&EPA
United States
Environmental Protection
Agency
Office of Research and
Development '
Washington, DC 20460
EPA/600/R-99/107
February 2000
Introduction to
Phytoremediation
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EPA/600/R-99/107
February 2000
Introduction to Phytoremediation
National Risk Management Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Cincinnati, Ohio 45268
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Notice
The EPA through its ORD produced this document. It has been subjected to the Agency's peer
and administrative review and has been approved for publication as an EPA document. Mention
of trade names or commercial products does not constitute endorsement or recommendation
for use.
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Foreword
The U.S. Environmental Protection Agency is charged by Congress with protecting the Nation's
land, air, and water resources. Under a mandate of national environmental laws, the Agency
strives to formulate and implement actions leading to a compatible balance between human
activities and the ability of natural systems to support and nurture life. To meet this mandate,
EPA's research program is providing data and technical support for solving environmental prob-
lems today and building a science knowledge base necessary to manage our ecological re-
sources wisely, understand how pollutants affect our health, and prevent or reduce environmen-
tal risks in the future.
The National Risk Management Research Laboratory is the Agency's center for investigation of
technicological and management approaches for reducing risks from threats to human health
and the environment. The focus of the Laboratory's research program is on methods for the
prevention and control of pollution to air, land, water and subsurface resources; protection of
water quality in public water systems; remediation of contaminated sites and ground water; and
prevention and control of indoor air pollution. The goal of this research effort is to catalyze
development and implementation of innovative, cost-effective environmental technologies; de-
velop scientific and engineering information needed by EPA to support regulatory and policy
decisions; and provide technical support and information transferto ensure effective implemen-
tation of environmental regulations and strategies.
This publication has been produced as part of the Laboratory's strategic long-term research
plan. It is published and made available by EPA's Office of Research and Development to assist
the user community and to link researchers with their clients.
E. Timothy Oppelt, Director
National Risk Management Research Laboratory
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Abstract
Phytoremediation is the name given to a set of technologies that use different plants as a
containment, destruction, or an extraction technique. Phytoremediation as a remediation tech-
nology that has been receiving attention lately as the results from field trials indicate a cost
savings compared to conventional treatments.
The U.S. EPA has a dual role in which it seeks to protect human health and the environment
associated with hazardous waste sites, while encouraging development of innovative technolo-
gies that might more efficiently clean up these sites.
This Introduction is intended to provide a tool for site regulators, owners, neighbors, and man-
agers to evaluate the applicability of phytoremediation to a site. This document defines terms
and provides a framework to understand phytoremediation applications. It is a compilation of
research and remediation work that has been done to date. The format is intended to be acces-
sible to EPA RPMs, state regulators, and others who need to choose between alternate tech-
nologies, as well for site owners, consultants, contractors, and students who are interested in
basic information. It is not a design manual, and is not intended to provide enough information
to choose, engineer, and install a phytoremediation application.
This work may also be used to help guide research, development, and regulation. Areas of
needed research have been identified. By compiling the published and unpublished work, re-
search repetition can be avoided, and areas of opportunity that need attention should be clear.
IV
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Table of Contents
Foreword iii
Acknowledgements xi
Chapter 1 Introduction 1
1.1 Objectives 1
1.2 Approach 1
1.3 Report Organization 1
Chapter 2 Overview of Phytoremediation 3
2.1 Background 3
2.2 Technical Considerations 7
2.3 Economic Considerations 7
2.4 Regulatory Considerations 9
2.5 Ecosystem Restoration 10
2.6 Current Research 12
Chapters Evaluation of Phytoremediation Technologies 14
3.1 Phytoextraction 14
3.2 Rhizofiltration 18
3.3 Phytostabilization 21
3.4 Rhizodegradation 23
3.5 Phytodegradation 28
3.6 Phytovolatilization 31
3.7 Hydraulic Control 34
3.8 Vegetative Cover Systems 35
3.9 Riparian Corridors/Buffer Strips 39
Chapter4 Phytoremediation System Selection and Design Considerations 41
4.1 Contaminated Media Considerations 42
4.2 Contaminant Considerations 43
4.3 Plant Considerations 44
4.4 Site Considerations 48
4.5 Treatment Trains 51
4.6 Additional Information Sources 51
Chapters Remedial Objectives, Treatability, and Evaluation 52
5.1 Remedial Objectives 52
5.2 Treatability Studies 53
5.3 Monitoring for Performance Evaluation 56
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Contents (continued)
Chapters Case Studies 58
6.1 Edgewood Area J-Field Toxic Pits Site Aberdeen Proving Grounds
Edgewood, Maryland 58
6.2 Carswell Site Fort Worth, Texas 60
6.3 Edward Sears Properties Site NewGretna, New Jersey 64
6.4 Bioengineering Management: U.S. Nuclear Regulatory Commission
Beltsville, MD 67
6.5 Lakeside Reclamation Landfill Beaverton, Oregon 68
6.6 Alternative Landfill Cover Demonstration Sandia National Laboratories
Albuquerque, NM 70
Appendix A Glossary A-1
AppendixB Phytoremediation Database B-1
Appendix C References C-1
AppendixD Common and Scientific Names of Referenced Plants D-1
VI
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Figures
Figure2-1. Mechanisms for phytoremediation
Figure 2-2. Example root depths
Figure 3-1. Phytoextraction
Figure 3-2. Rhizodegradation
Figure 3-3. Phytodegradation
Figure 3-4. Phytovolatilization
Figure 3-5. Hydraulic control of contaminated plume
Figure 3-6. Illustration of an Evapotranspiration (ET) cover.
Figure 3-7. Phytoremediation cover evolution
Figure 6-1. Experimental Design
Figure 6-2. TCE Concentrations
Figure 6-3. Site Map (Edward Sears Site)
Figure 6-4. Sampling Grid (Edward Sears Site)
.. 4
..6
16
24
29
32
34
36
37
61
63
65
66
VII
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Tables
Table 2-1. Phytoremediation Applications 5
Table 2-2. Root Depth for Selected Phytoremediation Plants 6
Table 2-3. Phytoremediation atSuperfund Sites 8
Table 3-1. Phytoremediation Overview 15
Table 4-1. Phytoremediation Technologies Applicable to Different Contaminant Types12 43
Table 4-2. Plant Selection Process 45
Table 5-1. Summary of Phytoremediation Technologies and Method of Contaminant Control 52
Table 5-2. Experimental Factors for Testing in Treatability Studies 54
Table 5-3. Information Needed fora Pilot Treatability Study 55
Table 5-4. Summary of Monitoring Parameters 57
Table 6-1. Monitoring Approaches at the J-Field Site 59
Table 6-3. Estimated Cost of Phytoremediation at the Carswell Site 63
Table 6-2. Average Concentrations of TCE, cis-DCE, and trans-DCE at Carswell Site 63
Table 6-4. Design Type and Completion Dates for the Experimental Covers 67
Table 6-5. Summary of Run-off, Evapotranspiration, and Deep Percolation From the
Bioengineered Plots 68
Table 6-6. Summary of Percolation and Precipitation Rates From
May 1997 Through March 1998 forthe Six Cover Designs 72
Table 6.7 Construction Costs for the Final Landfill Covers 72
VIM
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Acronyms
AAP Army Ammunition Plant
ACAP Alternative Cover Assessment Program (U.S. EPA)
ALCD Alternative Landfill Cover Demonstration
ANOVA Analysis of Variance
APG Aberdeen Proving Grounds
ARARs Applicable or Relevant and Appropriate Requirements
ASTM American Society for Testing and Materials
BTEX Benzene, Toluene, Ethylbenzene, Xylenes
CERCLA Comprehensive Environmental Response, Compensation, and Liability Act
Co Original Concentration
DEPH Diethylhexylphthalate
DNAPL Dense Nonaqueous Phase Liquid
DOD Department of Defense
DOE Department of Energy
EPA Environmental Protection Agency
ERT U.S. EPA Emergency Response Team
FFDCA Federal Food, Drug, and Cosmetic Act
FIFRA Federal Insecticide, Fungicide, and Rodenticide Act
GC/MS Gas Chromatography/Mass Spectroscopy
HCB Hexachlorobenzene
K^ octanol-water partition coefficient
LNAPL Light Nonaqueous Phase Liquid
MCL Maximum Contaminant Level
NPDES National Pollutant Discharge Elimination System
NPL National Priority List (Superfund)
NRMRL National Risk Management Research Laboratory
OSC On-Scene Coordinator
ORD U.S. EPA Office of Research and Development
OSWER U.S. EPA Office of Solid Waste and Emergency Response
PAH Polynuclear Aromatic Hydrocarbons
PCB Polychlorinated Biphenyls
PCE Perchloroethylene, tetrachloroethene
PCP Pentachlorophenol
PRP Potentially Responsible Party
PVC Polyvinyl Chloride
QA/QC Quality Assurance/Quality Control
RCRA Resource Conservation and Recovery Act
RD Remedial Design
RPM Remedial Project Manager
ROD Record of Decision
RTDF Remediation Technologies Development Forum
SITE Superfund Innovative Technology Evaluation Program (EPA)
TCA Tetrachloroethane
TCAA Trichloroaceticacid
TCE Trichloroethylene
TIO U.S. EPA Technology Innovation Office
TNT Trinitrotoluene
IX
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Acronyms (continued)
TPH Total Petroleum Hydrocarbons
TSCA Toxic Substances Control Act
USDA U.S. Department of Agriculture
UXO Unexploded Ordinance
VOC Volatile Organic Compounds
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Acknowledgements
This Introduction to Phytoremediationwas written by an EPA Team consisting of Nancy Adams,
Dawn Carroll, Kelly Madalinski, Steve Rock, and Tom Wilson as well as Bruce Pivetz of ManTech
Environmental Research Services Corporation. Additional assistance was also provided by Todd
Anderson, Jon Chappell, Scott Huling, Jessica Palmiotti, and Phil Sayre. The team would like to
thank Ed Earth, Michelle Laur, Ken Lovelace, Andrea Mclaughlin, Linda Fiedler, Susan Thornloe,
and Albert Venosa for their extensive review.
XI
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Chapter 1
Introduction
Phytoremediation is an emerging technology that uses
various plants to degrade, extract, contain, or immobilize
contaminants from soil and water. This technology has been
receiving attention lately as an innovative, cost-effective
alternative to the more established treatment methods used
at hazardous waste sites.
The U.S. Environmental Protection Agency (EPA) seeks
to protect human health and the environment from risks
associated with hazardous waste sites, while encouraging
development of innovative technologies such as
phytoremediation to more efficiently clean up these sites.
This document reports the results of phytoremediation
efforts as originally reported by researchers. No attempts
were made to validate data obtained from the literature.
1.1 Objectives
The objectives of this report are as follows:
• Provide an educational tool for site regulators, owners,
neighbors, and managers to evaluate the applicability
of phytoremediation to a site. Phytoremediation projects
have been proposed or applied to ecosystem restora-
tion and soil, surface water, groundwater, and sediment
remediation. This document identifies and defines
phytoremediation technologies, and provides a guide
to current research to aid in evaluation of proposed
phytoremediation applications.
• Develop a format that is accessible to EPA and state
regulators and others who need to evaluate alternate
remedial technologies, as well as to site owners, project
managers, consultants, contractors, and students who
are interested in basic information.
• Evaluate the various phytoremediation processes (e.g.,
phytodegradation, rhizofiltration, hydraulic control).
• Present phytoremediation system characteristics that
site managers and others might find useful in assess-
ing the potential applicability of phytoremediation to a
specific site.
• Present case studies illustrating field applications of
phytoremediation.
• Provide a detailed bibliography of additional resources for
those interested in learning more about phytoremediation.
• Provide access to general information on various re-
source applications. However, it should be noted that
this document is not a design manual and is not in-
tended to provide enough information to engineer and
install any phytoremediation application.
• Provide a guide for research, development, and regula-
tion, and identify areas of needed research. Through
the compilation of published and unpublished work, re-
search repetition can be avoided, and areas of opportu-
nity that need attention should be clear.
1.2 Approach
The following approach was used to compile and summa-
rize information on phytoremediation processes:
• Conduct a comprehensive literature search.
• Contact contractors and researchers to obtain informa-
tion on phytoremediation applications and cost.
• Review and evaluate existing research and field appli-
cations of current phytoremediation projects.
• Assemble a compilation of research and remedial work
that has been performed to date.
• Use the resources of the Internet to both gather and
disseminate information. The creators of this document
have written their sections so that they can be regularly
updated to keep them relevant as the technology
changes. This document may be accessed on the
Internet "www.clu-in.org".
1.3 Report Organization
This report has been designed to provide quick access to
information on the various phytoremediation processes and
associated information as follows:
• Chapter2 provides an overview of phytoremediation in-
cluding applications, limits, cost information, and regu-
latory concerns. Ecosystem restoration as it applies to
phytoremediation processes is also discussed.
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Chapters provides a literature review and evaluation of
the major phytoremediation processes. This chapter is
divided into subsections that present definitions, mecha-
nisms, site characteristics, applicable media, contami-
nants amenable to each phytoremediation process, and
the associated concentrations where available. The
advantages, disadvantages, and current status of each
process are also discussed. Finally, an annotated ref-
erence list is included at the end of the discussion of
each phytoremediation process to provide more detailed,
specific information. The purpose of this chapter is to
provide site managers with an overview of the various
phytoremediation processes as well as what can be
expected from each process and its limitations.
Chapter 4 discusses considerations involved in the se-
lection, design, and implementation of phytoremediation
systems. It presents information that will help a site
managerto identify whether phytoremediation maybe
appropriate for a site and to select a particular
phytoremediation technology, based on the conditions
occurring at, or applicable to, a site. This chapter intro-
duces issues and concepts that should be considered
in the design and implementation of a phytoremediation
system.
• Chapter 5 presents the remedial objectives for
phytoremediation as well as associated monitoring
needed to evaluate system performance. Information
on conducting treatability studies is also included in
this chapter.
• Chapter 6 presents six case studies where
phytoremediation has been applied. The six case stud-
ies presented in this chapter illustrate specific field ap-
plications of phytoremediation. This chapter includes
site descriptions, design considerations, monitoring rec-
ommendations, status, and costs of various
phytoremediation processes.
• Included as appendices are a glossary of
phytoremediation terms, references, common and sci-
entific names of referenced plants, and part of the da-
tabase that provides information on phytoremediation
projects available on the Internet or through EPA.
Please note that because phytoremediation is an emerg-
ing technology, standard performance criteria for
phytoremediation systems have not been developed. Data
are being gathered and assessed to develop performance
measures that can be used to predict the function and effi-
cacy of an individual system.
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Chapter 2
Overview of Phytoremediation
2.1 Background
Phytoremediation is the name given to a set of technolo-
gies that use plants to clean contaminated sites. Many tech-
niques and applications have been called phytoremediation,
possibly leading to confusion. This document uses the term
phytoremediation to refer to a set of plant-contaminant inter-
actions, and not to any specific application. Many of the
phytoremediation techniques involve applying information that
has been known for years in agriculture, silviculture, and
horticulture to environmental problems.
The term phytoremediation (phyto= plant and remediation
= correct evil,) is relatively new, coined in 1991. Basic infor-
mation for what is now called phytoremediation comes from
a variety of research areas including constructed wetlands,
oil spills, and agricultural plant accumulation of heavy met-
als. The term has been used widely since its inception, with
a variety of specific meanings. In this document
phytoremediation is used to mean the overall idea of using
plant-based environmental technologies, not any specific
application.
Research efforts into remediation can be roughly catego-
rized into two sets: exploration of mechanisms and evaluation
of claims. Mechanism work has centered on finding theoreti-
cal limits, and explanations for results observed in the field.
Pilot-scale field work has both preceded and followed explana-
tory laboratory research, and early successes have piqued
interest. Long-term, objective field evaluation is critical to un-
derstanding how well phytoremediation may work, what the
real cost of application will be, and how to build models to
predict the interaction between plants and contaminants. Most
of the projects are ongoing and thus provide only preliminary
data.
2.1.1 Applications
Phytoremediation applications (as shown in Figure 2-1 and
Table 2-1) can be classified based on the contaminant fate:
degradation, extraction, containment, ora combination ofthese.
Phytoremediation applications can also be classified based
on the mechanisms involved. Such mechanisms include ex-
traction of contaminants from soil orgroundwater; concentra-
tion of contaminants in plant tissue; degradation of contami-
nants by various biotic or abiotic processes; volatilization or
transpiration of volatile contaminants from plants to the air;
immobilization of contaminants in the root zone; hydraulic
control of contaminated groundwater (plume control); and
control of runoff, erosion, and infiltration by vegetative cov-
ers. A brief explanation ofthese application categories fol-
lows, with more detailed explanations in following chapters.
2.1.1.1 Degradation
Plants may enhance degradation in the rhizosphere (root
zone of influence). Microbial counts in rhizosphere soils can
be 1 or2 orders of magnitude greaterthan in nonrhizosphere
soils. It is not known whetherthis is due to microbial orfungal
symbiosis with the plant, plant exudates including enzymes,
or other physical/chemical effects in the root zone. There are,
however, measurable effects on certain contaminants in the
root zone of planted areas. Several projects examine the in-
teraction between plants and such contaminants as trinitro-
toluene (TNT), total petroleum hydrocarbons (TPH), pentachlo-
rophenol (PCP), and polynucleararomatic hydrocarbons (PAH).
Another possible mechanism for contaminant degradation
is metabolism within the plant. Some plants may be able to
take in toxic compounds and in the process of metabolizing
the available nutrients, detoxify them. Trichloroethylene (TCE)
is possibly degraded in poplar trees and the carbon used for
tissue growth while the chloride is expelled through the roots.
EPA has three projects underway in the field using populus
species to remediate TCE. Tests at the University of Wash-
ington are being developed to verify this degradation mecha-
nism under controlled conditions.
2.1.1.2 Extraction
Phytoextraction, orphytomining, is the process of planting
a crop of a species that is known to accumulate contami-
nants in the shoots and leaves of the plants, and then har-
vesting the crop and removing the contaminant from the site.
Unlike the destructive degradation mechanisms, this tech-
nique yields a mass of plant and contaminant (typically met-
als) that must be transported for disposal or recycling. This is
a concentration technology that leaves a much smaller mass
to be disposed of when compared to excavation and landfilling.
This technology is being evaluated in a Superfund Innovative
Technology Evaluation (SITE) demonstration, and may also
be a technology amenable to contaminant recovery and recy-
cling.
Rhizofiltration is similar to phytoextraction in that it
is also a concentration technology. It differs from phytoextraction
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Physical Effects - Transpiration of volatlles
hydraulic control of dissolved plume
Phylodegra elation - Metabolism within the
plant production of the dehalogenase and
oxygenase enzymes, which help catalyze
degradation
Accumulation in roots
translocated to shoots
and leaves
Enhanced "" ^
Rhizosphere
B.iodeg racial ion
Figure 2-1. Mechanisms for phytoremediation.
in that the mechanism is root accumulation and harvest
using hydroponic (soil-less) growing techniques. This is use-
ful for separating metal contaminants from water.
Rhizofiltration has been demonstrated on U.S. Department
of Energy (DOE) sites for radionuclides.
Volatilization or transpiration through plants into the at-
mosphere is another possible mechanism for removing a
contaminant from the soil or water of a site. It is often raised
as a concern in response to a proposed phytoremediation
project, but has not been shown to be an actual pathway for
many contaminants. Mercury (Hg) has been shown to move
through a plant and into the air in a plant that was geneti-
cally altered to allow it to do so. The thought behind this
media switching is that elemental Hg in the air poses less
risk than other Hg forms in the soil. However, the technol-
ogy orthe associated risk has not been evaluated.
2.1.1.3 Containment and Immobilization
Containment using plants either binds the contaminants
to the soil, renders them nonbioavailable, or immobilizes
them by removing the means of transport.
Physical containment of contaminants by plants can take
the form of binding the contaminants within a humic mol-
ecule (humification), physical sequestration of metals as
occurs in some wetlands, or by root accumulation in
nonharvestable plants. Certain trees sequester large con-
centrations of metals in their roots, and although harvesting
and removal is difficult or impractical, the contaminants
present a reduced human or environmental risk while they
are bound in the roots.
Risk reduction may also be achieved by transforming the
contaminant into a form that is not hazardous, or by render-
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Table 2-1. Phytoremediation Applications
Mechanism
Degradation
Degradation
Degradation
Degradation
Degradation
Extraction-Concentration
in shoot
Extraction-Concentration
in root
Extraction, Volatilization
Contaminant
Atrazine, nitrates
Landfill leachate
TCE
TNT
TPH
Lead
Uranium
Selenium
Media
Surface Water
Groundwater
Groundwater
Wetlands
Soil
Soil
Surface water
Soil, Surface Water
Plant
Poplar
Poplar
Poplar, cottonwood
Various
Grasses, crops
Indian mustard
Sunflower
Various
Status
Applied
Applied
Field demo
Field demo
Field demo
Field demo
Field demo
Applied
Reference
Schnoor 1995a
Licht 1990
Rock 1997
Bader 1996
Carreira 1996
McCutcheon 1995
Banks 1997
Drake 1997
Blaylock 1997
Dushenkov 1997
Banuelos 1996
Terry 1996
ing the contaminant nonbioavailable. EPA and the U.S. De-
partment of Agriculture (USDA) have ongoing research in
this area.
Hydraulic control is anotherform of containment. Ground-
water contaminant plume control may be achieved by wa-
ter consumption, using plants to increase the evaporation
and transpiration from a site. Some species of plants use
tremendous quantities of water, and can extend roots to
draw from the saturated zone. EPA is pursuing research in
this area at a number of sites, including the SITE demon-
strations at Ogden, UT and Ft. Worth, TX, and the Emer-
gency Response Team (ERT) lead projects at Aberdeen
Proving Grounds (Edgewood, MD) and the Edward Sears
Properties Site (New Gretna, NJ). Private companies have
installed trees as a hydraulic control at many sites.
Vegetative cover (evapotranspiration or water-balance
cover) systems are another remediation application utiliz-
ing the natural mechanisms of plants for minimizing infil-
trating water. Originally proposed in arid and semi-arid re-
gions, vegetative covers are currently being evaluated for
all geographic regions. The effectiveness in all regions and
climates needs to be assessed on a site-specific basis.
If there is potential for gas generation a vegetative cover
may not be an option. For example, a municipal solid waste
landfill can produce landfill gas that may be of concern to
human health and the environment. Sites with requirements
to collect and control landfill gas may not meet Federal
requirements under the Clean Air Act if a vegetative cover
is used.
Hydraulic control for groundwater plumes and water bal-
ance covers are two technologies that are being applied in
the field prior to model development predicting their be-
havior. Under an EPA initiative called Alternative Cover As-
sessment Program (ACAP), several of these field installa-
tions will be monitored carefully and consistently to gather
data to both evaluate performance and to build and verify
models to predict the performance of other proposed in-
stallations. Data from a national network of sites that have
similar measurement regimes will be a powerful tool for
evaluating the appropriateness of a proposed installation,
and help develop the tools for predicting the efficacy of
similar cover systems.
2.1.2 Limits of Phytoremediation at
Hazardous Waste Sites
As a result of the early information provided by some
research and reported by the media, site owners and citi-
zen groups are interested in phytoremediation as possibly
the cleanest and cheapest technology that may be em-
ployed in the remediation of selected hazardous sites. Al-
though current research continues to explore and push
the boundaries of phytoremediation applications, there are
certain limitations to plant-based remediation systems.
2.1.2.1 Root System
Root contact is a primary limitation on phytoremediation
applicability. Remediation with plants requires that the con-
taminants be in contact with the root zone of the plants.
Either the plants must be able to extend roots to the con-
taminants, or the contaminated media must be moved to
within range of the plants. This movement can be accom-
plished with standard agricultural equipment and practices,
such as deep plowing to bring soil from 2 or 3 feet deep to
within 8 to 10 inches of the surface for shallow-rooted crops
and grasses, or by irrigating trees and grasses with con-
taminated groundwater or wastewater. Because these ac-
tivities can generate fugitive dust and volatile organic com-
pound emissions, potential risks may need to be evalu-
ated. As shown in Table 2-2 and illustrated in Figure 2-2,
the effective root depth of plants varies by species and
depends on soil and climate condition.
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Table 2-2. Root Depth for Selected Phytoremediation Plants
Plant Maximum Root Depth Target Contaminants
Indian mustard
Grasses
Poplar trees
To 12 inches
To 48 inches
To 1 5 feet
Metals
Organics
Metals, organics,
chlorinated solvents
2.1.2.2 Growth Rate
Phytoremediation is also limited by the growth rate of the
plants. More time may be required to phytoremediate a site
as compared with other more traditional cleanup technolo-
gies. Excavation and disposal or incineration takes weeks
to months to accomplish, while phytoextraction ordegrada-
tion may need several years. Therefore, for sites that pose
acute risks for human and other ecological receptors,
Phytoremediation may not be the remediation technique of
choice.
