cost format is applicable
to full-scale and demonstration-scale projects, and for both ex-situ applications and in-situ
projects (such as phytoremediation).
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Table 1 : Suggested Parameters to Document Phytoremediation Applications
(adapted from FRTR, 1998)
Parameter
Contaminants and concentrations
Hydrogeology
Cleanup goals and requirements
Soil classification
Clay content and/or particle size distribution
Hydraulic conductivity
PH
Depth below ground surface or zone of interest
Total organic carbon
Nutrients and other soil amendments
Plants per unit area
Plant type
Climate
Size of site
Plant installation method and depth
Irrigation
Required monitoring and reporting
Matrix
Characteristics
X
X
X
X
X
X
X
X
X
X
X
X
Operating
Characteristics
X
X
X
X
X
X
Table 2 - Recommended Format for Reporting Technology Costs
(adapted from FRTR, 1998)
Capital Costs
Technology mobilization, setup, and demobilization
Planning and preparation
Site work
Equipment and appurtenances
Startup and testing
Other (including non-process equipment)
Operation and Maintenance Costs
Labor (including planting, replanting, fertilization, irrigation, pest control, harvesting, etc.)
• Materials
Utilities and fuel
Equipment ownership, rental, or lease
Other (including non-process equipment overhead and health and safety)
Other Technology-Specific Costs
Patent fee ( see http://www.ecolotree.com)
Compliance testing and analysis
Soil, sludge, and debris excavation, collection, and control
Disposal of residues
Other Project Costs
24
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PHYTOTECHNOLOGY WEB RESOURCES
U.S. Environmental Protection Agency
http://www.cluin.org/techfocus/
Interstate Technology and Regulatory Council
Phytoremediation Decision Tree
http://www.itrcweb.org/phyto 1 .pdf
Phytotechnology technical and regulatory guidance document
http://www.itrcweb.org/PHYTO2.pdf
International Journal of Phytoremediation
http://www.aehs.com/iournals/phytoremediation/
International Phytoremediation Electronic Network (U. Parma, Italy)
http://www.dsa.unipr.it/phvtonet
Remediation Technologies Development Forum
http://www.rtdf.org/public/phvto/default.htm
U.S. Department of Agriculture PLANTS National Database
http://plants.usda.gov
Wildlife Habitat Council
http ://www.wildlifehc.org
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APPENDIX FREQUENTLY ASKED QUESTIONS
The answer to most questions about phytoremediation begins, "It depends...." Because the use
of plants as tools for remediation relies on the organisms of the plants and the associated
rhizosphere microbes, the site conditions are always unique and every project must be custom
designed and installed. This section is intended to give brief answers to some commonly asked
questions, and to direct the reader to the relevant sections of the document for further
information.
MECHANISMS
How does phytoremediation work?
The use of plants, in general, and trees, in particular, to remove, degrade, or contain toxic
chemicals located in soils, sediments, ground or surface water, and air.
Section 2.0
What happens in winter when the plants are dormant?
Water consumption essentially stops when plants are dormant. Degradation by microbes and the
rhizosphere effect continues, but at a reduced rate.
Section 2.3, 2.4, 2.5
What happens if there is a catastrophic windstorm, fire, disease, beaver, or insect infestation
that kills the plants?
If the plants die or are damaged, the beneficial effects are lost or greatly diminished.
TREES
Which trees should be used and how do you decide?
Some trees have been studied for phytoremediation of groundwater. Various varieties have been
studied in various conditions and climates, and all plant selection must be made based on site
specific conditions. Climate, altitude, soil salinity, position and concentration of contaminant are
some of the determining elements.
Section 4.1
How are plants selected for a remediation?
Based on site data and published information about trees, a list of candidate plants can be
developed. Plants from that list can be tested to see if they survive under the site conditions.
Using a variety of plants decreases problems of monoculture.
Section 3.2
How fast do they grow? How long do they live?
Tree growth depends on species, soil, and climate. For example, Hybrid Poplars can grow 3
meters per year. In general, those trees that grow rapidly tend to be shorter lived and to be more
easily damaged in storm events.
(Dickmann et al, 2001; Dickmann and Stuart, 1983)
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What do you have to do for operations and maintenance (O &M)? How much does O &M
cost?
A new planting may require irrigation, weed control (mowing, mulching, or spraying), pest
control, and some percentage of replanting. Agricultural and silvacultural practices and costs
vary greatly from region to region.
Section 4.5
Costs would include the labor, materials, utilities, and equipment costs for irrigation, weed
control, pest control and replanting. At some point, after the vegetation is well established, O&M
costs are expected to be minimal.
Section 6.0
EFFICACY
Willphytoremediation work on my site?
It depends
Figure 1
How much groundwater can be cleaned? How many gallons? How deep?
It depends on groundwater flow, soil characteristics, and the type of plant used for the
remediation, as well as climate, altitude, and type of planting.
Section 3.0
Depth to groundwater is often variable from season to season, or from year to year, and that
influences the efficacy of trees to impact water quality. It is reasonably easy to plant trees to
influence groundwater that is 15 feet below ground surface. The deepest phytoremediation
impacted aquifer is at 40 feet below ground surface.
U.S. EPA, 2003; Hirsh et al, 2003; andNegri et al, 2004
How do you know it is working?
Monitoring the stabilization or decrease in concentration of the contaminant of concern (COC) in
water/soil, formation of metabolites for degradable compounds and detection of the COC and
their metabolites in the plant tissues will allow evaluation of phytoremediation.
Section 5.0
How long from planting until the groundwater is affected? How long until the groundwater is
clean?
The depth to groundwater and the method of planting determines how long before the trees
impact groundwater. Complete restoration of the groundwater will depend on the site, the type of
contaminant, the extent of contamination, and the phytoremediation technologies enhanced in the
design. The mass of contaminant starts decreasing once the trees impact groundwater. The plants
may have to be in place for the foreseeable future as they are only cleaning the soluble
contaminants that are passing the roots and not the source area that will continue to add the
contaminant to the aquifer.
U.S. EPA, 2003 and 2000
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SAFETY
Do the plants become contaminated in this process?
According to the current research, there is little or no accumulation of volatile contaminants in
plant roots, wood, leaves, or fruit. Plants may accumulate metals or other toxic materials, but
finding volatiles in plants is rare.
Section 2.0, Newman et al, 1999, andHirsh et al, 2003 (29).
Are fruit and nuts from these plants safe for humans and animals?
Probably not, but test them to be sure.
Is the wood usable?
Yes.
How can one tell if a plant is safe or not?
Analysis of plant and core tissue sampling (leaves and stems) will determine if the plant is safe.
Section 5.3
Do plants release contaminants into the air? If so, how much, and how often?
Possibly. Extensive sampling in the field shows minimal amounts of VOC can come from plant
leaves and bark.
Section 2.2
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January 2005
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