2.1.2.3 Contaminant Concentration
Sites with widespread, low to medium level contamina-
tion within the root zone are the best candidates for
phytoremediative processes. High concentrations of con-
taminants may inhibit plant growth and thus may limit ap-
plication on some sites or some parts of sites. This phyto-
toxicity could lead to a tiered remedial approach in which
high concentration waste is handled with expensive ex situ
techniques that quickly reduce acute risk, while in situ
Phytoremediation is used over a longer period of time to
clean the high volumes of lower contaminant concentra-
tions.
2.1.2.4 Impacts of Contaminated Vegetation
Some ecological exposure may occur whenever plants
are used to interact with contaminants from the soil. The
fate of the metals in the biomass is a concern. At one site,
sunflower plants that extracted cesium (Cs) and strontium
(Sr) from surface water were disposed of as radioactive
waste (Adler1996).
Although some forms of phytoremediation involve accu-
mulation of metals and require handling of plant material
embedded with metals, most plants do not accumulate sig-
nificant levels of organic contaminants. While metal accu-
mulating plants will need to be harvested and either re-
cycled or disposed of in compliance with applicable regu-
lations, most phytoremediative plants do not require fur-
ther treatment or disposal.
Often overlooked, however, is the possibility that natural
vegetation on the site is already creating very similar (but
often unrecognized) food chain exposures. In addition, even
on currently unvegetated sites, contaminants will be enter-
ing the food chain through soil organisms.
The remediation plan should identify and, if possible, quan-
tify potential avenues of ecological exposure, and deter-
mine if and where any accumulation of toxics in the se-
lected plants will occur. Accumulation in fruits, seeds, and
Poplar Trees 15 ft.
Alfalfa 4-6 ft.
Grasses 2 ft.
Indian
Mustard 1 ft.
>
*
Figure 2-2. Example root depths.
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leaves typically creates more exposure than accumulation
in stems and roots. Most organic contaminants do not ac-
cumulate in significant amounts in plant tissue.
Some plant-eating animals have been shown to avoid
eating plants with elevated metal levels (Pollard 1996). In
addition, the increased habitat provided by the plants may
in some cases offset any potential localized impacts.
If some organisms (e.g., caterpillars, rodents, deer, etc.)
seem likely to ingest significant amounts of the vegeta-
tion, and if harmful bioconcentration up the food chain is a
concern during the life of the remediation effort, appropri-
ate exposure control measures should be implemented
including perimeter fencing, overhead netting, and pre-flow-
ering harvesting. Phytoextraction techniques aim to har-
vest metal-laden crops just as the plants translocate met-
als into shoots, thereby limiting availability of contaminants
for consumption.
Transfer of the contaminants or metabolites to the atmo-
sphere might be the greatest regulatory concern. Transpi-
ration of TCE into the atmosphere has been measured
(Newman et al. 1997a), but little information is available
that would indicate any release of vinyl chloride.
Research being done on the bioavailability of contami-
nants and on human health and environmental risk as-
sessment is directly related to phytoremediation. Studies
are underway to determine if contaminants that are not
available to plants for uptake or that are not vulnerable to
plant remediation are less of a risk to human health and
the environment.
2.2 Technical Considerations
Several key factors to consider when evaluating whether
phytoremediation is a potential site remedy are described
below.
1. Determine whether evidence of the potential effec-
tiveness of phytoremediation is specific to the site
matrix and contaminants. If laboratory studies on the
plants and contaminants of interest are the primary
evidence used to support the use of phytoremediation
at the site, the studies should at least show that the
plants to be used at the site are capable of remediating
site contaminants.
2. Consider the protectiveness of the remedy during the
time it takes the plants associated with phytoremediation
to establish themselves at the site to a point where
they are containing/degrading the contaminants of in-
terest.
3. Consider whether phytoremediation is likely to clean
up the site in an acceptable time frame.
4. An adequate backup or contingency technology
should be identified in the event that phytoremediation
is attempted and does not succeed.
Additionally, monitoring the efficacy of any innovative
treatment may be more extensive than would be required
for a more accepted technology. Monitoring needs to ad-
dress both the decrease in the concentration of the con-
taminants in the media of concern, and examine the fate
of the contaminants. The monitoring plan must be tailored
to the site and plants.
2.2.1 Prior Applications of
Phytoremediation
One indication of acceptability of a technique is previ-
ous successful applications on similar sites. Because it is
a relatively new technology, phytoremediation does not have
a long history of completed cleanups. Table 2-3 lists 12
Superfund sites where phytoremediation has been ac-
cepted or is being field-tested for possible remediation of
soil or groundwater contamination. Appendix B lists ap-
proximately 180 sites where the technology has been ap-
plied or is being field-tested. The peer-reviewed field data
that are available on these projects are limited. More data
should become available in the next few years through the
efforts of programs such as the Superfund Innovative Tech-
nology Evaluation (SITE) program, the Remediation Tech-
nologies Development Forum (RTDF), and others.
Results of studies done in greenhouses and on field test
plots can be used to show proof of concept, and some of
that data may be directly applicable to site-specific consid-
eration. If time and funding permit, soil or water from the
site should be used in lab or greenhouse studies. Such
treatability studies can confirm the effectiveness of the site-
specific treatment. Chapter 5 provides more information
on treatability studies.
2.3 Economic Considerations
Because phytoremediation is an emerging technology,
standard cost information is not readily available. Subse-
quently, the ability to develop cost comparisons and to
estimate project costs will need to be determined on a site-
specific basis. Two considerations influence the econom-
ics of phytoremediation: the potential for application, and
the cost comparison to conventional treatments. Care must
be taken to compare whole system costs, which may in-
clude:
Design costs:
Site characterization
Work plan and report preparation
Treatability and pilot testing
Installation costs
Site preparation
Facilities removal
Debris removal
Utility line removal/relocation
Soil preparation
Physical modification: tilling
Chelating agents
pH control
Drainage
Infrastructure
Irrigation system
Fencing
Planting
Seeds, plants
Labor
Protection
Operating costs:
Maintenance
Irrigation water
Fertilizer
pH control
Chelating agent
Drainage water disposal
Pesticides
Fencing/pest control
Replanting
Monitoring
Soil nutrients
SoilpH
Soil water
Plant nutrient status
Plant contaminant status
roots, shoots, stems,
leaves)
Tree sap flow monitoring
Air monitoring (leaves,
branches, whole tree, area)
Weather monitoring
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Table 2-3. Phytoremediation at Superfund Sites
Site Name, State Date Planted
Plant
Contaminant/Matrix
Carswell Site, TX
Aberdeen Proving Grounds, MD
Edward Sears Site, NJ
Iowa Army Ammunition Depot, IA
FortWainwright.AK
Kaufman & Minteer, NJ
Calhoun Park, SC
Solvent Recovery Systems of
New England, CT
Twin Cities Army Ammunition
Plant, MN
Bofors-Nobel, Ml
Del Monte, HI
INEEL, ID
Spring 1996
Spring 1996
Fall 1996
Spring 1997
Spring 1997
Spring 1997
Fall 1998
Spring 1998
Spring 1998
Planting scheduled
Spring 1998
Spring 1999
Eastern cottonwood tree
Hybrid poplar trees
Hybrid poplartrees
Wetland and terrestrial plants
Felt leaf willow
Hybrid poplartrees
Local landscaping plants
Hybrid poplartrees
Corn, Indian mustard
Various trees and wetland plants
Koa haole
Kochia, willow
TCE/groundwater at 4-12 feet
TCE/groundwater
TCE/groundwater at 8 feet
TNT/soil and pond water
Pesticides/soil and groundwater
PCE/groundwater
PAH/groundwaterat1-4feet
Mixed solvents/groundwater
Metals/soil
Residual sludge in waste lagoons
Pesticides/soil and groundwater
Cesium, mercury in soil
The national potential for phytoremediation could be es-
timated by first totaling the number of sites that contain
organics and metals suitable for phytoremediation, i.e.,
those sites that contain contaminants in moderate con-
centrations in near-surface groundwater or in shallow soils.
Currently, such specific information about hazardous waste
sites in the United States is not available. Kidney (1997)
has estimated the current domestic market for
phytoremediation at only $2 to $3 million for organics re-
moval from groundwater, and $1 to $2 million for removal
of heavy metals from soils. The same study indicates that
by the year 2005, however, the market for phytoremediation
of organics in groundwater will be $20 to $45 million, of
metals in soils will be $40 to $80 million, and of radionu-
clides will be $25 to $50 million.
David Glass (1998) and others have estimated that total
system costs for some phytoremediation applications will
be 50 to 80% lower than alternatives. Each application of
plants will yield a separate performance evaluation includ-
ing rate and extent of cleanup and cost. Three actual cost
estimates of applications are compared to conventional
treatments in Table 2-4.
For some of phytoremediation applications, hypothetical
cost comparisons have been projected. These are esti-
mates based on laboratory and pilot scale work and tend
to reflect projected total project costs.
Phytoextraction Costs
(A) The estimated 30-year costs (1998 dollars) for
remediating a 12-acre lead site were $12,000,000 for ex-
cavation and disposal, $6,300,000 for soil washing,
$600,000 for a soil cap, and $200,000 for phytoextraction
(Cunningham 1996).
(B) Cost estimates made for remediation of a hypotheti-
cal case of a 20-in.-thick layer of sediments contaminated
with Cd, Zn, and 137Cs from a 1.2-acre chemical waste
disposal pond indicated that phytoextraction would cost
about one-third the amount of soil washing (Cornish et al.
1995).
(C) Costs were estimated to be $60,000 to $100,000
using phytoextraction for remediation of one acre of 20-
in.-thick sandy loam compared to a minimum of $400,000
for just excavation and storage of this soil (Salt et al. 1995).
Rhizofiltration Costs
The cost of removing radionuclides from water with sun-
flowers has been estimated to be $2 to $6 per thousand
gallons of water (Dushenkovet al. 1997).
Phytostabilization Costs
Cropping system costs have been estimated at $200 to
$10,000 per hectare, equivalent to $0.02 to $1.00 per cu-
bic meter of soil, assuming a 1-meter root depth
(Cunningham etal. 1995b).
Hydraulic Control Costs
Estimated costs for remediation of an unspecified con-
taminant in a 20-foot-deep aquifer at a 1-acre site were
$660,00 for conventional pump-and-treat, and $250,000
for phytoremediation using trees (Gatliff 1994).
Vegetative Cover Costs
Cost estimates indicate savings for an evapotranspira-
tion cover compared to a traditional cover design to be 20
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Table 2-4. Example Cost Comparisons
Problem
Lead in soil, 1 acre3
Phytoremediation
Application
Extraction, harvest
disposal
Cost
($ thousand)
$150-250
Conventional
Treatment
Excavate and landfill
Cost
($ thousand)
$500
Projected
Savings
50-65%
Solvents in groundwater, Degradation and
2.5 acresb hydraulic control
TPH in soil, 1 acrec
In situ degradation
$200 install and
initial maintenance
$50-100
Pump and treat
Excavate and landfill
incinerate
$700 annual running
cost
$500
50% cost saving
by third year
80%
a Phytotech estimate for Magic Marker site (Blaylock et al. 1997).
b PRP estimate for Solvent Recovery Systems of New England site.
CPERF estimate (Drake 1997)
to 50%, depending on availability of suitable soil (RTDF
1998).
2.4 Regulatory Considerations
While Federal regulations specific to phytoremediation
have not been developed, a range of existing Federal and
state regulatory programs may pertain to site-specific de-
cisions regarding the use of this technology. These pro-
grams include those established underthe: Resource Con-
servation and Recovery Act (RCRA); Comprehensive En-
vironmental Response, Compensation, and Liability Act
(CERCLA) referred to as "Superfund"; Clean AirAct (CAA);
Toxic Substances Control Act (TSCA); Federal Insecticide,
Fungicide, and Rodenticide Act (FIFRA); Federal Food,
Drug, and Cosmetic Act (FFDCA); and statutes enforced
by the U.S. Department of Agriculture. These programs
are discussed in the following sections.
2.4.1 RCRA
RCRA has nine sections (Subtitles) that deal with spe-
cific waste management activities. Two of these Subtitles
are most likely to pertain to the use of phytoremediation:
Subtitle C (Hazardous Waste Management), and Subtitle
D (Solid Waste Management).
EPA issued closure requirements for Subtitles C and D
treatment, storage, or disposal (TSD) units, which may be
closed by removal or decontamination ("clean closure") or
closed with waste in place ("landfill closure") (see 40 CFR
Parts 257, 258, 264, and 265). The regulations include
general closure requirements for all RCRA units and spe-
cific closure requirements for each type of TSD unit. The
requirements are performance-based, and therefore do not
stipulate any design standards. EPA delegates these regu-
latory programs to the states, which are responsible for
their implementation. The Federal requirements are mini-
mum requirements that must be incorporated into state
regulatory programs; however, states may promulgate clo-
sure requirements that are more stringent than those of
the Federal program. Site-specific evaluation of the use of
alternative covers at TSDs that close as a landfill will need
to include consideration of these requirements.
The Corrective Action Program, under RCRA, requires
corrective action, as necessary, to protect human health
and the environment for releases from solid waste man-
agement units at facilities seeking RCRA permits. This pro-
gram is implemented primarily through a series of policy
directives, and is similar in nature to the Superfund program's
remedy selection process contained in the National Contin-
gency Plan (NCP). EPA also delegates the Corrective Ac-
tion Program to the states. Policy directives pertinent to the
Corrective Action Program are available at http://
www.epa.gov/correctiveaction.
2.4.2 CERLCA (Superfund)
Remedial actions taken underthe Superfund program must
attain a general standard of cleanup that assures protection
of human health and the environment, must be cost effec-
tive, and must use permanent solutions and alternative treat-
ment technologies or resource recovery technologies to the
maximum extent practicable. The regulatory framework for
response actions under CERCLA is contained in 40 CFR
Part 300, the National Oil and Hazardous Substances Pol-
lution Contingency Plan, referred to as the NCP. The rem-
edy selection process outlined in the NCP includes a Fea-
sibility Study (FS), in which alternatives that represent vi-
able approaches are assessed against nine criteria. With
respect to phytoremediation, data collected from treatabil-
ity studies or other means will provide the necessary scien-
tific documentation to allow an objective evaluation (using
the nine criteria) of whether phytoremediation is the most
appropriate remedial option fora given site.
An important component in Superfund response actions
is the requirement that for any material remaining on-site,
EPA will attain or exceed any Federal or state limitation,
standard, or criteria that is considered to be applicable or
relevant and appropriate (ARAR) given the circumstances
of the site (for off-site actions, all applicable requirements
must be met). Further, on-site remedial actions must at-
tain promulgated state ARARs that are more stringent than
Federal ARARs. A requirement is applicable if the specific
terms of the law or regulations directly address the cir-
cumstances at the site. If not applicable, a requirement
may nevertheless be relevant and appropriate if circum-
stances at the site are sufficiently similar to the problems
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regulated by the requirement. The Maximum Contaminant
Levels (MCLs) under the Safe Drinking Water Act are an
example of relevant and appropriate requirements.
In order for phytoremediation to be selected as a rem-
edy at a CERLCA site, it will be necessary to meet or waive
the ARARs identified for the site. ARARs can be waived
under six specific circumstances described in the NCR
2.4.3 CAA
Sections 111 and 112 of the Clean Air Act (CAA) contain
the statutory basis for regulation of criteria and hazardous
air pollutant emissions from source categories. The New
Source Performance Standards and Emission Guidelines for
Municipal Solid Waste Landfills, 40 CFR Part 60 Subparts
WWW and Cc, are statutorily based on section 111 of the
CAA. These standards were promulgated in March 1996 and
regulate air emissions of non-methane organic compounds
(NMOCs) from municipal solid waste landfills. Specifically,
these standards require municipal solid waste landfills with a
waste capacity of at least 2.5 million megagrams and with
the potentially to emit at least 50 megagrams of NMOC to
collect and control landfill gas. A second standard, 40 CFR
Part 63 Subpart AAAA, statutorily based on section 112 of
the CAA is currently under development. This regulation will
regulate hazardous air pollutant emissions from municipal
solid waste landfills. These regulations set performance based
standards that allow owners and operators a number of op-
tions to achieve compliance. EPA delegates the implemen-
tation of these regulations to Federal, State and local gov-
ernments. These Federal standards establish minimum re-
quirements that must be implemented. However, State and
local governments may choose to increase the stringency of
these requirements. Any site contemplating the use of
Phytoremediation needs to consider the requirements es-
tablished by Federal, State and local government programs
to regulate air emissions from municipal solid waste landfills.
2.4.4 TSCA
Although the EPA does not currently regulate plants in-
tended for commercial bioremediation, EPA believes the
Toxic Substances Control Act (TSCA) gives it authority to
do so if such action is necessary to prevent unreasonable
risk to human health or the environment. TSCA gives EPA
authority to regulate "chemical substances."TSCAdefines
chemical substances broadly to mean all chemicals and
mixtures of chemical substances. Living organisms such
as plants are mixtures of chemical substances and thus
are subject to TSCA. Although TSCA could potentially be
applied to plants used in bioremediation, EPA has not yet
made a determination of whether such action is neces-
sary to protect the environment and human health. EPA
to date has only issued regulations for microorganisms
under Section 5 of TSCA (EPA, 1997). Further informa-
tion on TSCA and biotechnology products can be found at
http://www.epa.gov/opptintr/biotech/.
2.4.5 FIFRA/FFDCA
Certain plants engineered to contain sequences that af-
ford the plant resistance to pests to enhance the
remediation efficacy of the plant could be subject to re-
view by EPA under its authority to regulate pesticides. EPA
regulates pesticides under two statutes: the Federal In-
secticide, Fungicide, and Rodenticide Act (FIFRA), and
the Federal Food, Drug, and Cosmetic Act (FFDCA). Sub-
stances that plants produce to protect themselves against
pests and disease are pesticides under the definition of
FIFRA Section 2 (i.e., if they are "...intended for prevent-
ing, destroying, repelling, mitigating any pest....") regard-
less of whether the pesticidal capabilities evolved in the
plants orwere introduced by breeding orthrough the tech-
niques of modern biotechnology. These substances, along
with the genetic material necessary to produce them, are
designated "plant-pesticides"(EPA, 1994). Additional de-
tails about EPA plant pesticide regulations can be found at
http://www.epa.gov/fedrstr/EPA-PEST/1994/November/
Day-23/. The U.S. Food and Drug Administration has re-
sponsibility for food safety under the FFDCA. It is unlikely
FDA would be involved in the review of phytoremediation
plants since use of phytoremediation plants as food or as
components of food is an unlikely scenario.
2.4.6 Department of Agriculture Statutes
Plants used for phytoremediation could be potentially
regulated under several U.S. statutes. The U.S. Depart-
ment of Agriculture (USDA) administers several statutes
that could be used to regulate such plants: e.g., the Fed-
eral Plant Pest Act (7 U.S.C. 150aa et seq.), the Plant Quar-
antine Act (7 U.S.C. 151 etseq.), and the Federal Noxious
Weed Act (7 U.S.C. 2801 et seq.). Pertinent regulations
are found at 7 CFR Parts 319, 321, 330, 340, and 360,
respectively. Under USDA authority, one type of plant
(transgenic or naturally-occurring) potentially subject to
review would be a plant considered to be a plant pest. For
additional guidance on USDA regulations pertaining to
plants, refer to http://www.aphis.usda.gov/bbep/bp/.
2.5 Ecosystem Restoration
Vegetation is not only an aid to ecosystem restoration, it
is a key indicator. Plant species that are present on a site,
as well as their quantities and condition, describe a
watershed's health and resilience. Loss of vegetation
through clearing, building, and human land use has se-
vere ecosystem effects. After a human disturbance such
as mining, dumping, industrial, agricultural, or residential
use, revegetation may occur slowly. Recolonization of con-
taminated or disturbed ground by plants typically starts at
the edges of an impacted area. Natural revegetation may
take decades or hundreds of years because it is depen-
dent on animal and windborne seeding. If replanting is
designed and carried out in a way that includes the per-
spectives of engineers, botanists, ecologists, landscape
architects, and others, the environmental systems can
begin to be restored in a few years. In some cases
phytoremediation can help restore wild species diversity
through habitat growth in addition to aiding in remediation
of soil and water.
Despite the general ecological advantages of
phytoremediation, adverse ecological effects are possible
to both on- and off-site biological communities. In evaluat-
10
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ing such concerns, it is important to compare the relative
ecological risks posed by phytoremediation to those risks
already occurring on site or those risks posed by alterna-
tive cleanup methods. Actions needed to protect ecosys-
tems should be clearly specified in the site cleanup plan.
Overall ecological risks associated with remediation of
a site are often overlooked, even by interested parties who
may be familiarwith the human health and ecological risks
associated with current site conditions or with the general
risks posed by feasible alternative cleanup methods. These
overall ecological concerns may be expressed in a limited
context that does not help in the selection of an alterna-
tive. Often, however, a site visit will broaden the under-
standing of interested parties and thus enable them to better
assist in identifying options with the greatest overall eco-
logical benefits.
Listed below are issues that typically arise in such dis-
cussions. Also included are frequently overlooked factors
that should be considered in identifying the relative risks.
2.5.1 Introduction of Non-Native Plants
History is rife with disastrous examples of newly intro-
duced plant species spreading quickly to damage native
ecosystems (e.g., kudzu, Eurasian watermilfoil, etc.). Plants
that work best in remediating a particular contaminant may
or may not be native to a particular area. Although native
plants are most desirable, non-native species may be ac-
ceptable under the following circumstances:
• The plants have been previously introduced, and are
now so common that their proposed use would not
create a new ecological risk.
• The plants are unable to propagate effectively in the
wild (e.g., sterility, dependence on human cultivation,
etc.).
• Genetically altered plants have been introduced. Man-
kind has been using selective breeding to obtain de-
sired plant characteristics for at least 10,000 years.
Now, researchers in many fields are using new ge-
netic engineering techniques to replace selective breed-
ing, allowing them to achieve their desired results more
quickly and selectively. Decisions on the desirability of
using genetically engineered plants must be site spe-
cific.
2.5.2 Integration of Phytoremediation
Into the Site's Long-Term
Landscaping Objectives
Long-term phytoremediation-based treatment can be de-
signed into future site landscaping plans, e.g., tree borders
used for shading and visual screening can also provide on-
going groundwater remediation, etc. Such vegetation can
often create valuable ecological niches, particularly in ur-
ban industrial areas.
Public Uses
A number of contaminated sites are being converted to
parks and other low-intensity public uses. These sites,
particularly with their greater flexibility in the timing and
design of cleanup, frequently offer significant ecological
opportunities. Trees and shrubs do not have to be planted
in straight rows to be effective in remediation.
Commercial Uses
Businesses traditionally landscape for aesthetic reasons
and storm run-off control. These functions may be com-
bined with phytoremediation to offer significant opportuni-
ties. Properly designed and located, such landscaping could
also provide long-term treatment and enhanced ecologi-
cal habitats. A site owner may be willing to significantly
expand the land committed to phytoremediative landscap-
ing if that commitment would reduce overall cleanup costs
and allow quicker site redevelopment. A phased approach,
with intensive short-term treatment by one plant species
followed by permanent plantings with more beneficial veg-
etation, may maximize ecosystem benefits.
Wood Lot Uses
Short-rotation woody crops for pulp, fuel, or timber may
be grown on land with nonaccumulating organic contami-
nants. The trees could be grown and harvested while re-
calcitrant compounds slowly degrade.
2.5.3 Phytoremediation as an Interim
Solution
Phytoremediation is most suitable for remediating sites
or portions of sites with widespread, low-mid level con-
taminants that are often too expensive to remediate by
traditional means. Absent a cost-effective remediation
method, contaminants are often left in place, and contact
with the waste attempted to be minimized by fences, insti-
tutional controls and deed restrictions. Too often no one is
required or able to cleanup the site, and many sites have
been abandoned with no clean up or controls. Occasion-
ally a phytoremediation system may be inexpensive enough
that it could be installed during considerations and debate
regarding a permanent solution, and removed once a final
remedy is implemented. The interim ecosystem benefits,
coupled with improved aesthetics, and some containment
and/or degradation may combine to make even temporary
re vegetation worthwhile.
Although phytoremediation may not be the selected fi-
nal technique, the benefits of a well-designed, properly in-
stalled, and capably managed phytoremediation system
may be preferable to the risks posed by leaving a waste
site completely untreated during delays in implementing a
final remedy.
2.5.4 Ecosystem Restoration at
Phytoremediation Sites
Many sites with contamination support complex ecosys-
tems, primarily due to the low level of human activity on the
11
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site. Plants and animals recolonize some areas where de-
creased human traffic allows vegetation to take root. Phy-
toremediation could aid the natural revegetation underway
on such site. Phytoremediation can make use of various
types of plants, and it is possible to consider local or native
plants as part of a remediation system.
In orderto consider using native plants, the remediation
potential of plants already growing on the site must be
carefully assessed. It must be determined whether these
plants are just tolerating the contaminant or whether they
are already actively remediating it. A field or greenhouse
study may be needed to make this determination.
If non-native plants must be utilized, appropriate control
techniques (e.g., sterile plants) should be used to ensure
that genetic contamination or invasive spread does not
occur.
Phytoremediative plants with desirable ecological values could
provide diversified habitat where appropriate. A combination of
trees, understory shrubs, and grasses may provide shelter and
food for numerous species. Nonphytoremediative plants can be
added to supplement ecological values such as soil stabilization
or to provide a food source.
Evaluating the ecological recovery of a site is important,
but such an evaluation does not have to be expensive or
complex. Neighborhood environmental and school groups
could "adopt" a brownfield or similar distressed site and
provide data of mutual interest (e.g., the local Audubon
Society could assess bird habitat utilization; a biology class
could track plant species survival and growth, etc.). Prop-
erly done, such collaborative monitoring can build commu-
nity understanding and support, while also providing data
that would be otherwise unaffordable, and is seldom col-
lected during remediation. Many sites will not allow access
by untrained personnel. Personal protection must always
be a foremost concern in any such collaboration.
As noted earlier, phytoremediation offers significant eco-
logical promise, but it is not a perfect solution. Ecological
benefits in one area may create ecological impacts in oth-
ers. Negative impacts must be avoided. Some stakehold-
ers may disagree with this definition of ecosystem restora-
tion because it does not attempt to recreate a pristine eco-
system. Although it may not be possible or feasible to re-
turn a site to its condition before human impact,
phytoremediation may provide realistic opportunities to im-
prove the overall ecological health of a site.
2.6 Current Research
To assess the appropriateness of any phytoremediation
application, media- and contaminant-specific field data must
be obtained that can show the rate and extent of degrada-
tion or extraction. The existing knowledge base is limited,
and specific data are needed on more plants, contami-
nants, and climate conditions.
In addition, monitoring systems need to be standard-
ized. Currently there is no industry or research consensus
on which parameters are crucial to measure, and very few
projects can afford to sample, analyze, or monitor very
many parameters over the years needed for most
phytoremediation projects.
The EPAs Office of Research and Development (ORD)
and the Office of Solid Waste and Emergency Response
(OSWER) have several programs that investigate the effi-
cacy, risk, and cost of phytoremediation. These EPA ac-
tivities include EPA in-house laboratory research efforts,
support given to universities that are centers for
phytoremediation research, and joint EPA-private coopera-
tive efforts to field-test phytoremediation.
EPAs Office of Research and Development manages
various in-house research projects, and several EPA labo-
ratories have work underway to determine the fate of con-
taminants in phytoremediation applications. Steve
McCutcheon and Lee Wolfe at the EPA National Exposure
Research Laboratory (NERL) in Athens, GA, have explored
the degradation of TNT by wetland plants, and continue to
investigate plant enzyme and contaminant interactions.
Albert Venosa at the EPA National Risk Management Re-
search Laboratory (NRMRL) in Cincinnati, OH, is research-
ing the effect of plants on oil spills in salt and fresh water
wetlands. James Ryan, who is also at EPA-NRMRL, is
working with Rufus Chaney of USDA on using plants to
immobilize metals in soil. Richard Brenner, also at EPA-
NRMRL, is leading a team comparing the use of land farm-
ing and phytoremediation on the site of a former manufac-
tured gas facility. Harry Compton and George Prince of
the ERT are monitoring poplar tree plantings at Superfund
sites in MD and NJ (see Table 2-3). Larry Erickson's EPA-
supported Hazardous Substance Research Center at Kan-
sas State University has for many years sponsored re-
search and symposia on the interaction of plants and con-
taminants. Tom Wilson at EPA Region 10 continues to ex-
plore and encourage innovative applications and interac-
tions between phytoremediation and ecosystem restora-
tion.
The Superfund Innovative Technology Evaluation Pro-
gram (SITE) demonstrates field-ready technologies that
are initiated and installed by the developer of the technol-
ogy. SITE began evaluating phytoremediation projects in
1994. Currently four full demonstrations (including two at
Superfund sites), and one Emerging Program project have
been done or are underway using phytoremediation, coor-
dinated by Steve Rock at EPA-NRMRL in Cincinnati, OH.
Reports detailing the performance of the demonstrations
will be published at the conclusion of the field work. Infor-
mation on the SITE program or individual projects can be
found at http://www.epa.gov/ORD/SITE.
EPAs Office of Research and Development and the Of-
fice of Solid Waste and Emergency Response (OSWER)
jointly support the Remediation Technologies Development
Forum (RTDF). The RTDF was established in 1992 by the
EPA to foster collaboration between the public and private
sectors in developing innovative solutions to mutual haz-
12
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ardous waste problems. The RTDF has grown to include
partners from industry, several government agencies, and
academia who share the common goal of developing more
effective, less-costly hazardous waste characterization and
treatment technologies. There are currently seven RTDF
Action Teams, including the "Phytoremediation of Organics
Action Team."This Action Team was formed in early 1997,
and is currently comprised of three working groups that
are concerned with phytoremediation of three separate
pollution/matrix situations: petroleum compounds in shal-
low soils, chlorinated solvents in near-surface ground-
water, and the use of vegetation with high transpiration
rates as an alternative cap for hydraulic containment and/
or degradation of various pollutants. The Action Team
has held several meetings, and has regular conference
calls to select and implement field testing projects. Cur-
rent co-chairs in the subcommittees include representa-
tives from Chevron, Exxon, the Air Force, and Union
Carbide.
To access meeting and teleconference minutes, biblio-
graphic information on phytoremediation, and other infor-
mation, referto http://www.rtdf.org. The Technology Innova-
tion Office (TIO) within OSWER supports RTDF activities,
as well as other efforts aimed at bringing innovative site
characterization and treatment technologies to commercial-
ization. Further information on the Technology Innovation
Office and resources generated by TIO can be found at
http://www. clu-in. org.
In addition to EPA efforts, other Federal agencies, uni-
versities, consultants, and remediation contractors have
research underway in phytoremediation. All these projects
expand the knowledge base of what plants can be expected
to do consistently, and make the application of innovative
technologies more acceptable to regulators and consum-
ers.
Continuing research and policy discussions in the re-
lated areas of determining possible risk-based alternative
endpoints for cleanups, and measuring the intrinsic
remediative capacity of a site (natural attenuation) will im-
pact the applicability of many biological-based technolo-
gies, including plant-based systems.
Enhancements to the various phytoremediation pro-
cesses are continuing. Some applied research is directed
at selecting and breeding plants that have more of an at-
tractive quality such as hyperaccumulation of metal, pro-
duction of certain enzymes, and affinity or tolerance for
contaminants. Research continues in genetic engineering
of plants to combine positive traits, alter enzyme systems,
or increase a plant's natural range.
An engineering approach could be pursued by using ex-
isting plant traits as only a part of a remediation system of
combined planted systems and mechanical, thermal, or
chemical systems in treatment trains. Suggested combi-
nations include electrokinetics, bioventing, and surfactant
addition.
13
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Chapter 3
Evaluation of Phytoremediation Technologies
This chapter presents a literature review and evaluation
of the major phytoremediation processes or technologies.
The technologies presented represent the major, significant,
or widely studied forms of phytoremediation.
This chapter is divided into subsections that present defi-
nitions, mechanisms, site characteristics, applicable me-
dia, contaminants amenable to each process, and the as-
sociated concentrations where available. The advantages,
disadvantages, and current status of each process are also
discussed. Finally, an annotated reference list is included
at the end of the discussion of each process to provide
more detailed, specific information.
The purpose of this chapter is to provide site managers
with an overview of the various phytoremediation processes
as well as what can be expected from the process and its
limitations. Therefore, information on applicable contami-
nants/concentrations is included even though the informa-
tion may not be complete. Table 3-1 presents a summary of
the various phytoremediation processes.
3.1 Phytoextraction
3.1.1 Definition/Mechanism
Phytoextraction is the uptake of contaminants by plant
roots and translocation within the plants. Contaminants are
generally removed by harvesting the plants. This concen-
tration technology leaves a much smaller mass to be dis-
posed of than does excavation of the soil or other media.
This technology is most often applied to metal-contami-
nated soil as shown in Figure 3-1.
3.1.2 Media
Phytoextraction is primarily used in the treatment of soil,
sediments, and sludges. It can be used to a lesser extent
fortreatment of contaminated water.
3.1.3 Advantages
The plant biomass containing the extracted contaminant
can be a resource. For example, biomass that contains se-
lenium (Se), an essential nutrient, has been transported to
areas that are deficient in Se and used for animal feed
(Banuelos1997a).
3.1.4 Disadvantages
Phytoextraction has the following disadvantages:
• Metal hyperaccumulators are generally slow-growing with
a small biomass and shallow root systems.
• Plant biomass must be harvested and removed, fol-
lowed by metal reclamation or proper disposal of the
biomass. Hyperaccumulators may accumulate signifi-
cant metal concentrations — e.g., Thlaspirotund/folium
grown in a lead-zinc mine area contained 8,200 g/g Pb
(0.82%) and 17,300 g/g zinc (Zn) (1.73%), and Armeria
maritimavar. hallericontained 1,300 g/g Pb, dry weight
basis (Reeves and Brooks 1983).
• Metals may have a phytotoxic effect (Nanda Kumar et
al. 1995).
• Phytoextraction studies conducted using hydroponically-
grown plants, with the contaminant added in solution,
may not reflect actual conditions and results occurring
in soil. Phytoextraction coefficients measured under field
conditions are likely to be less than those determined
in the laboratory (Nanda Kumar etal. 1995).
3.1.5 Applicable Contaminants/
Concentrations
3.1.5.1 Applicable Contaminants
Constituents amenable to phytoextraction include:
• Metals: Ag, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Zn
The relative degree of uptake of different metals will
vary. Experimentally-determined phytoextraction coef-
ficients [ratio of g metal/g dry weight (DW) of shoot tog
metal/g DW of soil] for B. juncea (Nanda Kumar et al.
1995) indicate, for example, that lead was much more
difficult to take up than cadmium:
Metal
Cd2+
Ni2+
Cu2+
Pb2+
Cr*
Zn2+
Phvtoextraction Coefficient
58
52
31
7
1.7
0.1
17
Metalloids: As, Se
14
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Table 3-1. Phytoremediation Overview
Mechanism Process Goal
Media
Contaminants
Plants
Status
Phytoextraction
Rhizofiltration
Phytostabilization
Rhizodegradation
Phytodegradation
Phytovolatilization
Hydraulic control
(plume control)
Vegetative cover
(evapotranspiration
cover)
Riparian corridors
(non-point source
control)
Contaminant extraction
and capture
Contaminant extraction
and capture
Contaminant
containment
Contaminant
destruction
Contaminant destruction
Contaminant extraction
from media and release
to air
Contaminant degradation
or containment
Contaminant containment,
erosion control
Contaminant destruction
Soil, sediment,
sludges
Groundwater,
surface water
Soil, sediment,
sludges
Soil, sediment,
sludges,
groundwater,
Soil, sediment,
sludges,
groundwater
surface water
Groundwater,
soil, sediment,
sludges
Groundwater,
surface water
Soil, sludge,
sediments
Surface water,
groundwater
Metals: Ag, Cd, Co,
Cr, Cu, Hg, Mn, Mo, Ni,
Pb, Zn; Radionuclides:
90Sr, 137CS, 239PU, 238,234(J
Metals, radionuclides
As, Cd, Cr, Cu, Hs, Pb,
Zn
Organic compounds
(TPH, PAHs, pesticides
chlorinated solvents,
PCBs)
Organic compounds,
chlorinated solvents,
phenols, herbicides,
munitions
Chlorinated solvents,
some inorganics (Se,
Hg, and As)
Water-soluble organics
and inorganics
Organic and inorganic
compounds
Water-soluble organics
and inorganics
Indian mustard,
pennycress, alyssum
sunflowers, hybrid
poplars
Sunflowers, Indian
mustard, water
hyacinth
Indian mustard,
hybrid poplars,
grasses
Red mulberry,
grasses, hybrid
poplar, cattail, rice
Algae, stonewort,
hybrid poplar,
black willow, bald
cypress
Poplars, alfalfa
black locust,
Indian mustard
Hybrid poplar,
cottonwood, willow
Poplars, grasses
Poplars
Laboratory, pilot, and
field applications
Laboratory and pilot-
scale
Field application
Field application
Field demonstration
Laboratory and field
application
Field demonstration
Field application
Field application
• Radionuclides: 90Sr, 137Cs, 239Pu, 238|j,234U
• Nonmetals: B
• Organics: The accumulation of organics and subsequent
removal of biomass generally has not been examined
as a remedial strategy.
3.1.5.2 Contaminant Concentrations
Contaminated soil concentrations used in research
studies or found in field investigations are given below.
These are total metal concentrations; the mobile or avail-
able concentrations would be less.
• 1,250 mg/kg As (Pierzynski et al. 1994).
• 9.4 mg/kg Cd (Pierzynski et al. 1994).
• 11 mg/kg Cd (Pierzynski and Schwab 1992).
• 13.6 mg/kg Cd (Thlaspi caerulescens) (Baker et al.
1995).
2000 mg/kg Cd was used in studies of Cd uptake in
vegetables (Azadpourand Matthews, 1996).
110 mg/kg Pb (Pierzynski and Schwab 1992).
625 mg/kg Pb (Nanda Kumar et al. 1995).
40 mg/kg Se (Banuelos et al. 1997b).
444 mg/kg Zn (Thlaspicaerulescens) (Baker etal. 1995).
1,165 mg/kg Zn was suspected to have phytotoxic ef-
fects (Pierzynski and Schwab 1992).
Nanda Kumar et al. (1995) reported that the following
concentrations were not phytotoxic to Brassicajuncea
when added to soil mixtures:
2 mg/L Cd2+
50 mg/L Cr3+
3.5 mg/L Cr6*
10 mg/L Cu2+
100 mg/L Ni2+
500 mg/L Pb2+
100 mg/L Zn2+
15
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Physical Effects - Plant transpiration results in
contaminant being concentrated in plant
Contaminant uptake
Contaminant uptake
Figure 3-1. Phytoextraction.
The following solution concentrations were reported in
studies of phytoextraction that used hydroponically-grown
plants:
• Cd: Thlaspi caerulescens survived 63.2 M Cd without
evidence of chlorosis at 21 days in hydroponic solu-
tion, but was severely affected at 200 M (22 mg/L)
(Brown etal. 1995).
• Pb: 6,22,47, 98,188 mg/L. Root uptake of Pb became
saturated at Pb solution concentrations above 188 mg/
L (Nanda Kumar et al. 1995).
• Zn: Thlaspi caerulescens survived 3,160 M Zn without
evidence of chlorosis at 21 days in hydroponic solu-
tion, but was severely affected at 10,000 M (650 mg/L)
(Brown etal. 1995).
3.1.6 Root Depth
Phytoextraction is generally limited to the immediate zone
of influence of the roots; thus, root depth determines the
depth of effective phytoextraction. The root zones of most
metal accumulators are limited to the top foot of soil.
3.17 Plants
Hyperaccumulator plants are found in the Brassicaceae,
Euphorbiaceae, Asteraceae, Lamiaceae, orScrophulariaceae plant
families (Baker 1995). Examples include:
• Brassicajuncea (Indian mustard) - a high-biomass plant
that can accumulate Pb, Cr (VI), Cd, Cu, Ni, Zn, 90Sr,
B, and Se (Nanda Kumar et al. 1995; Salt et al. 1995;
Raskin etal. 1994). It has over 20 times the biomassof
Thlaspi caerulescens (Salt et al. 1995). Brassicas can
also accumulate metals. Of the different plant species
screened, B. juncea had the best ability to transport
lead to the shoots, accumulating >1.8% lead in the
shoots (dry weight). The plant species screened had
0.82 to 10.9% Pb in roots (with Brassicaspp. having
the highest), with the shoots having less Pb. Except for
sunflower (Helianthus annuus) and tobacco (Nicotiana
tabacum), other non-Brassica plants had phytoextraction
coefficients less than one. 106 B.y'unceacultivars var-
ied widely in their ability to accumulate Pb, with differ-
ent cultivars ranging from 0.04% to 3.5% Pb accumu-
lation in the shoots and 7 to 19% in the roots (Nanda
Kumar etal. 1995).
16
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• Thlaspi caerulescens (Alpine pennycress) for Ni and
Zn (Brown etal. 1994).
• Thlaspirotundifoliumssp. cepaeifolium, a noncrop Bras-
sica and one of the few Pb accumulators mentioned in
the literature (Nanda Kumar et al. 1995).
• AlyssumwulfenianumtorN\ (Reeves and Brooks 1983).
• Baker (1995) found 80 species of nickel-accumulating
plants in the Buxaceae (including boxwood) and
Euphoribiaceae (including cactus-like succulents) fami-
lies. Some euphorbs can accumulate up to 5% of their
dry weight in nickel.
• Indian mustard (Brassicajuncea) and canola (Brassica
napus) have been shown to accumulate Se and B. Kenaf
(Hibiscus cannabinus L. cv. Indian) and tall fescue
(Festuca arundinacea Schreb cv. Alta) also take up Se,
but to a lesser degree than canola (Banuelos et al.
1997b).
• Hybrid poplar trees were used in a field study in mine-
tailing wastes contaminated with As and Cd (Pierzynski
etal. 1994).
• Lambsquarter leaves had relatively higher As concen-
trations (14 mg/kg As) than other native plant or poplar
leaves (8 mg/kg) in mine-tailing wastes (Pierzynski et
al. 1994).
• Sunflowers took up Cs and Sr, with Cs remaining in the
roots and Sr moving into the shoots (Adler 1996).
• Metal accumulator plants such as the crop plants corn,
sorghum, and alfalfa may be more effective than
hyperaccumulators and remove a greater mass of met-
als due to their faster growth rate and larger biomass.
Additional study is needed to quantify contaminant re-
moval.
The number of taxonomic groups (taxa) of
hyperaccumulators varies according to which metal is
hyperaccumulated:
Metal Number of Taxonomic Groups of Hvperaccumulators
Ni >300
Co 26
Cu 24
Zn 18
Mn 8
Pb 5
Cd 1
3.1.8 Site Considerations
Because potentially toxic levels of metals can accumu-
late in the aboveground portion of the plant, access to the
plants must be controlled and plant debris must be moni-
tored more closely than with other phytoremediation tech-
nologies. Thus, care must be taken to restrict access of
browsing animals, and harvested plant material must be
properly disposed of.
3.1.8.1 Soil Conditions
Soil conditions must be appropriate for plant growth and
contaminant migration to the plant, yet not allow leaching of
the metals. The pH of the soil may need to be adjusted and/
orchelating agents may need to be added to increase plant
bioavailability and uptake of metals.
3.1.8.2 Ground and Surface Water
The primary considerations for phytoremediation in ground-
water are depth to groundwaterand depth to contamination
zone. Groundwater phytoremediation is essentially limited
to unconfined aquifers in which the water table depths are
within reach of plant roots.
3.1.8.3 Climatic Conditions
Hyperaccumulators are often found in specific geographic
locations and might not grow under other climatic condi-
tions.
3.1.9 Current Status
Both laboratory and field experiments have been con-
ducted. The first controlled field trial of Thlaspi caerulescens
in the UK was in 1994 (Moffat 1995). In this study, Thlaspi
caerulescens accumulated Zn and Cd to several percent
dry weight. A commercial operation, Phytotech, Inc., also
conducted field tests and small-scale field applications (in-
cluding the "Magic Marker" site in Trenton, NJ) with some
degree of success using Indian mustard (Brassicajuncea)
to remove lead from soil.
Plant selection, breeding, and genetic engineering forfast-
growing, high-biomass hyperaccumulators are active areas
of research. Information on uptake and translocation of met-
als has been assessed by Nellessen and Fletcher (1993a).
3.1.10 System Cost
The estimated 30-year costs (1998 dollars) for remediating
a 12-acre lead site are $12,000,000 for excavation and dis-
posal, $6,300,000 forsoil washing, $600,000 for a soil cap,
and $200,000 for phytoextraction (Cunningham 1996).
In a hypothetical study involving the remediation of a 20-
in.-thick layer of sediments contaminated with Cd, Zn, and
137Cs from a 1.2-acre chemical waste disposal pond,
phytoextraction cost was estimated to be about one-third
the cost of soil washing (Cornish etal. 1995).
Phytoextraction costs were estimated to be $60,000 to
$100,000 for remediation of one acre of 20-in.-thick sandy
loam, compared to a minimum of $400,000 for just excava-
tion and storage of this soil (Salt et al. 1995).
3.1.11 Selected References
Azadpour,A., and J. E. Matthews. 1996. Remediation of
Metal-Contaminated Sites Using Plants. Remed. Summer.
17
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In this literature review of research conducted on the
uptake of metals by plants, factors that affect metals
uptake are provided along with examples of plants ex-
amined for phytoextraction.
Chaney, R. L. 1983. Plant Uptake of Inorganic Waste Con-
stituents, pp. 50-76. In J. F. Parr, P. B. Marsh, and J. M. Kla
(eds.), Land Treatment of Hazardous Waste. Noyes Data
Corporation, Park Ridge, NJ.
This literature review of factors affecting metals uptake
by plants, metals tolerance, and metals impacts on plant
growth is written from the viewpoint of the impact of
land-applied waste on plants. It is also an early pro-
posal for the use of plants to remediate contaminated
sites.
Cornish, J. E., W C. Goldberg, R. S. Levine, and J. R.
Benemann. 1995. Phytoremediation of Soils Contaminated
with Toxic Elements and Radionuclides. pp. 55-63. In R. E.
Hinchee, J. L. Means, and D. R. Burris (eds.), Bioremediation
of Inorganics. Battelle Press, Columbus, OH.
This review focuses on the application of plants for
remediating U.S. Department of Energy sites contami-
nated with metals and radionuclides. It lists the con-
taminant ranges found at these sites, the phytotoxicity
threshold for the contaminants, and the number of
hyperaccumulating species for the contaminants. It
develops a hypothetical example to show the potential
cost savings of phytoextraction.
Nanda Kumar, P. B. A., V. Dushenkov, H. Motto, and I.
Raskin. 1995. Phytoextraction: The Use of Plants to Re-
move Heavy Metals from Soils. Environ. Sci. Technol.
29(5):1232-1238.
Experimental studies are described examining metals
uptake by a variety of plant species and different Bras-
s/cay'tynceacultivars. The experiments focused on lead
but also included other metals.
3.2 Rhizofiltration
3.2.1 Definition/Mechanism
Rhizofiltration is the adsorption or precipitation onto plant
roots, or absorption into the roots of contaminants that are
in solution surrounding the root zone, due to biotic or abiotic
processes. Plant uptake, concentration, and translocation
might occur, depending on the contaminant. Exudates from
the plant roots might cause precipitation of some metals.
Rhizofiltration first results in contaminant containment, in
which the contaminants are immobilized or accumulated on
or within the plant. Contaminants are then removed by physi-
cally removing the plant.
3.2.2 Media
Extracted groundwater, surface water, and waste water
can be treated using this technology. Rhizofiltration is gen-
erally applicable to low-concentration, high-water-content
conditions. This technology does not work well with soil,
sediments, or sludges because the contaminant needs to
be in solution in order to be sorbed to the plant system.
3.2.3 Advantages
Rhizofiltration has the following advantages:
• Eitherterrestrial or aquatic plants can be used. Although
terrestrial plants require support, such as a floating plat-
form, they generally remove more contaminants than
aquatic plants.
• This system can be either in situ (floating rafts on ponds)
or ex situ (an engineered tank system).
• An ex situ system can be placed anywhere because
the treatment does not have to be at the original loca-
tion of contamination.
3.2.4 Disadvantages
Rhizofiltration has the following disadvantages:
• The pH of the influent solution may have to be continu-
ally adjusted to obtain optimum metals uptake.
• The chemical speciation and interaction of all species
in the influent have to be understood and accounted
for.
• Awell-engineered system is required to control influent
concentration and flow rate.
• The plants (especially terrestrial plants) may have to
be grown in a greenhouse or nursery and then placed in
the rhizofiltration system.
• Periodic harvesting and plant disposal are required.
• Metal immobilization and uptake results from labora-
tory and greenhouse studies might not be achievable in
the field.
3.2.5 Applicable Contaminants/
Concentrations
Constituents amenable to phytoremediation include:
• Metals:
- Lead
(i) Pb2+ at a solution concentration of 2 mg/L, was
accumulated in Indian mustard roots with a bio-
accumulation coefficient of 563 after24 hours. Pb2+
(at solution concentrations of 35,70,150,300, and
500 mg/L) was accumulated in Indian mus-
tard roots, although root adsorption of Pb saturated
at 92 to 114 mg Pb/g DW root. Pb disappeared
from the 300- and 500-mg/L solutions due to pre-
cipitation of lead phosphate. Pb absorption by
roots was found to be rapid, although the amount
of time required to remove 50% of the Pb from
solution increased as the Pb concentration increased
(Dushenkov etal. 1995).
18
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(ii) Pb was accumulated in the roots of Indian mustard
(Brassica juncea) in water concentrations of ap-
proximately 20 to 2,000 g/L, with bioaccumulation
coefficients of 500 to 2,000 (Salt et al. 1997).
(iii) Pb at concentrations of 1 to 16 mg/L was accu-
mulated by water milfoil (Myriophyllum spicatum)
with a minimum residual concentration below 0.004
mg/L (Wang etal. 1996).
- Cadmium
Cd2+ (2 mg/L) was accumulated in Indian mustard
roots with a bioaccumulation coefficient of 134 after
24 hours (Dushenkov et al. 1995). Cd was accumu-
lated by the roots of Indian mustard (Brassica juncea)
in water concentrations of about 20 to 2,000 g/L, with
bioaccumulation coefficients of 500 to 2,000. The
seedlings removed 40 to 50% of the Cd within 24
hours at a biomass loading of 0.8 g dry weight/L so-
lution. The Cd went from 20 g/L to 9 g/L within 24
hours. After 45 hours, the Cd reached 1.4% in the
roots and 0.45% in the shoots. Cd saturation was
reached in the roots in 12 hours and in the shoots in
45 hours. Removal of competing ions in the solution
increased the uptake 47-fold (Salt et al. 1997). Cd at
concentrations of 1 to 16 mg/L was accumulated by
water milfoil (Myriophyllum spicatum) with a minimum
residual concentration of approximately 0.01 mg/L
(Wang etal. 1996).
- Copper
Cu2+ (6 mg/L) was accumulated in Indian mustard
roots with a bioaccumulation coefficient of 490 after
24 hours (Dushenkov et al. 1995). Cu at concentra-
tions of 1 to 16 mg/L was accumulated by water mil-
foil (Myriophyllumspicatum) with a minimum residual
concentration of approximately 0.01 mg/L (Wang et
al. 1996).
- Nickel
Ni2+ (10 mg/L) was accumulated in Indian mustard
roots with a bioaccumulation coefficient of 208 after
24 hours (Dushenkov et al. 1995). Ni was accumu-
lated by the roots of Indian mustard (Brassica juncea)
in water concentrations of about 20 to 2,000 g/L, with
bioaccumulation coefficients of 500 to 2,000 (Saltet
al. 1997). Ni at concentrations of 1 to 16 mg/L was
accumulated by water milfoil (Myriophyllum spicatum)
with a minimum residual concentration of approxi-
mately 0.01 mg/L (Wang et al. 1996).
- Zinc
Zn2+ (100 mg/L) was accumulated in Indian mustard
roots with a bioaccumulation coefficient of 131 after
24 hours (Dushenkov et al. 1995). Zn at concentra-
tions of 1 to 16 mg/L was accumulated by water mil-
foil (Myriophyllumspicatum) with a minimum residual
concentration of approximately 0.1 mg/L (Wang etal.
1996).
- Chromium
(i) Cr6* (4 mg/L) was accumulated in Indian mustard
roots with a bioaccumulation coefficient of 179
after 24 hours. The roots contained Cr3+, indicat-
ing reduction of Cr6* (Dushenkov et al. 1995).
(ii) Cr (VI) was accumulated by the roots of Indian
mustard (Brassica juncea) in water concentrations
of about 20 to 2000 g/L, with bioaccumulation co-
efficients of 100 to 250 (Salt et al. 1997).
Radionuclides:
- Uranium
U was studied using sunflowers in bench-scale and
pilot-scale engineered systems (Dushenkov et al.
1997).
Co = 56 g/L, reduced by >95% in 24 hours.
Co = 600 g/L, to 63 g/L in 1 hour, then down to 10 g/
L after 48 hours.
Co = 10, 30, 90, 810, or 2430 g/L with no signs of
phytotoxicity, and doubled their biomass.
Co = several hundred g/L, went to below regulatory
goal of 20 g/L.
Co = >1,000 g/L, could not reach 20 g/L goal; went
down to 40 to 70 g/L.
Average Co = 207 g/L, went to <20 g/L.
Influent concentrations at the field site were 21 to
874 g/L.
- Cesium
(i) Cs was used with sunflowers in bench-scale and
pilot-scale engineered systems (Dushenkov et
al. 1997). Co = 200 g/L, decreased noticeably after
6 hours, then went below 3 g/L after 24 hours.
(ii) Cs was accumulated in the roots of Indian mustard
(Brassica juncea) in water concentrations of ap-
proximately 20 to 2,000 g/L, with bioaccumulation
coefficients of 100 to 250 (Salt et al. 1997).
- Strontium
(i) Sr was used with sunflowers (Dushenkov et al.
1997). Co = 200 g/L, went to 35 g/L within 48 hours,
then down to 1 g/L by 96 hours.
(ii) Sr was accumulated in the roots of Indian mus-
tard (Brassica juncea) in water concentrations
of approximately 20 to 2,000 g/L (Salt et al. 1997).
19
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Rhizofiltration has not been evaluated for use with nutri-
ents or organics.
3.2.6 Root Depth
Rhizofiltration occurs within the root zone in water. For
rhizofiltration to occur, the water must come into contact
with the roots. Engineered systems can be designed to maxi-
mize this contact zone by matching the depth of the unit to
the depth of the roots. Groundwater may be extracted from
any depth and piped to an engineered hydroponic system
for ex-situ treatment. The depth of treatable groundwater is
a function of the extraction system, not the rhizofiltration
treatment system.
For in situ technologies, such as natural water bodies,
the depth of the roots might not be the same as the depth of
the water body. The water must be adequately circulated in
such cases to ensure complete treatment, which is likely to
become more difficult as the depth of the water increases.
3.2.7 Plants
The following are examples of plants used in rhizofiltration
systems:
• Terrestrial plants can be grown and used hydroponically
in rhizofiltration systems. These plants generally have
a greater biomass and longer, faster-growing root sys-
tems than aquatic plants (Dushenkovetal. 1995). Seed-
lings have been proposed for use instead of mature
plants because seedings do not require light or nutri-
ents for germination and growth for up to 2 weeks (Salt
etal. 1997).
• Under hydroponic conditions, 5 dicots (broadleaf crops),
3 monocots (cereals), 11 cool season grasses, and 6
warm season grasses were each effective in accumu-
lating Pb in their roots after three days of exposure to
300 mg/L Pb. The maximum lead concentration on a
dry weight basis was 17% in a cool season grass (colo-
nial bentgrass), and the minimum was 6% in a warm
season grass (Japanese lawngrass). The dicot Indian
mustard (Brassica juncea) was also effective in taking
up other metals (Dushenkovetal. 1995).
• Sunflowers (Helianthus annuus L.) removed concen-
trated Cr6*, Mn, Cd, Ni, Cu, U, Pb, Zn, and Sr in labora-
tory greenhouse studies (Salt et al. 1995). Sunflowers
also were more effective than Indian mustard (Brassica
juncea) and bean (Phaseolus coccineus) in removing
uranium. Bioaccumulation coefficients for uranium in
the sunflowers were much higher for the roots than for
the shoots (Dushenkovetal. 1997).
• At a field site in Chernobyl, Ukraine, sunflowers were
grown for 4 to 8 weeks in a floating raft on a pond.
Bioaccumulation results indicated that sunflowers could
remove 137Cs and 90Srfrom the pond.
• Aquatic plants have been used in water treatment, but
they are smaller and have smaller, slower-growing root
systems than terrestrial plants (Dushenkovetal. 1995).
Floating aquatic plants include water hyacinth
(Eichhornia crassipes), pennyworth (Hydrocotyle
umbellata), duckweed (Lemna minor), and water velvet
(Azollapinnata) (Saltetal. 1995).
• The floating aquatic plant water milfoil (Myriophyllum
spicatum), at a biomass density of 0.02 kg/L, rapidly
accumulated Ni, Cd, Cu, Zn, and Pb. The plant accu-
mulated up to 0.5% Ni, 0.8% Cd, 1.3% Cu, 1.3% Zn,
and 5.5% Pb by weight (Wang et al. 1996).
• Wetland plants can be used in engineered or constructed
beds to take up or degrade contaminants. Hydroponi-
cally-grown plants concentrated Pb, Cr(VI), Cd, Ni, Zn,
and Cu onto their roots from wastewater. Lead had the
highest bioaccumulation coefficient, and zinc the low-
est (Raskin et al. 1994).
3.2.8 Site Considerations
In situ applications in water bodies are not likely to repre-
sent a disturbance or limitation to the use of a site because
site activities generally do not occur in water.
3.2.8.1 Soil Conditions
Because this technology involves the hydroponic or
aquatic use of plants, soil use may be limited to raising
plants prior to installation. A layer of soil may be required on
a floating platform.
3.2.8.2 Ground and Surface Water
An ex-situ engineered system using rhizofiltration needs
to accommodate the predicted volume and discharge rate
of groundwater orsurface water. Groundwater and surface-
water chemistry must be assessed to determine the inter-
actions of the constituents in the water.
Groundwater must be extracted priorto rhizofiltration. Ex
situ rhizofiltration of groundwater orsurface water in an en-
gineered system might also require pretreatment of the in-
fluent. Pretreatment could include pH adjustment, removal
orsettling out of particulate matter, or other modification of
the water chemistry to improve the efficiency of rhizofiltration.
In situ applications such as the treatment of water bodies
might also require pretreatment, although this is likely to be
more difficult than with an engineered system due to the
potentially largerwatervolume and more complex configu-
ration.
3.2.8.3 Climatic Conditions
The amount of precipitation is not important in this tech-
nology because the plants are grown in water and often in
greenhouses. The treated media (water) supplies the water
requirements of the plants.
3.2.9 Current Status
Rhizofiltration applications are currently at the pilot-scale
stage.
20
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Scientists from Rutgers University and Phytotech, Inc.,
have conducted laboratory, greenhouse, and field pilot-scale
rhizofiltration studies. Phytotech tested a pilot-scale
rhizofiltration system in a greenhouse at a DOE uranium-
processing facility in Ashtabula, Ohio (Dushenkov et al.
1997). This engineered ex situ system used sunflowers to
remove uranium from contaminated groundwaterand/or pro-
cess water. Phytotech also conducted a small-scale field
test of rhizofiltration to remove radionuclides from a small
pond nearthe Chernobyl reactor, Ukraine, using sunflowers
floating on a raft.
The use of constructed wetlands for wastewater treat-
ment and/or acid mine drainage is a related technology that
has a significant history of research and application. Ex
situ rhizofiltration in engineered systems might be the
phytoremediation technology that most often uses traditional
engineering methods.
3.2.10 System Cost
The cost of removing radionuclides from water by using
sunflowers has been estimated to be $2 to $6 perthousand
gallons of water.
3.2.11 Selected References
Dushenkov, V., P. B. A. Nanda Kumar, H. Motto, and I.
Raskin. 1995. Rhizofiltration: The Use of Plants to Remove
Heavy Metals from Aqueous Streams. Environ. Sci. Technol.
29:1239-1245.
This study examined metals removal by roots of a vari-
ety of plant species. It provides bioaccumulation coefficents
and discusses the mechanisms of uptake. The study fo-
cuses on lead, but also provides information on other met-
als.
Dushenkov, S., D. Vasudev, Y. Kapulnik, D. Gleba, D.
Fleisher, K. C. Ting, and B. Ensley. 1997. Removal of Ura-
nium from Water Using Terrestrial Plants. Environ. Sci.
Technol. 31 (12):3468-3474.
This research included growth chamber, greenhouse, and
field-scale studies for remediation of uranium-contaminated
water. Continuous operation and optimization of an ex-situ
system were examined.
3.3 Phytostabilization
3.3.1 Definition/Mechanism
Phytostabilization is defined as (1) immobilization of a
contaminant in soil through absorption and accumulation
by roots, adsorption onto roots, or precipitation within the
root zone of plants, and (2) the use of plants and plant roots
to prevent contaminant migration via wind and water ero-
sion, leaching, and soil dispersion.
Phytostabilization occurs through root-zone microbiology
and chemistry, and/or alteration of the soil environment or
contaminant chemistry. Soil pH may be changed by plant
root exudates or through the production of CO2.
Phytostabilization can change metal solubility and mobility
or impact the dissociation of organic compounds. The plant-
affected soil environment can convert metals from a soluble
to an insoluble oxidation state (Salt et al. 1995).
Phytostabilization can occurthrough sorption, precipitation,
complexation, or metal valence reduction (EPA 1997a).
Plants can also be used to reduce the erosion of metal-
contaminated soil.
The term phytolignification has been used to refer to a
form of Phytostabilization in which organic compounds are
incorporated into plant lignin (Cunningham etal. 1995b). Com-
pounds can also be incorporated into humic material in soils
in a process likely related to Phytostabilization in its use of
plant material.
3.3.2 Media
Phytostabilization is used in the treatment of soil, sedi-
ments, and sludges.
3.3.3 Advantages
Phytostabilization has the following advantages:
• Soil removal is unnecessary.
• It has a lower cost and is less disruptive than other
more-vigorous soil remedial technologies.
• Revegetation enhances ecosystem restoration.
• Disposal of hazardous materials or biomass is not re-
quired.
3.3.4 Disadvantages
Phytostabilization has the following disadvantages:
• The contaminants remain in place. The vegetation and
soil may require long-term maintenance to prevent re-
release of the contaminants and future leaching.
• Vegetation may require extensive fertilization or soil
modification using amendments.
• Plant uptake of metals and translocation to the
aboveground portion must be avoided.
• The root zone, root exudates, contaminants, and soil
amendments must be monitored to prevent an increase
in metal solubility and leaching.
• Phytostabilization might be considered to only be an
interim measure.
• Contaminant stabilization might be due primarily to the
effects of soil amendments, with plants only contribut-
ing to stabilization by decreasing the amount of water
moving through the soil and by physically stabilizing
the soil against erosion.
3.3.5 Applicable Contaminants/
Concentrations
Phytostabilization has not generally been examined in
terms of organic contaminants. The following is a discus-
21
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sion of metals and metal concentrations, with implications
for phytostabilization:
• Arsenic: As (as arsenate) might be taken up by plants
because it is similar to the plant nutrient phosphate,
although poplar leaves in a field study did not accumu-
late significant amounts of As (Pierzynski et al. 1994).
Poplars were grown in soil containing an average of
1250 mg/kg As (Pierzynski et al. 1994).
• Cadmium: Cd might be taken up by plants because it
is similarto the plant nutrients Ca, Zn, although poplar
leaves in a field study did not accumulate significant
amounts of Cd (Pierzynski et al. 1994). Poplars were
grown in soil containing an average of 9.4 mg/kg Cd.
Plants were grown in mine waste containing up to 160
mg/kg Cd (Pierzynski et al. 1994).
• Chromium: Indian mustard (Brassicajuncea) might be
able to reduce Cr6* to Cr3+.
• Copper: Mine wastes containing copper were stabi-
lized by grasses (Salt et al. 1995).
• Mercury: Mercury might be one of the leading candi-
dates for the phytostabilization of metals, although ad-
ditional study is required (EPA 1997b).
• Lead: Pb in leachate was 22 g/mL in soil containing
Indian mustard (Brassicajuncea) compared to 740 u,g/
ml in soil without plants (Salt et al. 1995). Mine wastes
containing lead were stabilized by grasses (Salt et al.
1995). 625 u,g/g Pb was used in a sand-Perlite mixture
that supported Indian mustard (Brassicajuncea) (Salt
et al. 1995). Soil with 1660 mg/kg Pb had less than
50% plant cover. Plants in soil with 323 mg/kg Pb ex-
hibited heavy chlorosis. Plants were grown in mine
waste containing up to 4500 mg/kg (Pierzynski et al.
1994).
• Zinc: Mine wastes containing zinc were stabilized by
grasses (Salt et al. 1995). Soil with 4230 mg/kg Zn
had less than 50% plant cover. Plants in soil with 676
mg/kg Zn exhibited heavy chlorosis. Plants were grown
in mine waste containing up to 43,750 mg/kg Zn
(Pierzynski etal. 1994).
3.3.6 Root Depth
The root zone is the primary area affecting chemically-
moderated immobilization or root precipitation. Plants can
be selected fortheir root depth; for example, poplars can be
used for remediation of soil to a depth of 5 to 10 feet. The
impact of the roots may extend deeper into the soil, de-
pending on the transport of root exudates to lower soil depths.
3.3.7 Plants
Metal-tolerant plants are required for heavy-metal-contami-
nated soils. Brassica juncea has been shown to reduce
leaching of metals from soil by over 98% (Raskin et al.
1994).
The following grasses have been used to reduce metals
leaching (Salt et al. 1995):
• Colonial bentgrass (Agrostis tenuis cv Goginan) for acid
lead and zinc mine wastes.
• Colonial bentgrass (Agrostis tenuis cv Parys) for cop-
per mine wastes.
• Red fescue (Festuca rubra cv Merlin) for calcareous
lead and zinc mine wastes.
Native and tame grasses and leguminous forbs including
big bluestem (AndropogongerardiVA.), tall fescue (Festuca
amncf/naceaSchreb.), and soybean [Glycinemax(L.) Merr.]
were studied to determine their effectiveness in remediating
mine wastes (Pierzynski et al. 1994). In addition, hybrid
poplars were evaluated in a field study at a Superfund site
to determine their metal tolerance (Pierzynski et al. 1994).
3.3.5 Site Considerations
Plants used will require long-term maintenance if site-
specific constraints prohibit reversal of the stabilization pro-
cess.
3.3.8.1 Soil Conditions
Phytostabilization might be most appropriate for heavy-
textured soils and soils with high organic matter content
(Cunningham et al. 1995a). Phytostabilization can be per-
formed after more active soil treatment technologies have
been tried. "Hot spots" of higher contaminant concentra-
tions can be excavated and treated using other technolo-
gies, or landfilled. Soil amendments can also be used to
stabilize metals in soils. Amendments should be selected
that will maximize the growth of vegetation, which then also
helps to phytostabilize the soil (Berti and Cunningham, 1997).
3.3.8.2 Ground and Surface Water
Soil water content, which can affect redox conditions in
the soil, must be appropriate for plant growth.
3.3.8.3 Climatic Conditions
As discussed in Chapter 4, plans for remedial activities
must take into account the fact that phytoremediation sys-
tems can be severely impacted by weather conditions.
3.3.9 Current Status
The following are examples of typical phytostabilization
studies:
• Land affected by mining activities has been revegetated
with potentially useful plants. For example, a stabilizing
cover of vegetation was successfully established on
metalliferous mine wastes in the United Kingdom (Salt
etal. 1995).
• Phytostabilization using metal-tolerant grasses is be-
ing investigated for large areas of Cd- and Zn-contami-
nated soils at a Superfund site in Palmerton, PA. Ex-
perimental plots of poplars have been studied at the
22
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Whitewood Creek Superfund site, SD, and vegetative
remediation has been proposed as part of the
remediation at the Galena Superfund site in southeast-
ern KS (Pierzynski et al. 1994).
• The IINERT (In-Place Inactivation and Natural Eco-
logical Restoration Technologies) Soil-Metals Action
team coordinated by EPA's Jim Ryan and Dupont's
Bill Berti under the RTDF program has used plants to
physically stabilize metal-contaminated soil in orderto
decrease the off-site movement of contaminants.
• Researchers at Kansas State University and Montana
State University, among others, are actively examin-
ing the use of vegetation in reclaiming sites contami-
nated by mining wastes.
3.3.10 System Cost
Cropping system costs have been estimated at $200 to
$1 0,000 per hectare, equivalent to $0.02 to $1 .00 per cubic
meter of soil, based on a 1 -meter root depth (Cunningham
etal. 1995b).
3.3.11 Selected References
Azadpour,A., and J. E. Matthews. 1996. Remediation of
Metal-Contaminated Sites Using Plants. Remed. Summer.
This is a literature review of factors that affect metals
uptake by plants. It discusses plant tolerance to heavy
metals and summarizes work done on the use of plants
in soils that contain high levels of metal.
Cunningham, S. D., W. R. Berti, and J. W. Huang. 1995b.
Remediation of Contaminated Soils and Sludges by Green
Plants, pp. 33-54. In R.E. Hinchee, J. L. Means, and D. R.
Burris (eds.), Bioremediation of Inorganics. Battelle Press,
Columbus, OH.
The chemistry of metals is discussed in this paper,
with a focus on lead. The article examines the stabili-
zation and bioavailability of lead using sequential ex-
tractions. Phytoextraction of metals and
phytoremediation of organic contaminants are also dis-
cussed.
Pierzynski, G. M., J. L. Schnoor, M. K. Banks, J. C. Tracy,
L. A. Licht, and L. E. Erickson. 1994. Vegetative Remedia-
tion at Superfund Sites. Mining and Its Environ. Impact
(Royal Soc. Chem. Issues in Environ. Sci. Technol. 1). pp.
49-69.
This paper discusses in detail the chemical and mi-
crobiological aspects of metal-contaminated soils. Two
case studies of the phytoremediation of mine waste
sites are presented along with a modeling discussion
of the fate of heavy metal in vegetated soils.
Salt, D. E., M. Blaylock, P. B. A. Nanda Kumar, V.
Dushenkov, B. D. Ensley, I. Chet, and I. Raskin. 1995. Phy-
toremediation: A Novel Strategy for the Removal of Toxic
Metals from the Environment Using Plants. Biotechnol.
13:468-474.
This article is an introduction to the use of
phytoremediation technologies for reducing metals con-
tamination. Field research is presented on the use of
plants to immobilize metals in soils. Bioavailability is-
sues and mechanisms of plant accumulation are dis-
cussed in detail.
3.4 Rhizodegradation
3.4.1 Definition/Mechanism
Rhizodegradation is the breakdown of an organic contami-
nant in soil through microbial activity that is enhanced by
the presence of the root zone (Figure 3-2). Rhizodegradation
is also known as plant-assisted degradation, plant-assisted
bioremediation, plant-aided in situ biodegradation, and en-
hanced rhizosphere biodegradation.
Root-zone biodegradation is the mechanism for implement-
ing rhizodegradation. Root exudates are compounds pro-
duced by plants and released from plant roots. They include
sugars, amino acids, organic acids, fatty acids, sterols,
growth factors, nucleotides, flavanones, enzymes, and other
compounds (Shimp et al. 1993; Schnoor et al. 1995a). The
microbial populations and activity in the rhizosphere can be
increased due to the presence of these exudates, and can
result in increased organic contaminant biodegradation in
the soil. Additionally, the rhizosphere substantially increases
the surface area where active microbial degradation can be
stimulated. Degradation of the exudates can lead to
cometabolism of contaminants in the rhizosphere.
Plant roots can affect soil conditions by increasing soil
aeration and moderating soil moisture content, thereby cre-
ating conditions more favorable for biodegradation by indig-
enous microorganisms. Thus, increased biodegradation
could occur even in the absence of root exudates. One study
raised the possibility that transpiration due to alfalfa plants
drew methane from a saturated methanogeniczone up into
the vadose zone where the methane was used by
methanotrophs that cometabolically degraded TCE
(Narayanan et al. 1995).
The chemical and physical effects of the exudates and
any associated increase in microbial populations might
change the soil pH or affect the contaminants in other ways.
3.4.2 Media
3.4.3 Advantages
Rhizodegradation has the following advantages:
• Contaminant destruction occurs in situ.
• Translocation of the compound to the plant or atmo-
sphere is less likely than with other phytoremediation
technologies since degradation occurs at the source of
the contamination.
• Mineralization of the contaminant can occur.
23
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Enhanced rhizosphere biodegradation
- Supply of nutrients, cometabolites
- Transport and retention of water
- Aeration
Soil dessication
Root respiration
Root intrusion
Sloughing
Enzymes
dehalogenase
nitroductase
Uptake
Figure 3-2. Rhizodegradation.
• Low installation and maintenance cost as compared
to other remedial options.
3.4.4 Disadvantages
Rhizodegradation has the following disadvantages:
• Development of an extensive root zone is likely to re-
quire substantial time.
• Root depth can be limited due to the physical structure
or moisture conditions of the soil.
• The rhizosphere might effect an increase in the initial
rate of degradation compared to a nonrhizosphere soil,
but the final extent or degree of degradation might be
similar in both rhizosphere and nonrhizosphere soil.
• Plant uptake can occur for many of the contaminants
that have been studied. Laboratory and field studies
need to account for other loss and phytoremediation
mechanisms that might complicate the interpretation
of rhizodegradation. For example, if plant uptake oc-
curs, phytodegradation or phytovolatilization could oc-
cur in addition to rhizodegradation.
• The plants need additional fertilization because of mi-
crobial competition for nutrients.
• The exudates might stimulate microorganisms that are
not degraders, at the expense of degraders.
• Organic matter from the plants may be used as a car-
bon source instead of the contaminant, which could
decrease the amount of contaminant biodegradation. In
laboratory sediment columns, debris from the salt marsh
plant Spartina alterniflora decreased the amount of oil
biodegradation. This could have been due to competi-
tion for limited oxygen and nutrients between the indig-
enous oil-degrading microorganisms and the microor-
ganisms degrading plant organic matter (Molina et al.
1995).
3.4.5 Applicable Contaminants/
Concentrations
The following contaminants are amenable to
rhizodegradation:
• TPH (total petroleum hydrocarbons)
- Several field sites contaminated with crude oil, diesel,
a heavier oil, and other petroleum products were stud-
ied for phytoremediation by examining TPH disappear-
ance. Rhizodegradation and humification were the most
important disappearance mechanisms, with little plant
uptake occurring. Phytoremediation was able to bring
TPH levels to below the plateau level found with normal
(non-plant-influenced) bioremediation (Schwab 1998).
- High initial petroleum hydrocarbon contents (2,000
to 40,000 mg/kg TPH) were studied at several field
24
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sites. Plant growth varied by species, but the pres-
ence of some species led to significantly greaterTPH
disappearance than with other species or in
unvegetated soil (Schwab 1998).
PAHs (polycyclic aromatic hydrocarbons)
- Chrysene, benzo(a)anthracene, benzo(a)pyrene, and
dibenzo(a,h)anthracene had greaterdisappearance
in vegetated soil than in nonvegetated soil (Aprill and
Sims 1990).
- Anthracene and pyrene had greaterdisappearance
in vegetated soils than in unvegetated soil (Reilley
etal. 1996).
- Pyrene was mineralized at a greater rate in a planted
system than in an unplanted system (Ferro et al.
1994a).
- Pyrene at 150 mg/kg was used in an experiment
with crested wheatgrass (Ferro et al. 1994b).
- Anthracene and pyrene at 100 mg/kg were used in
a study with grasses and a legume (Reilley et al.
1996).
- 10 mg/kg PAH (chrysene, benzo(a)anthracene,
benzo(a)pyrene, dibenzo(a,h)anthracene) had
greater disappearance in vegetated soil than in
nonvegetated soil (Aprill and Sims 1990).
- PAHs at 1,450 to 16,700 mg/kg (in soil also con-
taminated with PCP) strongly inhibited germination
and growth of eight species of grasses (Pivetz et al.
1997).
BTEX (Benzene, toluene, ethylbenzene, and xylenes)
- Soil from the rhizosphere of poplar trees had higher
populations of benzene-, toluene-, and o-xylene-de
grading bacteria than did nonrhizosphere soil. Root
exudates contained readily biodegradable organic
macromolecules (Jordahl et al. 1997).
Pesticides
- Atrazine, metolachlor, andtrifluralin herbicides: Soil
from the rhizosphere had increased degradation
rates compared to nonrhizosphere soil. The experi-
ments were conducted in the absence of plants to
minimize effects of root uptake (Anderson et al.
1994).
- Parathion and diazinon organophosphate insecti-
cides: Mineralization rates of the radiolabeled com-
pounds were higher in rhizosphere soil (soil with
roots) than in nonrhizosphere soil (soil without roots).
Diazinon mineralization in soil without roots did not
increase when an exudate solution was added, but
parathion mineralization did increase (Hsu and
Barthal 979).
- Propanil herbicide: An increased number of gram-
negative bacteria were found in rhizosphere soil. It
was hypothesized that the best propanil degraders
would benefit from the proximity to plant roots and
exudates (Hoagland et al. 1994).
- 2,4-D herbicide: Microorganisms capable of degrad-
ing 2,4-D occurred in elevated numbers in the rhizo
sphere of sugar cane, compared to nonrhizosphere
soil (Sandmann and Loos 1984). The rate constants
for 2,4-D biodegradation were higher in rhizosphere
soil than in nonrhizosphere soil (Boyle and Shann
1995).
- 2,4,5-T herbicide: The rate constants for2,4,5-T bio-
degradation were higher in rhizosphere soil than in
nonrhizosphere soil (Boyle and Shann 1995).
- Increased degradation of 0.3g/g trifluralin, 0.5g/g
atrazine, and 9.6 g/g metolachlor occurred in rhizo-
sphere soil compared to nonrhizosphere soil (Ander-
son etal. 1994).
- Parathion and diazinon at 5 g/g had greater mineral-
ization in rhizosphere soil than in nonrhizosphere soil
(Hsu and Barthal 979).
- Rhizosphere soil with 3 g/g propanil had increased
numbers of gram-negative bacteria that could rap-
idly transform propanil (Hoagland etal. 1994).
Chlorinated solvents
- Greater TCE mineralization was measured in veg-
etated soil as compared to nonvegetated soil (Ander-
son and Walton 1995).
- TCE and TCA dissipation was possibly aided by
rhizosphere biodegradation enhanced by the plant
roots (Narayanan et al. 1995).
- TCE at 100and200g/Lingroundwaterwas used in
a soil and groundwater system (Narayanan et al.
1995).
- TCAat50 and 100 g/L in groundwater was used in
a soil and groundwater system (Narayanan et al. 1995).
PCP (pentachlorophenol)
- PCP was mineralized at a greater rate in a planted
system than in an unplanted system (Ferro et al.
1994b).
- 100 mg PCP/kg soil was used in an experiment with
hycrest crested wheatgrass [Agropyron desertorum
(Fisher ex Link) Schultes] (Ferro et al. 1994b).
- Proso millet (Panicum miliaceum L.) seeds treated
with a PCP-degrading bacterium germinated and
grew well in soil containing 175 mg/L PCP, compared
to untreated seeds (Pfender 1996).
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- PCP at 400 to 4100 mg/kg (in soil also contaminated
with PAHs) strongly inhibited germination and growth
of eight species of grasses (Pivetz et al. 1997).
• PCBs (polychlorinated biphenyls)
- Compounds (such as flavonoids and coumarins)
found in leachate from roots of specific plants stimu-
lated the growth of PCB-degrading bacteria (Donnelly
et al. 1994; Gilbert and Crowley 1997).
• Surfactants
- Linear alkylbenzene sulfonate (LAS) and linear al-
cohol ethoxylate (LAE) had greater mineralization
rates in the presence of root microorganisms than
in nonrhizosphere sediments (Federle and Schwab
1989).
- LAS and LAE at 1 mg/L had greater mineralization
rates in the presence of root microorganisms than
in nonrhizosphere sediments (Federle and Schwab
1989).
3.4.6 Root Depth
Because the rhizosphere extends only about 1 mm from
the root and initially the volume of soil occupied by roots is
a small fraction of the total soil volume, the soil volume
initially affected by the rhizosphere is limited. With time,
however, new roots will penetrate more of the soil volume
and other roots will decompose, resulting in additional exu-
dates to the rhizosphere. Thus, the extent of rhizodegradation
will increase with time and with additional root growth. The
effect of rhizodegradation might extend slightly deeperthan
the root zone. If the exudates are water soluble, not strongly
sorbed, and not quickly degraded, they may move deeper
into the soil. Contaminated groundwatercan be affected if it
is within the influence of roots.
3.4.7 Plants
Plants that produce exudates that have been shown to
stimulate growth of degrading microorganisms orstimulate
cometabolism will be of more benefit than plants without
such directly useful exudates. The type, amount, and effec-
tiveness of exudates and enzymes produced by a plant's
roots will vary between species and even within subspe-
cies or varieties of one species.
The following are examples of plants capable of
rhizodegradation:
• Red mulberry (Moms rubra L.), crabapple [Malus fusca
(Raf.) Schneid], and osage orange [Madura pomifera
(Raf.) Schneid] produced exudates containing relatively
high levels of phenolic compounds, at concentrations
capable of stimulating growth of PCB-degrading bacte-
ria (Fletcher and Hegde 1995).
• Spearmint (Mentha spicata) extracts contained a com-
pound that induced cometabolism of a PCB (Gilbert
and Crowley 1997).
Alfalfa (Medicago saliva) appears to have contributed
to the dissipation of TCE and TCA through exudates on
soil bacteria (Narayanan etal. 1995).
A legume [Lespedeza cuneata (Dumont)], Loblolly pine
[Pinus taeda (L.)], and soybean [Glycine max(L.) Merr.,
cv Davis] increased TCE mineralization compared to
nonvegetated soil (Anderson and Walton 1995).
At a Gulf Coast field site, the use of annual rye and St.
Augustine grass led to greaterTPH disappearance af-
ter 21 months than that experienced with the use of
sorghum or an unvegetated plot (Schwab 1998).
At one field site, although white clover did not survive
the second winter, concentrations of TPH were reduced
more than with tall fescue or bermudagrass with annual
rye, or bare field (Schwab 1998).
PAH degradation occurred through the use of the fol-
lowing mix of prairie grasses: big bluestem (Andropogon
gerardi), little bluestem (Schizachyrium scoparius),
Indiangrass (Sorghastrumnutans), switchgrass (Pani-
cum virgatum), Canada wild rye (Elymus canadensis),
western wheatgrass (Agropyron smithil), side oats grama
(Bouteloua curtipendula), and blue grama (Bouteloua
gracilis) (Aprill and Sims 1990).
Fescue (Festuca arundinacea Schreb), a cool-season
grass; sudangrass (Sorghum vulgare L.) and switch-
grass (Panicum virgatumL), warm-season grasses; and
alfalfa (Medicago saliva L.), a legume, were used to
study PAH disappearance; greater disappearance was
seen in the vegetated soils than in unvegetated soils
(Reilley etal. 1996).
Hycrest crested wheatgrass [Agropyron desertorum
(Fischer ex Link) Schultes] increased mineralization
rates of PCP and pyrene relative to unplanted controls
(Ferroetal. 1994a, 1994b).
In PAH- and PCP-contaminated soil, a mix of fescues
[hard fescue (Festuca ovina var. duriuscula), tall fes-
cue (Festuca arundinacea), and red fescue (Festuca
rubra)] had higher germination rates and greater biom-
ass relative to controls than did a mix of wheatgrasses
[western wheatgrass (Agropyron smithii) and slender
wheatgrass (Agropyron trachycaulum)] and a mix of little
bluestem (Andropogon scoparius), Indiangrass
(Sorghastrum nutans), and switchgrass (Panicum
virgatum) (Pivetz et al. 1997).
Bush bean (Phaseolus vulgarism. "TenderGreen") rhizo-
sphere soil had higher parathion and diazinon mineral-
ization rates than nonrhizosphere soil (Hsu and Bartha
1979).
Rice (Oryza saliva L.) rhizosphere soil had increased
numbers of gram-negative bacteria, which were able to
rapidly transform propanil (Hoagland etal. 1994).
• Kochiasp. rhizosphere soil increased the degradation
of herbicides relative to nonrhizosphere soil (Anderson
etal. 1994).
26
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• Cattail (Typha latifolia) root microorganisms produced
greater mineralization rates of LAS and LAE than did
nonrhizosphere sediments (Federle and Schwab
1989).
• Hybrid poplar tree (Populus deltoidesX nigra DN-34,
Imperial Carolina) rhizosphere soil contained signifi-
cantly higher populations of total heterotrophs,
denitrifiers, pseudomonads, BTXdegraders, and atra-
zine degraders than did nonrhizosphere soil (Jordahl et
al. 1997).
3.4.8 Site Considerations
3.4.8.1 Soil Conditions
The physical and chemical soil conditions must allow for
significant root penetration and growth.
3.4.8.2 Ground and Surface Water
Although rhizodegradation is primarily soil-based, ground-
water movement can be induced by the transpiration of plants
bringing contaminants from the groundwater into the root
zone.
3.4.8.3 Climatic Conditions
Field studies that include rhizodegradation as a compo-
nent have been conducted under in a wide variety of cli-
mates including the humid south, arid west, and the cold
north.
3.4.9 Current Status
The following list provides information on the status or ap-
plication of rhizodegradation studies:
• Rhizodegradation was first extensively studied in relation
to the biodegradation of pesticides in agricultural soils.
• Numerous laboratory and greenhouse studies and sev-
eral field studies have been conducted, including a field
study conducted at the McCormick & Baxter Superfund
Site.
• "Hot spots" of higher contaminant concentrations can be
excavated and treated using other technologies, or
landfilled. Rhizodegradation could be applied as a polish-
ing orfinal step after active land treatment bioremediation
has ended.
• ATPH/PAH subgroup has been established as part of
the RTDF Phytoremediation of Organics Action Team to
examine rhizodegradation. The Petroleum Environmental
Research Forum is also examining rhizodegradation in
the phytoremediation of petroleum hydrocarbons.
3.4.10 System Cost
Cost information for rhizodegradation is incomplete at this
time.
3.4.11 Selected References
Anderson, T A., and J. R. Coats (eds.). 1994. Bioremediation
Through Rhizosphere Technology, ACS Symposium Series,
Volume 563. American Chemical Society, Washington, DC.
249 pp.
This is a collection of 17 articles examining
rhizodegradation. The papers introduce the concepts
involved in rhizodegradation; discuss interactions be-
tween microorganisms, plants, and chemicals; and pro-
vide examples of rhizodegradation of industrial chemi-
cals and pesticides.
Anderson, T. A., E. A. Guthrie, and B. T. Walton. 1993.
Bioremediation in the Rhizosphere. Environ. Sci. Technol.
27:2630-2636.
This literature review summarizes research work con-
ducted on a variety of contaminants (pesticides, chlori-
nated solvents, petroleum products, and surfactants).
Anderson, T. A., and B. T. Walton. 1995. Comparative Fate
of [14c]trichloroethylene in the Root Zone of Plants from a
Former Solvent Disposal Site. Environ. Toxicol. Chem.
14:2041-2047.
Exposure chambers within an environmental chamber
were used with a variety of plant types and with radiola-
beled TCE. Mineralization rates were greater in vegetated
soils than in unvegetated soils.
Aprill, W, and R. C. Sims. 1990. Evaluation of the Use of
Prairie Grasses for Stimulating Polycyclic Aromatic Hydro-
carbon Treatment in Soil. Chemosphere. 20:253-265.
Eight prairie grasses were examined using chambers
constructed of 25-cm-diameterPVC pipe. PAH-spiked
soil at 10 mg PAH/kg soil was added to the chambers
priorto seeding. Soil, leachate, and plant tissue samples
were collected during the study. PAH disappearance was
greater in planted chambers compared to unplanted
chambers.
Ferro,A. M., R. C. Sims, and B. Bugbee. 1994a. Hycrest
Crested Wheatgrass Accelerates the Degradation of Pen-
tachlorophenol in Soil. J. Environ. Qual. 23:272-279.
Agrowth-chamberstudy conducted using radiolabeled
pentachlorophenol indicated that mineralization was
greater in planted systems than in unplanted systems.
Fletcher, J. S.,and R. S. Hegde. 1995. Release of Phe-
nols by Perennial Plant Roots and their Potential Impor-
tance in Bioremediation. Chemosphere. 31:3009-3016.
Greenhouse studies identified chemical and microbio-
logical evidence forthe occurrence of rhizodegradation.
The potential for biodegradation within the root zone
was determined to be dependent on the particular plant
species and exudates produced by the plant.
Schnoor, J. L, L. A. Licht, S. C. McCutcheon, N. L. Wolfe,
and L. H. Carreira. 1995a. Phytoremediation of Organic and
Nutrient Contaminants. Environ. Sci. Technol. 29:318A-323A.
27
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This paper introduces the important concepts for
rhizodegradation and phytodegradation, including the
role of plant enzymes. Laboratory and field research for
TNT, pesticides, and nutrient contaminants is summa-
rized. Applications and limitations of phytoremediation
are discussed, and field applications of phytoremediation
are tabulated.
Schwab, A. P. 1998. Phytoremediation of Soils Contami-
nated with PAHs and Other Petroleum Compounds. Pre-
sented at: Beneficial Effects of Vegetation in Contaminated
Soils Workshop, Kansas State University, Manhattan, KS,
January 7-9, 1998. Sponsored by Great Plains/Rocky
Mountain Hazardous Substance Research Center.
This presentation summarizes the methods and re-
sults of field test plots at a variety of geographic and
climatic regions. Dissipation of TPH was greater in
planted plots than in unplanted plots, and differences
were seen in the growth and effectiveness of different
plant species.
3.5 Phytodegradation
3.5.1 Definition/Mechanism
Phytodegradation (also known as phytotransformation)
is the breakdown of contaminants taken up by plants
through metabolic processes within the plant, orthe break-
down of contaminants external to the plant through the
effect of compounds (such as enzymes) produced by the
plants. As shown in Figure 3-3, the main mechanism is
plant uptake and metabolism. Additionally, degradation may
occur outside the plant, due to the release of compounds
that cause transformation. Any degradation caused by mi-
croorganisms associated with or affected by the plant root
is considered rhizodegradation.
3.5.1.1 Uptake
For phytodegradation to occur within the plant, the com-
pounds must be taken up by the plant. One study identi-
fied more than 70 organic chemicals representing many
classes of compounds that were taken up and accumu-
lated by 88 species of plants and trees (Paterson et al.
1990). A database has been established to review the
classes of chemicals and types of plants that have been
investigated in regard to their uptake of organic compounds
(Nellessen and Fletcher 1993b).
Uptake is dependent on hydrophobicity, solubility, and
polarity. Moderately hydrophobic organic compounds (with
log kow between 0.5 and 3.0) are most readily taken up by
and translocated within plants. Very soluble compounds
(with low sorption) will not be sorbed onto roots or translo-
cated within the plant (Schnoor et al. 1995a). Hydrophobic
(lipophilic) compounds can be bound to root surfaces or
partitioned into roots, but cannot be further translocated
within the plant (Schnoor et al. 1995a; Cunningham et al.
1997). Nonpolar molecules with molecular weights <500
will sorb to the root surfaces, whereas polar molecules will
enter the root and be translocated (Bell 1992).
Plant uptake of organic compounds can also depend on
type of plant, age of contaminant, and many other physi-
cal and chemical characteristics of the soil. Definitive con-
clusions cannot always be made about a particular chemi-
cal. For example, when PCP was spiked into soil, 21%
was found in roots and 15% in shoots after 155 days in the
presence of grass (Qiu et al. 1994); in another study, sev-
eral plants showed minimal uptake of PCP (Bellin and
O'Connor 1990).
3.5.1.2 Metabolism
Metabolism within plants has been identified for a diverse
group of organic compounds, including the herbicide atra-
zine (Burken and Schnoor 1997), the chlorinated solventTCE
(Newman et al. 1997a), and the munition TNT (Thompson et
al. 1998). Other metabolized compounds include the insecti-
cide DDT, the fungicide hexachlorobenzene (HCB), PCP, the
plasticizerdiethylhexylphthalate (DEHP), and PCBs in plant
cell cultures (Komossa et al. 1995).
3.5.1.3 Plant-Formed Enzymes
Plant-formed enzymes have been identified for their po-
tential use in degrading contaminants such as munitions,
herbicides, and chlorinated solvents. Immunoassay tests
have been used to identify plants that produce these en-
zymes (McCutcheon 1996).
3.5.2 Media
Phytodegradation is used in the treatment of soil, sedi-
ments, sludges, and groundwater. Surface water can also
be remediated using phytodegradation.
3.5.3 Advantages
Contaminant degradation due to enzymes produced by a
plant can occur in an environment free of microorganisms
(for example, an environment in which the microorganisms
have been killed by high contaminant levels). Plants are
able to grow in sterile soil and also in soil that has concen-
tration levels that are toxic to microorganisms. Thus,
phytodegradation potentially could occur in soils where bio-
degradation cannot.
3.5.4 Disadvantages
Phytodegradation has the following disadvantages:
• Toxic intermediates or degradation products may form.
In a study unrelated to phytoremediation research, PCP
was metabolized to the potential mutagen
tetrachlorocatechol in wheat plants and cell cultures
(Komossa et al. 1995).
• The presence or identity of metabolites within a
plant might be difficult to determine; thus con-
taminant destruction could be difficult to confirm.
3.5.5 Applicable Contaminants/
Concentrations
Organic compounds are the main category of con-
taminants subject to phytodegradation. In general,
28
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Phytodegradation
- Metabolism within tie plant
• Production of the dehalogenase and oxygenase enzymes,
which help catalyze degradation
Contaminant uptake
Figure 3-3. Phytodegradation.
organic compounds with a log kow between 0.5 and 3.0 can
be subject to phytodegradation within the plant. Inorganic
nutrients are also remediated through plant uptake and me-
tabolism. Phytodegradation outside the plant does not de-
pend on log kow and plant uptake.
3.5.5.1 Organics
• Chlorinated solvents
- The plant-formed enzyme dehalogenase, which
can dechlorinate chlorinated compounds, has
been discovered in sediments (McCutcheon
1996).
- TCE was metabolized to trichloroethanol, trichloro-
acetic acid, and dichloroacetic acid within hybrid
poplar trees (Newman et al. 1997a). In a similar
study, hybrid poplar trees were exposed to wa-
ter containing about 50 ppm TCE and metabo-
lized the TCE within the tree (Newman et al. 1997a).
- Minced horseradish roots successfully treated waste-
water containing up to 850 ppm of 2,4-dichlorophenol
(Dec and Bollag 1994).
Herbicides
- Atrazine in soil was taken up by trees and then hy-
drolyzed and dealkylated within the roots, stems, and
leaves. Metabolites were identified within the plant
tissue, and a review of atrazine metabolite toxicity
studies indicated that the metabolites were less toxic
than atrazine (Burken and Schnoor 1997).
- The plant-formed enzyme nitrilase, which can de-
grade herbicides, has been discovered in sedi-
ments (Carreira 1996).
29
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- A qualitative study indicated that the herbicide
bentazon was degraded within black willow
trees, as indicated by bentazon loss during a nurs
ery study and by identification of metabolites
within the tree. Bentazon was phytotoxicto six tree
species at concentrations of 1000 and 2000 mg/L.
At 150 mg/kg, bentazon metabolites were detected
within tree trunk and canopy tissue samples (Con-
ger and Portier 1997).
- Atrazine at 60.4 g/kg (equivalent to about 3 times
field application rates) was used to study phytodeg-
radation in hybrid poplars (Burken and Schnoor
1997).
- The herbicide bentazon was phytotoxic at concen-
trations of 1,000 to 2,000 mg/L, but allowed growth
at 150 mg/L (Conger and Portier 1997).
• Insecticides
- The isolation from plants of the enzyme phos-
phatase, which can degrade organophosphate
insecticides,may have phyotodegradation ap-
plications (McCutcheon 1996).
• Munitions
- The plant-formed enzyme nitroreductase, which
can degrade munitions, has been discovered in
sediments; this enzyme, from parrot feather, de-
graded TNT (McCutcheon 1996).
- Hybrid poplar trees metabolized TNT to 4-amino-
2,6-dinitrotoluene (4-ADNT), 2-amino-4,6-
dinitrotoluene (2-ADNT), and other unidentified
compounds (Thompson et al. 1998).
- TNT concentrations in flooded soil decreased from
128 to 10 ppm with parrot feather (Schnoor et
al. 1995b).
• Phenols
- Chlorinated phenolic concentrations in wastewa-
ter decreased in the presence of oxidoreductase
enzymes in minced horseradish roots (Dec and
Bollag 1994).
3.5.5.2 Inorganics
• Nutrients
- Nitrate will be taken up by plants and transformed to
proteins and nitrogen gas (Licht and Schnoor 1993).
3.5.6 Root Depth
Phytodegradation is generally limited to the root zone,
and possibly below the root zone if root exudates are
soluble, nonsorbed, and transported below the root zone.
The degree to which this occurs is uncertain.
3.5.7 Plants
The aquatic plant parrot feather (Myriophyllum
aquaticum) and the algae stonewort (Nitella) have been
used for the degradation of TNT. The nitroreductase en-
zyme has also been identified in other algae, ferns, mono-
cots, dicots, and trees (McCutcheon 1996).
Degradation of TCE has been detected in hybrid pop-
lars and in poplar cell cultures, resulting in production of
metabolites and in complete mineralization of a small por-
tion of the applied TCE (Gordon et al. 1997; Newman et
al. 1997a). Atrazine degradation has also been confirmed
in hybrid poplars (Populus deltoides x nigra DN34, Impe-
rial Carolina) (Burken and Schnoor 1997). Poplars have
also been used to remove nutrients from groundwater (Licht
and Schnoor 1993).
Black willow (Salix nigra), yellow poplar (Liriodendron
tulipifera), bald cypress (Taxodium distichum), river birch
(Betula nigra), cherry bark oak (Quercus falcata), and live
oak (Quercus viginiana) were able to support some deg-
radation of the herbicide bentazon (Conger and Portier
1997).
3.5.8 Site Considerations
3.5.8.1 Soil Conditions
Phytodegradation is most appropriate for large areas of
soil having shallow contamination.
3.5.8.2 Ground and Surface Water
Groundwater that can be extracted by tree roots or that
is pumped to the surface may be treated by this system.
Phytodegradation can also occur in surface water, if the
water is able to support the growth of appropriate plants.
3.5.8.3 Climatic Conditions
Phytoremediation studies involving phytodegradation
have been conducted under a wide variety of climatic con-
ditions.
3.5.9 Current Status
Research and pilot-scale studies have been conducted
primarily at Army Ammunition Plants (AAPs). These dem-
onstrations include field studies at the Iowa AAP, Volun-
teer AAP, and Milan AAP (McCutcheon 1996).
3.5.10 System Costs
Cost information has not been reported.
3.5.11 Selected References
Bell, R. M. 1992. Higher Plant Accumulation of Organic
Pollutants from Soils. Risk Reduction Engineering Labo-
ratory, Cincinnati, OH. EPA/600/R-92/138.
This paper includes an extensive literature review of the
behavior of organic contaminants in plant-soil systems
and the uptake of contaminants by plant. A wide variety
of plant species and contaminant types are covered in
30
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the paper. Tables and graphs in the reviewed literature
provide quantitative information on plant uptake. Experi-
ments conducted on plant uptake of hexachlorobenzene,
phenol, toluene, and TCE are described in depth.
Burken, J. G., and J. L. Schnoor. 1997. Uptake and Me-
tabolism ofAtrazine by PoplarTrees. Environ. Sci. Technol.
31:1399-1406.
This presentation describes poplartrees grown in soil or
sand that took up, hydrolyzed, and dealkylated radiola-
beled atrazineto less-toxic compounds. Metabolism was
found to occur in roots, stems, and leaves, and the
amount of metabolism increased with increased time in
plant tissue. In leaves, the atrazine parent compound
was found to be 21% of the radiolabel at 50 days, and
10% at 80 days. In the sand planting, uptake of the ra-
diolabel was 27.8% at 52 days and 29.2% at 80 days.
Less than 20% of radiolabel remained as bound residue
in plant tissue. Atrazine degradation in unplanted soil
was similar to degradation in planted soil. A model for
atrazine metabolism was also presented.
McCutcheon, S. 1996. Phytoremediation of Organic Com-
pounds: Science Validation and Field Testing. In W. W.
Kovalick and R. Olexsey (eds.), Workshop on
Phytoremediation of Organic Wastes, December 17-19,
1996, Ft. Worth, TX. An EPA unpublished meeting summary.
An overview of the uses, advantages, and disadvantages
of phytoremediation are presented along with the identi-
fication and use of plant-derived enzymes for
photodegradation. Field demonstrations at several Army
Ammunition Plants are also discussed.
Newman, L. A., S. E. Strand, N. Choe, J. Duffy, G. Ekuan,
M. Ruszaj, B. B. Shurtleff, J. Wilmoth, P. Heilman, and M.
P. Gordon. 1997a. Uptake and Biotransformation ofTrichlo-
roethylene by Hybrid Poplars. Environ. Sci. Technol.
31:1062-1067.
Discussions are presented of axenic poplar tumor cell
cultures dosed with TCE and samples that were ana-
lyzed for degradation products and 14CO2. The cells stud-
ied metabolized TCE to trichloroethanol and di- and
trichloracetic acid. The cell cultures oxidized 1 to 2% of
the TCE to CO2 in 4 days. Whole trees were exposed to
50 ppm TCE. Leaves were bagged and the entrapped
air sampled for TCE. Plant parts were harvested and
analyzed for TCE and metabolites. TCE-exposed trees
had significant TCE in stems but minimal amounts in
leaves. Equal concentrations of trichloroethanol and TCE
were found in leaves, but a smaller concentration of
trichloroethanol than TCE was found in stems. Trichlo-
roacetic acid appeared in stems and leaves. Roots con-
tained TCE, trichloroacetic acid, dichloroacetic acid, and
trichloroethanol. TCE was transpired from the trees.
Paterson, S., D. Mackay, D. Tarn, and W. Y. Shiu. 1990.
Uptake of Organic Chemicals by Plants: A Review of Pro-
cesses, Correlations and Models. Chemosphere. 21:297-
331.
The routes of entry (root uptake and foliar uptake) of
organic compounds into plants are discussed. Equations
are presented that correlate the concentration in vari-
ous parts of a plant to the octanol-water partition coeffi-
cient, molecular weight, or Henry's Law constant. A re-
view of plant uptake models is also included. Crossed-
referenced tables are included that identify the literature
citations for plant uptake research conducted on differ-
ent plant species and on different chemical compounds.
Thompson, P. L., L. A. Ramer, and J. L. Schnoor. 1998.
Uptake and Transformation of TNT by Hybrid PoplarTrees.
Environ. Sci. Technol. 32:975-980.
In these laboratory experiments, hybrid poplars and ra-
diolabeled TNT were used in hydroponic and soil sys-
tems. Much of the TNT was bound in the roots, with rela-
tive little (<10%) translocation within the tree. Metabo-
lites of TNT were found within the plant tissue.
3.6 Phytovolatilization
3.6.1 Definition/Mechanism
Phytovolatilization (Figure 3-4) is the uptake and tran-
spiration of a contaminant by a plant, with release of the
contaminant or a modified form of the contaminant to the
atmosphere from the plant through contaminant uptake,
plant metabolism, and plant transpiration.
Phytodegradation is a related phytoremediation process
that can occur along with Phytovolatilization.
3.6.2 Media
Phytovolatilization has mainly been applied to ground-
water, but it can be applied to soil, sediments, and slud-
ges.
3.6.3 Advantages
Phytovolatilization has the following advantages:
• Contaminants could be transformed to less-toxic forms,
such as elemental mercury and dimethyl selenite gas.
• Contaminants or metabolites released to the atmosphere
might be subject to more effective or rapid natural deg-
radation processes such as photodegradation.
3.6.4 Disadvantages
Phytovolatilization has the following disadvantages:
• The contaminant or a hazardous metabolite (such as
vinyl chloride formed from TCE) might be released into
the atmosphere. One study indicated TCE transpira-
tion, but other studies found no transpiration.
• The contaminant or a hazardous metabolite might ac-
cumulate in vegetation and be passed on in later prod-
ucts such as fruit or lumber. Low levels of metabolites
have been found in plant tissue (Newman etal. 1997a).
31
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Physical effects
Transpiration of volatile compounds or their metabolic products
Transpiration of
volatiles and H2O
Contaminant uptake
Figure 3-4. Phytovolatilization.
3.6.5 Applicable Contaminants/
Concentrations
3.6.5.1 Organics
Chlorinated solvents include TCE, 1,1,1 -trichloroethane
(TCA) and carbon tetrachloride (Newman et al. 1997a,
1997b; Narayanan et al. 1995). In two years, hybrid pop-
lars removed >97% of the 50-ppm TCE from the water
(Newman etal. 1997b). 100 and 200 u,g/LTCE in ground-
water was studied using alfalfa (Narayanan et al. 1995).
50 and 100 u,g/LTCE in groundwaterwere studied using
alfalfa (Narayanan et al. 1995). In one year, 95% of 50-
ppm carbon tetrachloride was removed by hybrid poplars
(Newman etal. 1997b).
3.6.5.2 Inorganics
The inorganic contaminants Se and Hg, along with
As, can form volatile methylated species (Pierzynski et
al. 1994). Selenium has been taken up and transpired
at groundwater concentrations of 100 to 500 u,g/L
(Banuelos et al. 1997a) and at soil concentrations of 40
mg/L (Banuelos et al. 1997b). Genetically engineered
plants were able to germinate and grow in 20-ppm Hg++
and then volatilize the Hg; 5 to 20 ppm Hg++ was phyto-
toxic to unaltered plants (Meagher and Rugh 1996).
3.6.6 Root Depth
The contaminant has to be within the influence of the
root of the plant. Since groundwater is the target me-
32
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dia, contaminated groundwater upgradient of the plants
may flow into the area of influence of the plants. Con-
taminated water may also be pumped and watered on
plants.
3.6.7 Plants
Plants used for phytovolatilization include:
• University of Washington researchers have extensively
studied the use of poplars in the phytoremediation of
chlorinated solvents. In these studies, transformation
of TCE was found to occur within the trees (Newman
etal. 1997a).
• Alfalfa (Medicago sativa) has been studied by Kansas
State University researchers for its role in the
phytovolatilization of TCE.
• Black locust species were studied for use in
remediating TCE in groundwater (Newman et al.
1997b).
• Indian mustard (Brassica juncea) and canola (Bras-
sica napus) have been used in the phytovolatilization
of Se. Selenium (as selenate) was converted to less-
toxic dimethyl selenite gas and released to the atmo-
sphere (Adler 1996). Kenaf (Hibiscus cannabinus L.
cv. Indian) and tall fescue (Festuca arundinacea
Schreb cv. Alta) have also been used to take up Se,
but to a lesser degree than canola (Banuelos et al.
1997b).
• A weed from the mustard family (Arabidopsis thaliana)
genetically modified to include a gene for mercuric re-
ductase converted mercuric salts to metallic mercury
and released it to the atmosphere (Meagherand Rugh
1996).
3.6.8 Site Considerations
Because phytovolatilization involves the transfer of con-
taminants to the atmosphere, the impact of this contami-
nant transfer on the ecosystem and on human health needs
to be addressed.
3.6.8.1 Soil Conditions
For significant transpiration to occur, the soil must be
able to transmit sufficient water to the plant.
3.6.8.2 Ground and Surface Water
Groundwater must be within the influence of the plant
(usually a tree) roots.
3.6.8.3 Climatic Conditions
Climatic factors such as temperature, precipitation, hu-
midity, insolation, and wind velocity can affect transpiration
rates.
3.6.9 Current Status
Several research groups are performing active labora-
tory and field studies of TCE phytovolatilization and other
chlorinated solvents. A SITE demonstration project has
been started at the Carswell Site, Fort Worth, TX using
poplars to phytoremediate TCE-contaminated groundwa-
ter and to examine the possible fate of the TCE, including
volatilization.
A significant amount of research, including field testing
and application, has been conducted on selenium volatil-
ization.
3.6.10 System Costs
Cost information is being collected as part of the SITE
demonstration project at the Carswell Site.
3.6.11 Selected References
Banuelos, G. S., H. A. Ajwa, N. Terry, and S. Downey.
1997a. Abstract: Phytoremediation of Selenium-Laden Ef-
fluent. Fourth International In Situ and On-Site
Bioremediation Symposium, April 28 - May 1, 1997, New
Orleans, LA. 3:303.
This abstract summarizes the methods used in field in-
vestigations of the use of Brassica napus (canola) to
remediate water contaminated with selenium. These field
studies included an investigation of the volatilization of
selenium by the plants.
Banuelos, G. S., H. A. Ajwa, B. Mackey, L. L. Wu, C.
Cook, S. Akohoue, and S. Zambrzuski. 1997b. Evaluation
of Different Plant Species Used for Phytoremediation of
High Soil Selenium. J. Environ. Qual. 26:639-646.
This evaluation discusses three plant species (canola,
kenaf, and tall fescue) grown in seleniferous soil un-
der greenhouse conditions. Total soil selenium was
significantly reduced by each species. A partial mass
balance indicated that some selenium was lost by a
mechanism that was not measured. Selenium volatil-
ization was hypothesized as the cause of the decrease
in soil concentration.
Meagher, R. B., and C. Rugh. 1996. Abstract: Phytore-
mediation of Mercury Pollution Using a Modified Bacterial
Mercuric Ion ReductaseGene. International Phytoremediation
Conference, May 8-10, 1996, Arlington, VA. International
Business Communications, Southborough, MA.
This abstract describes transgenic plants developed
to reduce mercuric ion to metallic mercury, which was
then volatilized, and additional plants developed to
process methyl mercury to metallic mercury.
Newman, L. A., S. E. Strand, N. Choe, J. Duffy, G. Ekuan,
M. Ruszaj, B. B. Shurtleff, J. Wilmoth, P. Heilman, and M.
P. Gordon. 1997a. Uptake and Biotransformation ofTrichlo-
roethylene by Hybrid Poplars. Environ. Sci. Technol.
31:1062-1067.
Whole trees were exposed to 50 ppm TCE and bags
were placed around leaves. Analysis of the entrapped
air indicated that TCE was transpired from the trees.
33
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Newman, L. A., C. Bod, N. Choe, R. Crampton, R.
Cortellucci, D. Domroes, S. Doty, J. Duffy, G. Ekuan, D.
Fogel, R. Hashmonay, P. Heilman, D. Martin, I.A.
Muiznieks, T. Newman, M. Ruszaj, T. Shang, B. Shurtleff,
S. Stanley, S. E. Strand, X. Wang, J. Wilmoth, M. Yost,
and M. P. Gordon. 1997b. Abstract: Phytoremediation of
Trichloroethylene and Carbon Tetrachloride: Results from
Bench to Field. Presentation 55. In 12th Annual Confer-
ence on Hazardous Waste Research -Abstracts Book, May
19-22, 1997, Kansas City, MO.
Axenic poplar cell culture, metabolic chamber-grown
rooted cuttings, and pilot-scale systems for the
phytoremediation of TCE and carbon tetrachloride are
briefly described. Oxidation of TCE, update and transpi-
ration of TCE and carbon tetrachloride, and removal of
TCE and carbon tetrachloride from water under field
conditions are discussed.
3.7 Hydraulic Control
3.7.1 Definition/Mechanism
Hydraulic control is the use of plants to remove ground-
water through uptake and consumption in order to contain
or control the migration of contaminants (Figure 3-5). Hy-
draulic control is also known as phytohydraulics or hydraulic
plume control.
3.7.2 Media
Hydraulic control is used in the treatment of groundwa-
ter, surface water, and soil water.
3.7.3 Advantages
Hydraulic control has the following advantages:
• An engineered pump-and-treat system does not need
to be installed.
• Costs will be lower.
• Roots will penetrate into and be in contact with a much
greater volume of soil than if a pumping well is used.
3.7.4 Disadvantages
Hydraulic control has the following disadvantages:
• Water uptake by plants is affected by climatic and sea-
sonal conditions; thus, the rate of water uptake will not
be constant. Water uptake by deciduous trees will slow
considerably during winter.
• Groundwater removal is limited by the root depth of
the vegetation.
Figure 3-5. Hydraulic control of contaminated plume.
34
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3.7.5 Applicable Contaminants/
Concentrations
Water-soluble leachable organics and inorganics are
used at concentrations that are not phytotoxic. Poplar trees
were used to form a barrier to groundwater movement at a
site contaminated with gasoline and diesel (Nelson 1996).
3.7.6 Root Depth
Hydraulic control by plants occurs within the root zone
or within a depth influenced by roots, for example:
• The effective rooting depth of most crops is 1 to 4 feet.
Trees and other vegetation can be used to remediate
groundwater in water table depths of 30 feet or less
(Gatliff 1994).
• Plant roots above the water table can influence con-
taminants in the groundwater by interfacing through
the capillary fringe. Fe, Tc, U, and P diffused upward
from the water table and were absorbed by barley roots
that were 10 cm (3.9 in) above the water table inter-
face (Sheppard and Evenden 1985).
• The placement depth of roots during planting can be
varied. Root depth, early tree growth, and nitrogen ac-
cumulation were enhanced by placing poplar tree root
balls closer to shallow groundwater during planting
(Gatliff 1994).
3.7.7 Plants
The following plants are used in hydraulic control:
• Cottonwood and hybrid poplar trees were used at
seven sites in the East and Midwest to contain and
treat shallow groundwater contaminated with heavy
metals, nutrients, or pesticides (Gatliff 1994). Poplars
were used at a site in Utah to contain groundwater
contaminated with gasoline and diesel (Nelson 1996).
Passive gradient control was studied at the French
Limited Superfund site using a variety of phreatophyte
trees; native nondeciduous trees were found to per-
form the best (Sloan and Woodward 1996).
3.7.5 Site Considerations
The establishment of trees or other vegetation is likely
to require a larger area than would be required for the in-
stallation of a pumping well.
3.7.8.1 Soil Conditions
The primary considerations for selecting hydraulic con-
trol as the method of choice are the depth and concentra-
tion of contaminants that affect plant growth. Soil texture
and degree of saturation are influential factors. Planting tech-
nique and materials can extend the influence of plants
through non-saturated zones to water-bearing layers.
3.7.8.2 Ground and Surface Water
The amount of water transpired by a tree depends on
many factors, especially the size of the tree. Some esti-
mates of the rate of water withdrawal by plants are given
below.
• Poplartrees on a landfill in Oregon transpired 70 acre-
inches of water per acre of trees (Wright and Roe
1996).
• Two 40-foot-tall cottonwood trees in southwestern Ohio
pumped 50 to 350 gallons perday (gpd) pertree, based
on calculations using observed water-table drawdown
(Gatliff 1994).
• A 5-year-old poplar tree can transpire between 100
and 200 L water per day (Newman et al. 1997a).
• Young poplars were estimated to transpire about 8 gpd
pertree, based on the observed watertable drawdown
(Nelson 1996).
• Mature phreatophyte trees were estimated to use 200
to 400 gpd (Sloan and Woodward 1996).
3.7.8.3 Climatic Conditions
The amount of precipitation, temperature, and wind may
affect the transpiration rate of vegetation.
3.7.9 Current Status
Several U.S. companies have installed phytoremediation
systems that have successfully incorporated hydraulic con-
trol.
3.7.70 System Cost
Estimated costs for remediating an unspecified contami-
nant in a 20-foot-deep aquifer at a 1 -acre site were $660,00
for conventional pump-and-treat, and $250,000 for
phytoremediation using trees (Gatliff 1994).
3.7.11 Selected References
Gatliff, E. G. 1994. Vegetative Remediation Process Of-
fers Advantages Over Traditional Pump-and-Treat Tech-
nologies. Remed. Summer. 4(3):343-352.
A summary is presented of the impact of poplar or cot-
tonwood trees to influence a shallow water table at sites
along the East Coast and in the Midwest that were con-
taminated with pesticides, nutrients, or heavy metals. The
contribution of the trees to water table drawdown was
measured at some sites. Information is presented on the
decrease in contaminant concentrations at some of the
sites.
Wright, A. G., and A. Roe. 1996. It's Back to Nature for
Waste Cleanup. ENR. July 15. pp. 28-29.
A poplar tree system for landfill leachate collection and
treatment is described. The trees use up to 70 inches of
water per acre per year. A proposed project at another
landfill is presented.
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3.8 Vegetative Cover Systems
3.8.1 Definition/Mechanism
A vegetative cover is a long-term, self-sustaining sys-
tem of plants growing in and/or over materials that pose
environmental risk; a vegetative cover may reduce that
risk to an acceptable level and, generally, requires mini-
mal maintenance. There are two types of vegetative cov-
ers: the Evapotranspiration (ET) Cover and the
Phytoremediation Cover.
• Evapotranspiration Cover: A cover composed of soil
and plants engineered to maximize the available stor-
age capacity of soil, evaporation rates, and transpira-
tion processes of plants to minimize water infiltration.
The evapotranspiration cap is a form of hydraulic con-
trol by plants. Risk reduction relies on the isolation of
contaminants to prevent human or wildlife exposure
and the reduction of leachate formation or movement.
Fundamentally, an ET cover is a layer of monolithic
soil with adequate soil thickness to retain infiltrated
water until it is removed by evaporation and transpira-
tion mechanisms. Mechanisms include the uptake and
storage of water in soil and vegetation. An ET cover is
one type of a water-balance cover, illustrated in Fig-
ure 3-6.
• Phytoremediation Cover: A cover consisting of soil and
plants to minimize infiltration of water and to aid in the
degradation of underlying waste. Risk reduction relies on
the degradation of contaminants, the isolation of contami-
nants to prevent human or wildlife exposure, and the re-
duction of leachate formation or movement. Mechanisms
include water uptake, root-zone microbiology, and plant
metabolism. The phytoremediation cover incorporates cer-
tain aspects of hydraulic control, phytodegradation,
rhizodegradation, phytovolatilization, and perhaps
phytoextraction. Figure 3-7 presents the evolution of a
phytoremediation cover as it moves from a remediation func-
tion to a water exclusion function.
In limited cases, vegetative covers may be used as an
alternative to traditional covers that employ a resistive bar-
rier (i.e., a multilayered cover with a relatively imperme-
able component). Vegetative covers may be appropriate
to address contaminated surface soil or sludge, certain
waste disposal units, waste piles, and surface impound-
ments.
In general, the application of any cover system should
provide the following functions:
• isolate underlying waste from direct human or wildlife
exposure (e.g., prevent burrowing animals from reach-
ing the contaminants);
• minimize the percolation of water into the underlying
waste;
Vegetative grasses
and legumes soil = 6"
Clay
Waste up to 60'
Conventional Cover
Root
I system f|j|
Vegetative Cover
Figure 3-6. Illustration of an Evapotranspiration (ET) cover.
36
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0
2'
4'
8'—
60' C
Waste
T=0
Trees planted
T=1
Tree roots penetrate waste
Remediation
T= Mature
Soil created
Water balance established
Figure 3-7.
• achieve long-term performance and minimize mainte-
nance needs (e.g., control surface water runoff and
reduce soil erosion); and
• prevent the migration or release of significant quanti-
ties of gas produced.
The acceptability of vegetative covers as a final cover
for certain waste disposal units, such as landfill cells, is
dependent on applicable regulatory requirements (e.g.,
RCRA). EPA's minimum technical requirements for landfill
cover systems have evolved within a framework referred
to as the "liquids management strategy." The two primary
objectives of the strategy are: (1) to minimize leachate for-
mation by keeping liquids out of the landfill (orsource area);
and (2) to detect, collect, and remove the leachate that is
generated (EPA, 1987, 1991). A vegetative cover must
demonstrate equivalent performance with generic cover
designs specified in EPA guidance [i.e., Design and Con-
struction of RCRA/CERCLA Final Covers (EPA/625/4-91/
025); Design, Operation, and Closure of Municipal Solid
Waste Landfills (EPA/625/R-94/008); and Technical Guid-
ance For RCRA/CERCLA Final Covers (EPA/OSWER
Draft)].
Vegetative covers are not appropriate for certain landfill
units, such as municipal solid waste (MSW) landfills, that
generate gas in chronic, large, or uncontrolled amounts.
As reported by Flower et al. (1981), landfill gases can be
toxic to plants and therefore must be considered. To date,
vegetative cover systems have not been shown to prevent
the diffusion of gases from landfills. Gas emissions from
MSW landfills are governed by two sets of regulations.
• 40 CFR §258.23, under RCRA Subtitle D, addresses
the personal and fire/explosion safety aspects of landfill
gas.
• New Source Performance Standards (NSPS) and
Emissions Guidelines (EG) promulgated under the
Clean AirAct (CAA), 40 CFR Part 60 Subparts Cc and
WWW,, regulate emissions of non-methane organic
compounds (NMOCs) as a surrogate to total landfill
gas emissions.
3.8.2 Media
ET and phytoremediation covers are used in the uptake
of infiltrating surface water. A phytoremediation cover can
37
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also be used in the treatment of soil, sludge, and sedi-
ments.
3.8.3 Advantages
A vegetative cover may have the following advantages:
• May reduce maintenance needs and requirements,
such as minimizing surface erosion by establishing a
self-sustainable ecosystem.
• The use of vegetative covers is generally considered
cost-effective, as evaluated in the Alternative Landfill
Cover Demonstration (Dwyer, 1997a) for an ET cover.
• Vegetation has been shown to be an effective final layer
for hazardous waste site covers (EPA 1983; McAneny
etal. 1985).
• Vegetation may encourage aerobic microbial activity
in the root zone; such activity could discourage forma-
tion of anaerobic landfill gases or degrade them.
• Phytoremediation covers have the potential to enhance
the biodegradation of contaminants in soils, sludges,
and sediments.
3.8.4 Disadvantages
A vegetative cover may have any or all of the following
disadvantages:
• Proper long-term inspection and maintenance may be
required to ensure appropriate plant cover. Natural suc-
cession of plants may lead to a predominance by plant
species other than those originally planted as part of
the cover.
• Surface water may have a tendency to follow macropores
opened by decaying roots and consequently flow down-
ward to underlying waste orgroundwater.
• For a phytoremediation cover, contaminants may be
taken up by plants intended or used for human, domes-
tic animal, or wild animal consumption, and potential
adverse effects on the food chain could occur.
• Most plant based cover designs will be effective only in
a specific climate. Universally applicable designs may
not be possible.
• If trees planted as part of a vegetative cover are toppled
by wind, buried waste may be exposed.
• Most alternative cover designs do not contain and col-
lect landfill gas.
3.8.5 Applicable Contaminants/
Concentrations
3.8.5.1 Organics and Inorganics
• Evapotranspiration Cover: The concentration of the
contaminants in the underlying material is not a con-
cern, as long as the plants are not in contact with ma-
terials having phytotoxic concentrations.
• Phytoremediation Cover: Contaminants in the waste
materials should not be at phytotoxic levels because
for degradation to occur, the plant roots need to be in
contact with the contaminated waste.
3.8.6 Root Depth
For an evapotranspiration cover, the depth of the under-
lying waste is generally not a factor because the mecha-
nisms (i.e, water evaporation, transpiration, and storage)
occur above the waste.
The effective depth of contaminant degradation for the
phytoremediation cover is the root depth of the plants.
3.5.7 Plants
Poplartrees and grasses have been used commercially
to construct vegetative covers. Ideally, the vegetation se-
lected for the system should be a mixture of native plants
and consist of warm- and cool-season species.
3.8.8 Site Considerations
Several factors should be evaluated when considering
the use of a vegetative cover such as soil physical proper-
ties, plant community activities, the potential for gas pro-
duction from the biodegradation of waste, and climatic vari-
ables (e.g., precipitation quantity, type, intensity, and sea-
sonality, temperature, humidity).
3.8.8.1 Soil Conditions
Soils most suitable for a vegetative cover should have a
high water storage capacity. The soil should be a high mix-
ture of clays and silts (e.g., fine-grain soils). Soils with rapid
drainage are to be avoided, although a carefully designed
and maintained cover may include a coarser-grained ma-
terial.
3.8.8.2 Ground and Surface Water
Water tables that are relatively high may result in soils
with less available water storage capacity, if evaporation
and transpiration processes are not sufficient. However,
with an appropriate thickness of soil to provide a sufficient
water storage capacity, the water table may not be a factor
in the performance of the cover.
3.8.8.3 Climatic Conditions
Areas with high precipitation rates require more water to
be transpired or stored in the soil. In humid regions (i.e.,
more than 20 inches of annual precipitation), inadequate
evapotranspiration may occur seasonally, and soil layers
will need to be thickerthan in arid and semi-arid regions to
provide adequate water storage capacity.
3.8.9 Current Status
Vegetative covers have been constructed, including nu-
merous testing facilities as described in the Alternative
Covers Assessment Project's "Phase I Report." There are
no performance evaluations at present; each installation
must be approved on a site by site basis.
38
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3.8.10 System Costs
In general, vegetative covers are considered cost-effec-
tive remedies. Cost estimates indicate notable savings for
an evapotranspiration cover compared to a traditional cover
design (RTDF1998).
3.8.11 Selected References
Dobson, M. C., and A. J. Moffat, 1993. The Potential for
Woodland Establishment on Landfill Sites. HMSO Press.
The information presented in this report focuses prima-
rily on the effects of the landfill environment on tree
growth, the typical rooting pattern of trees, the likelihood
of windthrow, and the possible effects of trees on landfill
hydrology.
Flower, F. B., E. F. Gilman, and IA Leone. 1981. Land-
fill Gas, What It Does to Trees and How Its Injurious Ef-
fects may be Prevented. J. of Arboriculture. 7(2):February
1981.
Methods are suggested for preventing the entry of land-
fill gases into the root zones of the trees and in accom-
modating other tree growth problems found to be asso-
ciated with former refuse dumping areas. These meth-
ods include gas venting and blocking, irrigation, plant-
ing adaptable species, using small sized specimens in
preference to large, and providing adequate mainte-
nance.
Environmental Science and Research Foundation Con-
ference Proceedings. 1997. Landfill Capping in the Semi-
Arid West: Problems, Perspectives, and Solutions. May
21-27, 1997. Grand Teton National Park.
These conference proceedings address the following:
• Regulatory performance and monitoring requirements
for landfills and caps
• Perspectives and problems with landfill closure
• What landfill covers do and how they do it
• Different approaches to landfill caps
• Perspective and alternative cap designs
• Economic issues
RTDF. 1998. Summary of the Remediation Technologies
Development Forum Alternative Covers Assessment Pro-
gram Workshop. February 17-18, 1998, Las Vegas, NV.
http://www. rtdf. org.
The meeting minutes of these workshops on alternative
covers include a discussion of the technical and regula-
tory issues relating to the use of vegetative covers. Regu-
latory and industry participants present their views on
the use of alternative covers for a variety of geographic
regions and on the research needs required to validate
this technology. Models used to assess landfill covers
are included in the discussion.
EPA. 1991. Design and Construction of RCRA/CERCLA
Final Covers. (Seminar Publication, EPA 625-4-91-025).
This seminar publication provides regulatory and design
personnel with an overview of design, construction, and
evaluation requirements for cover systems for RCRA/
CERCLA waste management facilities.
EPA. 1994. Design, Operation, and Closure of Munici-
pal Solid Waste Landfills. (Seminar Publication, EPA625-
R-94-008).
This seminar publication provides a documented sum-
mary of technical information presented at a series of
2-day seminars. The goal of the seminars were to
present state-of-the-art information on the proper de-
sign, construction, operation, and closure of Municipal
Solid Waste Landfills.
EPA. Draft December 1998. Technical Guidance For
RCRA/CERCLA Final Covers.
The purpose of this guidance document is to provide
information to facility owners/operators, engineers, and
regulators regarding the regulatory standards, perfor-
mance monitoring, and maintenance of final cover sys-
tems for municipal solid waste and hazardous waste
landfills regulated under RCRA, and sites being
remediated under CERCLA. When released (approxi-
mately December 1999), it will be an update to the 1991
EPA document entitled, Design and Construction of
RCRA/CERCLA Final Covers (EPA-625-4-91-025).
3.9 Riparian Corridors/Buffer Strips
3.9.1 Definition/Mechanism
Riparian corridors/buffer strips are generally applied along
streams and river banks to control and remediate surface
runoff and groundwater contamination moving into the river.
These systems can also be installed to prevent downgradient
migration of a contaminated groundwater plume and to de-
grade contaminants in the plume. Mechanisms for
remediation include water uptake, contaminant uptake, and
plant metabolism. Riparian corridors are similar in concep-
tion to physical and chemical permeable barriers such as
trenches filled with iron filings, in that they treat groundwater
without extraction containment. Riparian corridors and buffer
strips may incorporate certain aspects of hydraulic control,
phytodegradation, rhizodegradation, phytovolatilization, and
perhaps phytoextraction.
3.9.2 Media
Riparian corridors/buffer strips are used in the treatment
of surface water and groundwater.
3.9.3 Advantages
Secondary advantages include the stabilization of stream
banks and prevention of soil erosion. Aquatic and terres-
39
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trial habitats are greatly improved by riparian forest corri-
dors.
3.9.4 Disadvantages
The use of buffer strips might be limited to easily assimi-
lated and metabolized compounds. Land use constraints
may restrict application.
3.9.5 Applicable Contaminants/
Concentrations
Nutrient and pesticide contaminants are among the wa-
ter-soluble organics and inorganics studied the most often
using this technology. The nitrate concentration in ground-
water was 150 mg/L at the edge of a field, 8 mg/L below a
poplar buffer strip, and 3 mg/L downgradient at the edge
of a stream (Licht and Schnoor 1993).
3.9.6 Root Depth
Uptake occurs within the root zone or the depth of influ-
ence of the roots.
3.9.7 Plants
Poplars have been used in riparian corridors and buffer
strips.
3.9.5 Site Considerations
Sufficient land must be available for the establishment
of vegetation. Typically a triple row of trees is installed,
using 10 meters at minimum. Larger corridors increase
capacity, and wider areas allow for more diverse ecosys-
tem and habitat creation. Native Midwestern songbirds,
for example, prefer corridors 70 meters and more.
3.9.8.1 Soil Conditions
The primary considerations for this technology are the
depth and concentration of contaminants that affect plant
growth. Soil texture and degree of saturation are factors to
be considered for use of this system. Planting technique
can mitigate unfavorable soil conditions.
3.9.8.2 Ground and Surface Water
Groundwater must be within the depth of influence of
the roots.
3.9.8.3 Climatic Conditions
The amount of precipitation, temperature, and wind may
affect the transpiration rate of the plants.
3.9.9 Current Status
Buffer strips have been researched and installed com-
mercially with success.
3.9.70 System Cost
Cost information is not available.
3.9.11 Selected References
Licht, L. A. 1990. Poplar Tree Buffer Strips Grown in Ri-
parian Zones for Biomass Production and Nonpoint Source
Pollution Control. Ph.D. Thesis, University of Iowa, Iowa City,
IA.
This thesis describes the use of poplar trees to control
nitrate-nitrogen contamination from agricultural fields.
Methods and results of field work are presented. Poplar
trees successfully established in riparian zones removed
nitrate-nitrogen from soil and groundwater.
Licht, L. A., and J. L. Schnoor. 1993. Tree Buffers Protect
Shallow Groundwater at Contaminated Sites. EPA Ground
Water Currents, Office of Solid Waste and Emergency Re-
sponse. EPA/542/N-93/011.
Uptake of nitrates by poplars planted between a
stream and a corn field was studied at an agricultural
field site. The poplars decreased nitrate levels from
150 mg/L in the field to 3 mg/L at the stream. Poplar
trees were used with atrazine and volatile organic
compounds in toxicity studies conducted in labora-
tory chambers and in the field. Atrazine was mineral-
ized, and deep-rooted poplars slowed migration of
volatile organics.
40
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Chapter 4
Phytoremediation System Selection and Design Considerations
This chapter discusses considerations involved in the
selection, design, and implementation of phytoremediation
systems. It presents information that will help a site man-
ager to identify whether phytoremediation may be appro-
priate for a site and to select a particular phytoremediation
technology, based on the conditions occurring at or appli-
cable to a site. This chapter introduces issues and con-
cepts that should be considered in the design and imple-
mentation of a phytoremediation system. Because
phytoremediation is not yet a developed technology, this
discussion is not intended to serve as a design manual.
Rather, it is a foundation on which to develop a
phytoremediation system in consultation with
phytoremediation professionals on a site-specific basis.
The main questions when considering phytoremediation
as a remedial alternative are (1) What are my
phytoremediation choices?; (2) Will phytoremediation be
effective and economical in remediating the site?; and (3)
What will it take to implement phytoremediation? This chap-
ter discusses the considerations that need to be evalu-
ated when answering these questions; however, economic
considerations and potential costs of phytoremediation are
discussed in Chapter 2.
The main considerations in the evaluation of
phytoremediation as a possible remedial alternative for a
site are the type of contaminated media, the type and con-
centration of contaminants, and the potential for effective
vegetation to grow at the site. Recommendations for the
selection of a particular type of phytoremediation technol-
ogy are provided where appropriate, such as fora particu-
lar media and contaminant encountered at a site. Informa-
tion on selection of appropriate vegetation is provided.
Other site-specific factors that need to be considered to
determine if a potential phytoremediation technology will
work at a site are also discussed. This site-specific evalu-
ation of phytoremediation considerations will lead to a de-
cision regarding the selection of a phytoremediation tech-
nology to be used at a particular site. The more positive
responses encountered when going through this list of con-
siderations, the more phytoremediation is likely to work at
the site.
This discussion can be considered as a checklist of items
to evaluate that are specific to a particular site, and also
as a reality check for the use of phytoremediation at a site.
It is not meant to encourage or discourage the use of
phytoremediation; rather, it indicates that the use of
phytoremediation should be based on thorough and sound
evaluation.
This discussion does not include a comparison of
phytoremediation to other technologies, since the only in-
tent is to provide information on phytoremediation. It is as-
sumed, however, that such a comparison of effectiveness,
cost, and time frames will be conducted. It is possible that
other technologies might remediate a site more effectively.
A decision-making process for evaluating whether or not
phytoremediation is a viable option is provided by the fol-
lowing outline of the steps for applying phytoremediation:
• Define Problem
- Conduct site characterization
- Identify the problem: media/contaminant
- Identify regulatory requirements
- Identify remedial objectives
- Establish criteria for defining the success of the
phytoremediation system
• Evaluate site for use of phytoremediation
- Perform phytoremediation-oriented site charac-
terization
- Identify phytoremediation technology that addresses
media/contaminant/goals.
- Review known information about identified phytore-
mediation technology
- Identify potential plant(s)
• Conduct preliminary studies and make decisions
- Conduct screening studies
- Perform optimization studies
- Conduct field plot trials
- Revise selection of phytoremediation technology, if
necessary
- Revise selection of plant(s), if necessary
• Evaluate full-scale phytoremediation system
- Design system
- Construct system
- Maintain and operate system
- Evaluate and modify system
- Evaluate performance
41
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• Achieve objectives
- Perform quantitative measurement
- Meet criteria for success
When planning any remediation system, it is important
to first define the desired remedial objectives: the desired
fate of the contaminant(s) and the desired target
concentration(s). An appropriate remediation technology,
or different technologies as part of a treatment train, can
then be selected based on the characteristics and perfor-
mance of that technology in meeting the remedial goals.
Remedial objectives are discussed in Chapter 5.
4.1 Contaminated Media Considerations
Phytoremediation can be used for in situ or ex situ appli-
cations. Phytoremediation is generally considered for in
situ use by establishing vegetation in areas of contami-
nated soil orgroundwater. However, soil can be excavated
and placed into a treatment unit where phytoremediation
will be applied. Groundwater or surface water can be
pumped into a treatment unit established for
phytoremediation or it can be sprayed onto vegetation.
4.1.1 Soil, Sediment, and Sludge
The following phytoremediation technologies are used
in the treatment of these media:
• Phytoextraction
• Phytostabilization
• Rhizodegradation
• Phytodegradation
• Phytovolatilization (to a lesser degree)
• Vegetative cover
The primary considerations for phytoremediation of soil
are the depth and volume of contamination and soil char-
acteristics that affect plant growth, such as texture and
water content (degree of saturation).
Phytoremediation is most appropriate for large areas of
low to moderately contaminated soil that would be prohibi-
tively expensive to remediate using conventional technolo-
gies. The contaminated soil should be within the root zone
depth of the selected plant. Small volumes of contaminated
soil concentrated in just a few areas are likely to be more
efficiently remediated using other technologies.
4.1.2 Groundwater
The following phytoremediation technologies are used
in the treatment of groundwater:
• Phytodegradation
• Phytovolatilization
• Rhizofiltration
• Hydraulic control (plume control)
• Vegetative cover
• Riparian corridors/buffer strips
Groundwater, surface water, and wastewater have been
treated using constructed wetlands orsimilartechnologies;
however, a discussion of those technologies is beyond the
scope of this document.
Plants useful for groundwater phytoremediation include
trees (especially Salix family — poplars, willows, cotton-
woods), alfalfa, and grasses. The plant transpiration rate
is an important consideration for groundwater
phytoremediation (see Section 4.3).
The primary considerations for remediation of ground-
water contamination are the depth to groundwater and the
depth to the contaminated zone. Groundwater
phytoremediation is essentially limited to unconfined aqui-
fers in which the water table is within the reach of plant
roots and to a zone of contamination in the uppermost
portion of the water table that is accessible to the plant
roots. Plant roots are very unlikely to reach through clean
groundwaterto a deeper contaminated zone. The seasonal
fluctuation of the watertable will affect the root depth: rela-
tively little fluctuation is desirable to establish a root zone.
If remediation of deeper contaminated water is desired,
careful modeling must be done to determine if the water
table can be lowered by the plants or through pumping, or
if groundwater movement can be induced toward the roots.
Another consideration for groundwater phytoremediation
is the rate of water movement into the root zone of the
area to be treated. Groundwater remediation will be slow
when the rate of water movement is low. Soil water con-
tent will also affect the rate of phytoremediation. Although
root hairs can reach into relatively small pores and plant
roots can extract water held at relatively high matrix suc-
tions (about 15 bars), the low hydraulic conductivity of rela-
tively dry soils will decrease the rate at which dissolved
contaminants are moved toward the plant. Sufficient
groundwater and precipitation must be available to serve
the water requirements of the plants, or irrigation will be
necessary.
For groundwater containment, the rate of groundwater
flow should be matched by the rate of water uptake by the
plants to prevent migration past the vegetation. Generally,
the greaterthe groundwater flow rate, the largerthe plants
need to be, and/or the greaterthe density of the planting.
Groundwater geochemistry also must be conducive to
plant growth. For example, saline waters will be detrimen-
tal to the plants unless halophytes (salt-tolerant plants) are
used.
Canopy closure is the shading of soils by plant leaves,
and total canopy limits evapotranspiration. Large plants
provide a larger area of shading than small plants. There-
fore, the mature size of the plants selected for remediation
should be considered in the design of a treatment plot.
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4.2 Contaminant Considerations
The applicability of phytoremediation has been re-
searched for some of the most significant and widespread
contaminant classes. Table 4-1 indicates the applications
of phytoremediation and provides the relevant
phytoremediation technology for different types of contami-
nants.
The following sections contain additional discussion on
particular contaminants or classes of contaminants. After
a potential phytoremediation technology is identified
through a review of the information in this chapter, addi-
tional, specific information on the technology can be ob-
tained in Chapters.
4.2.1 Organic Contaminants
The hydrophobicity of an organic compound (as indi-
cated by the octanol-water partition coefficient, kow) will
affect the uptake and translocation of the compound. In
general, moderately hydrophobic organic compounds (with
log k w between 0.5 and 3.0) are most readily taken up by
and translocated within plants. Hydrophobic (lipophilic)
compounds also can be bound to root surfaces or partition
into roots but not be further translocated within the plant
(Schnoor et al. 1995a; Cunningham et al. 1997).
4.2.2 Inorganic Contaminants
Phytoextraction coefficients describe the relative ease
of extraction of different metals; for example, one study
showed that the easiest to most difficult to extract were:
Cr6*, Cd2+, Ni2+, Zn2+, Cu2+, Pb2+, and Cr3+ (Nanda Kumar et
al. 1995). Phytoremediation may be different for mixtures
of metals than for one metal alone (Ebbs et al. 1997). The
interaction of the metals in a mixture might need to be in-
vestigated, especially in terms of the ability to take up one
or more metals and nutrients.
4.2.3 Waste Mixtures
Most phytoremediation research has focused on indi-
vidual classes of contaminants and not on mixtures of dif-
ferent types of contaminants. Although there is some evi-
dence that plants can tolerate mixed organic and metal
contamination, it has generally not been investigated if one
type of vegetation can successfully remediate different
classes of contaminants (for example, heavy metals and
chlorinated solvents at the same time). The use of several
types of vegetation, each to remediate a different contami-
nant, might be required either at the same time or sequen-
tially.
4.2.4 Contaminant Concentrations
The primary consideration in this area is that the con-
taminant concentrations cannot be phytotoxic or cause un-
acceptable impacts on plant health or yield. A literature re-
view or a preliminary laboratory or field plot screening study
will be needed to determine if the given concentrations are
phytotoxic.
The contaminant concentrations necessary for success-
ful phytoremediation must be determined in comparison to
the concentrations that could be treated by other, more ef-
fective remedial technologies. In general, the highest
concentrations will comprise relatively small hot spots that
Table 4-1. Phytoremediation Technologies Applicable to Different Contaminant Types1-
Technology
Media
Chlorinated
solvents
Metals3
Metalloids
Munitions
Nonmetals
Nutrients
PAHs
PCBs
PCP
Pesticides
Petroleum
hydrocarbons
Radionuclides4
Surfactants
Phytoextraction Rhizofiltration
Soil Water Water
T
F F F
T F (Se)
T
F5
T
G F F
Phytostabilization Rhizodegradation Phytodegradation Phytovolatilization
Soil Soil Soil Water Soil Water
F
F
T
G
G
F
T
G F
F
F
G
T
G F T T
T(Hg)
G F (Se)
G F
F/F
F T
F F T
1The applicability of a particular method of phytoremediation to each contaminant type has been judged by the current state or stage of the
application.
This is indicated in the table by the following designations:
T - The application is at the theoretical stage.
G - The application has been researched in the greenhouse or laboratory.
F - The application has been researched using field plots or has been applied in full-scale field systems.
2AII contaminants can be controlled using vegetative covers. The vegetative cover, riparian corridors, buffer strips, and hydraulic control are not
included in the table because they can be considered combinations of the other phytoremediation technologies.
3Reevesand Brooks 1983; Baker 1995, Salt et al. 1995; Nanda Kumar et al. 1995; Cornish et al. 1995.
4Saltetal. 1995; Nanda Kumar etal. 1995; Cornish etal. 1995.
5ln constructed wetlands.
43
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could be more effectively treated through excavation and
other treatment means. However, costs and remedial time
frames need to be considered as well as concentrations.
Higher concentrations of organics and nutrients might
be tolerated more readily by plants than by soil microor-
ganisms (Schnooretal. 1995a). In addition, plants (as mea-
sured by seed germination tests) were less sensitive to
heavy metals than were bacteria in toxicity screening (Miller
et al. 1985). Thus, it might be possible that
phytoremediation (except for microbially-based
rhizodegradation) could be effective in cases where
bioremediation fails due to the presence of metals or to
high toxic levels of contaminants. This is speculative, how-
ever, because the relative tolerance of plants and microor-
ganisms to high contaminant concentrations might be dif-
ferent under field conditions as compared to laboratory
toxicity screening, due to acclimation of microorganisms
in the field.
4.2.5 Contaminant Depth and Distribution
in the Soil Profile
The contaminated soil must be within the root zone of
the plants in order for the vegetation to directly impact the
contamination. The depth to contamination is of less con-
cern in the use of a vegetated cover designed to prevent
infiltration.
4.2.6 Contaminant Characteristics
The contaminant type, pH, physical form of light non-
aqueous phase liquid (LNAPL) or dense nonaqueous
phase liquid (DNAPL), mixtures, or oily contamination can
adversely affect the water movement, air movement, or
uptake of nutrients necessary for plant growth. An NAPL
or oily contaminant can significantly decrease plant growth.
Aged compounds in soil can be much less bioavailable.
This may decrease phytotoxicity, but may also decrease
the effectiveness of phytoremediation technologies that rely
on the uptake of the contaminant into the plant. To judge
the effectiveness of an actual phytoremediation design, it
is important that treatability studies use contaminated soil
from the site rather than uncontaminated soil spiked with
the contaminant.
4.3 Plant Considerations
It must be remembered that engineers, hydrogeologists,
and other professionals typically involved in site
remediation are not farmers, and that it might be difficult to
have vegetation conform to standard engineering practices
or expectations. Phytoremediation adds an additional level
of complexity to the remediation process because plants
comprise a complex biological system that has its own char-
acteristics.
4.3.1 Phytoremediation Plant Selection
The goal of the plant selection process is to choose a
plant species with appropriate characteristics for growth
under site conditions that meet the objectives of
phytoremediation. There are several starting points for
choosing a plant:
(1) Plants that have been shown to be effective or that
show promise for phytoremediation. These plants
have been discussed in this handbook, they can be
found in research publications on phytoremediation,
orthey can be enumerated by phytoremediation spe-
cialists.
(2) Native, crop, forage, and other types of plants that
can grow under regional conditions. A list of these
plants can be obtained from the local agricultural ex-
tension agent.
(3) Plants can also be proposed based on those plants
growing at the site, extrapolations from phytoreme-
diation research, inferences drawn from unrelated re-
search, or other site-specific knowledge. The efficacy
of these plants for phytoremediation would need to be
confirmed through laboratory, greenhouse, or field
studies or through screening.
Ideally, there would be a plant common to lists (1) and
(2), or there would be evidence that a plant common to
lists (2) and (3) would be effective. These lists of plants
can be narrowed down according to the criteria discussed
in the outline of the steps forselecting a suitable plant (see
Table 4-2). Following the Plant Selection Process outline,
additional information on topics discussed in the plant se-
lection process is provided.
During the plant selection process, additional informa-
tion should be gathered regarding candidate plants. Infor-
mation can be obtained by telephone or the Internet from
local, state, or Federal agencies and offices, or from uni-
versities. The Internet has numerous locations with this
information. One very useful source is the Plant Materials
program of the USDA Natural Resources Conservation
Service (http://Plant-Materials.nrcs.usda.gov/).
4.3.2 Root Type
A fibrous root system has numerous fine roots spread
throughout the soil and will provide maximum contact with
the soil due to the high surface area of the roots. Fescue is
an example of a plant with a fibrous root system (Schwab
et al. 1998). A tap root system is dominated by a central
root. Alfalfa is an example of a plant with a tap root system
(Schwab etal. 1998).
4.3.3 Root Depth
Root depth can vary greatly among different types of
plants. It can also vary significantly for one species de-
pending on local conditions such as depth to water, soil
water content, soil structure, soil density, depth of a hard
pan, soil fertility, cropping pressure, or other conditions.
The bulk of root mass will be found at shallower depths,
with much less root mass at deeper depths. The deeper
roots will also provide a very small proportion of the water
needed by the plant, except in cases of drought.
The depth of in situ contamination or of excavated soil
generally should not exceed the root zone depth. Excep-
tions to this could be made if it is verified that upward move-
ment of dissolved contaminants can be induced toward
44
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Table 4-2. Plant Selection Process
1. Identify phytoremediation technology and remedial goals.
2. Gather site information.
• Location (also relative to plant/vegetation/ecosystem zones)
• Temperatures: averages, range
• USDA plant hardiness zone (range of average annual minimum temperature)
• Precipitation: amount, timing
• Length of growing season
• Amount of sun/shade
• Soil texture, salinity, pH, fertility, water content, structure (hardpans, etc.)
• Contaminant type, concentration, form
• Site-specific conditions or considerations
• Identify plants growing in contaminated portion of site. (Optional)
- Do these provide a clue as to what plants to select?
- If not, will these plants compete with the selected plant?
- If the native plants do compete with the selected plant, are they easily removed?
• Identify local plants and crops. (Optional)
- Do these plants provide a clue as to what plants to select?
- Will a selected plant interfere with local plants?
3. Identify important criteria for plant selection.
• Disease resistance
• Heat tolerance
• Cold tolerance
• Insect tolerance
• Drought resistance
• Salt tolerance
• Chemical tolerance
• Stress tolerance
• Legume/nonlegume
• Annual/biennial/perennial
• Cultural requirements: Due to the added stress of a contaminated soil environment, the cultivation and maintenance factors may have to
be carefully monitored.
- Seed pretreatment before germination (such as for some prairie grasses)
- Planting method (seeds, sod, sprigs, whips, plugs, transplants), timing, density, depth (of seeds, root ball, or whips)
- Mulching, irrigation, soil pH control, fertilization, protection from pests and disease
- Fallen leaves, debris
- Harvesting requirements
- Labor and cost requirements should not be excessive
• Invasive, undesirable, or toxic characteristics
• Plant/seed source
• Establishment rate
• Reproduction method/rate
• Growth rate/biomass production
• Competitive or allelopathic effects
• Value of plant as cash crop
Phvtoremediation-related:
• Demonstrated efficacy of plant: The plant can take up and/or degrade contaminants, produce exudates that can stimulate the soil microbes,
or possess enzymes that are known to degrade a contaminant. The potential for the success of phytoremediation can be increased by
screening plants for useful enzymes (Fletcher et al. 1995).
• Phytotoxicity of contaminant: The contaminant should not be phytotoxic at the concentrations found at the site. Contaminant phytotoxicity and
uptake information can be found in the phytoremediation and agricultural literature, or determined through preliminary germination and phyto-
toxicity screening studies. Chapter 3 provides examples of applicable contaminant concentrations. Databases such as PHYTOTOX or UTAB
have been used to summarize and investigate phytotoxicity and uptake information (Fletcher et al. 1988; Nellessen and Fletcher 1993a,
1993b), although these databases might not be readily accessible.
• Root type and shape: Fibrous root system versus tap root system.
• Root depth: The range of root depths of a given plant must be considered.
• Contaminant depth and distribution: The contaminant depth must be similar to the root depth. The distribution of the contamination at various
soil depths is also important in planning the plant type and planting method. The contaminant concentrations in the seed bed layer of the soil
profile may have a strong effect on the ability to establish vegetation. A surface layer with minimal contamination underlain by greater contami-
nant concentrations might allow more successful seed germination than if the surface layer is heavily contaminated. Root growth into the
more contaminated layer is then desired, and since it is not guaranteed, must be verified. Make general decisions.
• Deciduous/nondeciduous: Deciduous trees will be dormant for part of the year, resulting in lowered transpiration rates.
• Monoculture vs. mixed species: The use of mixed species of vegetation can lead to more success due to the increased chance that at least
one species will find a niche. However, there could be competition between plants for nutrients and space. A monoculture relies on just one
plant type, possibly requiring more management to ensure its growth against adverse conditions. Despite this, a well-established stand of
one plant that has been shown to be effective could be the most efficient means of phytoremediation.
• Native vs. non-native: Native, nonagricultural plants are desirable for ecosystem restoration. In most applications, plants that are adapted to
local conditions will have more chance of success than nonadapted plants.
• Growing season: Warm season and cool season grasses could be used in combination to address different seasonal conditions, prolonging
a vegetative cover throughout more of the year.
(Continued)
45
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Table 4-2.
(continued)
• Sterile/male/female: The ability to reproduce is necessary for the long-term establishment of vegetation. In cases where the spread of the
plant to surrounding areas is undesirable, however, the plants should be selected to prevent reproduction.
• Plant rotation; planned or natural plant succession: The long-term establishment of vegetation at a site is dependent on the project goals and
future uses of a site. For long-term, no-maintenance vegetation establishment as part of ecosystem restoration, it is likely that there will be a
succession of plants at a site. If so, this succession could be planned when considering the types and timing of vegetation. Plant rotation
could conceivably be important when short-lived vegetation is used that does not reach remedial goals and that should not be replanted in the
same place.
5. Match above criteria with list of available/proposed plants.
• Select all appropriate candidates (eliminate all inappropriate plants).
• Conduct detailed evaluation of remaining candidates against criteria in items 1 to 4 of this table.
• Conduct cost/benefit analysis of top candidates:
- Plant costs
- Plant effectiveness in reaching goal
- Plant value (cash crop)
• Conduct preliminary studies to assess germination, survivability, and biomass. It might not be possible to assess the success of some forms
of phytoremediation (i.e., rhizodegradation) due to the insufficient time for preliminary testing. Because phytoremediation can be a long-term
process, however, spending one or two years in preliminary trials won't substantially increase the overall remediation time.
- Germination screening studies for phytotoxicity
- Small-scale greenhouse or laboratory chamber studies
- Field plot trials
6. Select plant and implement phytoremediation.
• Monitor and evaluate plant growth and phytoremediation success.
• Reevaluate plant selection on basis of observations: Variability in phytoremediation efficacy in varieties, cultivars, or genotypes of a given
species has been encountered in alfalfa for hydrocarbon rhizodegradation (Wiltse et al. 1998). A screening of cultivars/varieties might be
required.
• Reseed/replant as necessary with same or different plant.
the roots, or if soluble root exudates can be transported
deeper into the soil.
The root depth ranges provided below represent maxi-
mum depths:
• Legumes: Alfalfa roots can go quite deep, down to
about 30 feet, given the proper conditions.
• Grasses: Some grass fibrous root systems can extend
8 to 10 feet deep (Sloan and Woodward 1996). The
roots of major prairie grasses can extend to about 6 to
10 feet.
• Shrubs: The roots of phreatophytic shrubs can extend
to about 20 feet (Woodward 1996).
• Trees: Phreatophyte roots will tend to extend deeper
than other tree roots. Phreatophytic tree roots can be
as deep as 80 feet. Some examples are mesquite tap
roots which range from 40 to 100 feet and river birch
tap roots which go to 90 to 100 feet (Woodward 1996).
• Other plants: Indian mustard roots generally are about
6 to 9 inches deep.
These maximum depths are not likely to be reached in
most situations, due to typical site conditions such as soil
moisture being available in the surface soils or poorer soil
conditions at greater depths. A review of the literature found
that maximum depths of tree roots were generally 3 to 6
feet, with almost 90% of the roots in the top 2 feet (Dobson
and Moffat1995).
The effective depth for phytoremediation by most
nonwoody plant species is likely to be only 1 or 2 feet. The
effective depth of tree roots is likely to be relatively shal-
low, less than 10 or 20 feet. Gatliff (1994) indicates that for
practical purposes, trees are useful for extraction of ground-
water less than 30 feet deep. In addition, a contaminated
zone below the water table is not likely to be reached by
roots, as the roots will obtain water from above the water
table.
4.3.4 Growth Rate
The growth rate of a plant will directly affect the rate of
remediation. Growth rates can be defined differently for
different forms of phytoremediation. For rhizodegradation,
rhizofiltration, and phytostabilization, for example, it is de-
sirable to have fast growth in terms of root depth, density,
volume, surface area, and lateral extension. For
phytoextraction, a fast growth rate of aboveground plant
mass is desirable.
A large root mass and large biomass are desired for an
increased mass of accumulated contaminants, for greater
transpiration of water, greater assimilation and metabo-
lism of contaminants, or for production of a greater amount
of exudates and enzymes. A fast growth rate will minimize
the time required to reach a large biomass.
Metal hyperaccumulators are able to concentrate a very
high level of some metals; however, their generally low
biomass and slow growth rate means that the total mass
of metals removed will tend to be low. For phytoextraction
of metals, the metals concentration in the biomass and
the amount of biomass produced must both be consid-
ered. A plant that extracts a lower concentration of metals,
but that has a much greater biomass than many
hyperaccumulators, is more desirable than the
hyperaccumulator because the total mass of metals re-
moved will be greater.
46
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Poplars have been widely used in phytoremediation re-
search and applications due to their fast growth rate. They
can grow 9 to 15 feet/year (Gordon 1997).
4.3.5 Transpiration Rate
The transpiration rate of vegetation will be important for
those phytoremediation technologies that involve contami-
nant uptake, and for hydraulic control. The transpiration
rate depends on factors such as species, age, mass, size,
leaf surface area, canopy cover, growth stage, and climatic
factors, and will vary seasonally. Thus, well-defined num-
bers for a given type of vegetation cannot be assumed.
Estimates for certain cases, however, provide a rough
guide to the order of magnitude that might be expected.
For Populus species, approximately 26 gpd for a 5-year-
old tree was estimated (Gordon 1997). It was reported that
5000 gpd was transpired by a single willow tree, which is
comparable to the transpiration rate of 0.6 acre of alfalfa
(Gatliff 1994). Individual cottonwood trees were estimated
to transpire between 50 and 350 gpd, based on analysis
of drawdown near the trees (Gatliff 1994).
These transpiration rates, given in terms of gpd for indi-
vidual trees, should be viewed with caution because the
transpiration rate varies with tree size and other factors,
as mentioned above. A more appropriate measure would
be to look at the total water usage in a given area of veg-
etation.
4.3.6 Seed/Plant Source
Important considerations for the source of the plant in-
clude:
• Are the plants/seeds local or from a comparable cli-
mate? It is important to have the seed or plant sup-
plier verify where the seeds were produced since a
supplier may sell seeds that have been collected from
a wide variety of geographic locations. It is generally
best to use seeds or plants (and varieties) that are
local or from the region of the site so that the plants
are adapted to the particular climatic conditions. The
seed supplier or local agricultural extension agents can
provide information regarding local seeds and plants.
A comment during a phytoremediation presentation at
the Fourth International In Situ and On-Site
Bioremediation Symposium (April 28 - May 1, 1997,
New Orleans, LA, sponsored by Battelle Memorial In-
stitute) indicated that poplars were purchased for a
project; however, the source was in a different climate,
and all the trees died.
• Can the source supply the quantity needed when they
are needed (whether in season or out of season)? Is
the supplier reliable?
• Are there any transport/import/quarantine restrictions
or considerations?
• Can the supplier provide information on the cultivation
of the plants?
• Are the seeds/plants viable or healthy?
• The seeds/plants must be high quality: no undesirable
weed seeds, diseases, etc.
4.3.7 Allelopathy
Allelopathy refers to the inhibition of growth of one plant
species due to the presence of chemicals produced by a
different plant species. Allelopathic effects could be inves-
tigated when considering co-establishment of several spe-
cies of vegetation to ensure that one species won't hinder
the growth of another. Allelopathic effects could also be
due to plant residue that is incorporated into the soil in an
attempt to increase the fertility of the soil. For example,
root, stem, and leaf residues from canola inhibited the
growth of corn, wheat, and barley (Wanniarachchi and
Voroney 1997).
The phenomenon of allelopathy might also provide clues
as to the usefulness of a particular plant species. The al-
lelopathic chemicals produced by the plant could be in-
vestigated to determine if they are suitable chemical sub-
strates for microbial cometabolism of soil contaminants.
Allelopathy also indicates that compounds exuded by plant
roots influence the surrounding soil. The distance that this
influence extends could be estimated by examining the
spacing between such allelopathic plants and their neigh-
bors. This would provide a clue as to how far soil
phytoremediation could reach.
4.3.5 Forensic Phytoremediation
Areas with contaminated soils or groundwater can be-
come revegetated through the establishment of naturally-
occurring plants. Forensic phytoremediation refers to the
investigation of naturally-revegetated contaminated areas
to determine which plants have become established and
why, and to determine the impact of these plants on the
contamination. This investigation can identify plants that
are capable of surviving in contaminated areas, some of
which might also be capable of contributing to the degra-
dation of the contaminants.
Because the vegetation has often been present at the
site for a relatively long period compared to the time inter-
val for planned phytoremediation field studies, a researcher
has the additional benefit of not having to wait many more
years to investigate the effects of the revegetation (which
are evident now at the naturally-revegetated site). Natural
revegetation of a site is essentially a form of intrinsic
bioremediation. Phytoremediation intrinsic bioremediation
and forensic phytoremediation approaches have been in-
tensively investigated at a petroleum refinery sludge im-
poundment that was naturally revegetated (Fletcher et al.
1997; Wong 1996).
4.3.9 Plants Used in Phytoremediation
A compilation of plants used in phytoremediation re-
search or application is given in Appendix D. This Appen-
dix includes a table giving the common name followed by
the scientific name, and a table with the scientific name
followed by the common name.
47
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The following are examples of commonly-investigated
or used plants:
• Trees:
- Poplars (hybrids)/cottonwoods
- Willows
• Grasses:
- Prairie grasses
- Fescue
• Legumes:
- Alfalfa
• Metal-accumulators:
- Hyperaccumulators
Thlaspi caerulescens
Brassica juncea
- Accumulators
Sunflower
• Aquatic plants:
- Parrot feather
- Phragmites reeds
- Cattails
4.3.10 Optimum Plant
The following general plant characteristics are optimum
for different forms of phytoremediation:
• Rhizofiltration and phytostabilization:
- Able to remove metals.
- No translocation of metals from the roots to the
shoots.
- Rapidly growing roots.
• Phytoextraction:
- Tolerates, translocates, and accumulates high con-
centrations of heavy metals in the shoots and leaves.
- Rapid growth rate and high biomass production.
- Is not favored for consumption by animals (this de-
creases risk to the ecosystem).
• Rhizodegradation:
- Possesses appropriate enzymes and should not take
up the contaminant.
- Appropriate root growth (depth and or extent).
• Phytodegradation:
- Able to take up the contaminant.
- Degradation products are not toxic.
• Phytovolatilization:
- Able to take up the contaminant.
4.4 Site Considerations
4.4.1 Site Activities
4.4.1.1 Former Site Activities
Former site activities will affect the selection of plants
for phytoremediation. The location, extent, degree, and age
of contaminant are the primary considerations. However,
any former incidental use of chemicals could affect
phytoremediation, such as herbicide use at the site to sup-
press vegetation during site activities. The former or exist-
ing vegetation at the site can negatively influence the es-
tablishment of vegetation. Examples include allelopathic
plants, well-established undesired vegetation, and soil
pathogens in former vegetation. The former activities and
vegetation could be investigated to determine if any of these
factors could increase the difficulty of establishing the re-
medial vegetation.
4.4.1.2 Current Site Activities
The site must have sufficient open space, and no physi-
cal structures or site activities could interfere with the veg-
etation. In addition, site facilities or debris may have to be
removed. For ex-situ soil treatment, the volume of soil to
be remediated is divided by the depth of the root zone to
find the land area required for phytoremediation. The re-
quired land area must remain undisturbed by site activi-
ties, uses, or traffic.
The impact of tree roots on foundations and subsurface
utility lines must be considered as well as the impact of
tree branches on overhead utility lines. Potentially impacted
utilities may have to be removed or relocated.
Phytoremediation using trees would have to be curtailed if
such removal or relocation cannot be done.
Fencing might have to be installed around the
phytoremediation system to keep out animal pests that
might damage the vegetation. Additionally, the site must
have access to a water supply (groundwater, surface wa-
ter, or municipal) if irrigation is required.
4.4.1.3 Proposed Site Activities
Future activities planned for the site can impact the se-
lection of a phytoremediation technology. Portions of the
site might need to remain undisturbed to allow long-term
plant growth. If use of the site is required in the near fu-
ture, the establishment of trees orthe use of slow-growing
metal accumulators is not desirable. Fast-growing trees
such as hybrid poplars might be grown for a short period,
however, and then harvested if the remediation is predicted
to be relatively short-term.
Future use of the site might be for industrial, residential,
or recreational purposes. Different remedial criteria could
apply to these different uses. Institutional controls might
be necessary, depending on the proposed use of the site.
Agricultural uses of the site such as for grazing or for
crops will entail more concern regarding accumulation of
toxic compounds within the plants. Ecosystem or habitat
restoration uses will also raise concerns about possible
effects on the food chain.
4.4.2 Climatic Considerations
Climatic factors cannot be predicted with certainty, and
their effects cannot always be controlled. As a complex
48
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biological system, a phytoremediation system can be se-
verely impacted by extreme weather events; thus, this pos-
sibility must be considered during the planning of remedial
activities.
Precipitation. The amount and timing of rainfall and snow-
melt will determine the time of soil preparation, time of plant-
ing, and need for irrigation.
Air temperature. The mean, extremes, and fluctuations
in air temperature will affect the ability of plants to grow.
Sunlight. The amount of sunlight affects plant growth,
air temperature, and evapotranspiration.
Shade. The amount of shading from nearby buildings or
from mixed vegetation can affect the ability of plants to
grow.
Length of growing season. Phytoremediation processes
are more likely to be active during the growing season.
The length of the growing season must be considered in
predicting overall remedial timeframes.
Wind. The amount of wind affects evaporation, causes
damage to plants, and disperses volatiles and debris. Wind-
breaks may need to be installed.
Location. Regional and local weather patterns will affect
the factors described above.
4.4.3 Water Considerations
Surface water drainage and runoff will affect how soon
the soil can be worked, the soil temperature, and the sta-
bility of the soil, seeds, and plants. Subsurface water drain-
age will affect soil water content and soil temperature. Ar-
tificial drainage might need to be provided to encourage
remedial success. Poor growth and shallow roots in
buffalograss (Buchloe dactyloides) and warm season prai-
rie grasses resulted from water-logging during a field test
of phytoremediation in soils contaminated with relatively
low levels of PAHs and PCP (Qiu et al. 1997).
Irrigation is likely to be necessary during phytoremediation.
The source, availability, volume, cost, quality, and timing of
the irrigation water need to be considered.
4.4.4 Potential Adverse Effects/
Neighborhood Concerns
Phytoremediation could potentially have adverse impacts
on the site or surroundings. The list of potential adverse
impacts listed below is not meant to discourage the poten-
tial use of phytoremediation, but rather to make the reader
aware of potential pitfalls. Possible adverse impacts or dis-
advantages of phytoremediation include:
• Dust from tilling operations.
• Odor: Soil preparation that generates odors from vola-
tile contaminants might be required during
phytoremediation, but not with other remedial technolo-
gies. The selected plant might be odorous at certain
stages of growth or decay.
• Aesthetics: The presence of weeds and plant debris
can affect the perception and acceptance of a
phytoremediation site.
• Inappropriate plant introduction: The introduction and
spreading of a potentially undesirable plant (noxious or
invasive weeds) that will take over local vegetation must
be avoided. Plants should not have an adverse effect
on the local ecosystem. The vulnerability of the sur-
rounding area to the selected vegetation and the
vegetation's impact must be examined.
• Pollen and allergies: If plants are used that would con-
tribute an unacceptable amount to local allergen load-
ings, the plants must be harvested before release of
the allergen or treated to decrease the impact.
• Effect on nearby crops and vegetation:
- Pesticide drift: If pesticides are used during prepa-
ration or maintenance of the system, the impact of
any spraying must be carefully monitored, and nega-
tive impacts on nearby crop or residential areas pre
vented.
- Interbreeding: The impact of the selected plants on the
surrounding vegetation must be examined to ensure that
hybridization does not occur in a nearby crop.
- Airborne plant diseases could impact nearby veg-
etation.
• Attraction of pests: The plants might attract unwelcome
animals that become pests, such as birds (noise and
droppings), poisonous snakes (dangerto humans), rats
(disease-carriers or food-destroyers), or insects (dis-
ease-carrying vectors).
• Safety issues: blocked vision/sightlines, tree limbs, con-
cealment, fire hazard due to accumulated plant mat-
ter.
• Plant toxicity to people, birds, mammals (such as for-
aging animals), other plants (through allelopathy), or
beneficial insects such as honeybees. The inherent
toxicity of useful plants, such as that of Datura innoxia
(thornapple), must be considered in any risk analysis.
Potential bioconcentration of toxic contaminants in
plants is a concern, and the fate of the plant must be
controlled to prevent chemical or toxin ingestion by
animals or humans.
• Root damage to foundations, underground utilities, or
other structures.
• Impact on contaminant transport: The interactions of
the plants and all contaminants at the site could be
studied. Fertilizer application to optimize plant growth
may result in an increase in the mobility of some met-
als in the soil because many common nitrogen-con-
taining fertilizers lowerthe pH of soil. This might result
in leaching of metals to groundwater.
Phytoremediation might positively or negatively impact
other remediation activities. A potential positive impact is
that vegetation could be a visual, odor, dust, and noise
49
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barrier to block other site activities from the surrounding
areas. Potential negative impacts include the need to have
healthy vegetation, which requires that the plants not be
significantly disturbed. Thus, vehicles or equipment should
not be used or stored on the vegetated areas, which might
limit activities that could otherwise occur at the site.
4.4.5 Agronomic Considerations for
System Installation and
Maintenance
All of the factors that must be considered in successful
agriculture also must be considered during
phytoremediation. These factors will be more critical or more
difficult to control due to the additional stress placed on
the system by the contamination.
4.4.5.1 Pre-Plant Selection
Pre-plant selection includes the following:
• Soil must have a pH range that will allow plant growth.
The soil pH might need to be modified and controlled
through liming to increase pH or through acidification
to lower pH.
• Soil fertility and nutrient content
• Soil structure
• Soil tilth
• Soil salinity
• Soil water content
• Air-filled porosity: Affects aeration.
• Soil texture: Affects moisture content and drainage.
• Soil temperature: Affects germination of seeds.
• Soil depth: The depth to bedrock, a hardpan, or infer-
tile soil (along with soil water contents and soil nutri-
ents) can control the maximum depth of roots.
• Irrigation requirements
• Control of plant pests: birds, grazers, insects
4.4.5.2 Post-Plant Selection
Post-plant selection includes the following:
• Soil preparation: This preparation can include screen-
ing out debris or rocks, and (if desired) mixing and
diluting of the contaminated soil. Bulking agents and
organic matter amendments might need to be added
to improve the fertility or moisture-holding capacity of
the soil. Adding metal-chelating agents (such as
EDTA), maintaining a moderately acid pH, and adding
reducing organic acids to alter the redox status of the
soil can all increase the bioavailability of metals.
• Seed bed preparation: Preparation of the soil will likely
be required before seeding or planting. Various types
of tilling might need to be done to prepare the seed
bed. A good seed bed will increase the chances for
the establishment of a healthy stand of vegetation. Dust
will need to be suppressed during soil preparation work
or tilling.
• Planting considerations include the density, timing, and
method of applying seeds or plants.
• pH maintenance
• Mulching
• Fertilization: Fertilizers or organic matter amendments
might be necessary. The effect of fertilizers on soil pH
(for phytoremediation of metals) and on soil microbes
(for rhizodegradation) could be assessed.
• Irrigation equipment and scheduling
• Control of plant pests/desirable animals/undesirable
pests
- Birds: netting
- Grazers, vermin: fences, trapping
- Insects: pesticides
- Plant competitors: herbicides
- Diseases: pesticides, nutrients, pH, drainage
• Aesthetics/debris cleanup (wind damage, fallen leaves,
etc.)
• Odor control: from plant, from soil prep, from contami-
nant
• Biomass disposal
- Harvesting: determine method
- Plant debris (uncontaminated): occasional, periodic
- Plant debris (contaminated, biomass with metals)
• Effect of contaminant on nutrient or toxin availability
(some metals, or modification to enhance metal solu-
bility or chelation, may make nutrients unavailable or
enhance adverse impacts of toxins).
4.4.6 Disposal Considerations
Due to the growth of vegetation, the mass of plant mate-
rial will increase with time. Depending on the type of
phytoremediation, the biomass that must be removed from
the active system will vary. Relatively permanent long-term
systems that rely on the establishment of mature vegeta-
tion (e.g., poplar trees or grass for rhizodegradation) will
not require periodic planned removal of the biomass. In all
phytoremediation systems, however, some biomass such
as dead or diseased plants, fallen leaves, fallen limbs, or
pruned material might have to be removed occasionally to
maintain good operation of the system. These uncontami-
nated plant materials will need to be harvested, stored,
and disposed of as necessary. It will be important to con-
firm that the plant material does not contain any hazard-
ous substances. Afterthis confirmation, the material could
be composted or worked into the soil on site. If that is not
possible, off-site disposal will be required.
50
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The operation of some phytoremediation systems, such
as with phytoextraction and rhizofiltration, does depend
on the periodic removal of biomass. In these cases, proper
harvesting, storage, and disposal of contaminated biom-
ass (e.g., containing heavy metals or radionuclides) will be
necessary to prevent potential risk pathways such as in-
troduction to the food chain. An appropriate disposal facil-
ity must be identified, and it is likely that costs will be greater
than with uncontaminated biomass. Regulatory require-
ments for the handling and disposal of this material will
ha veto be followed.
If the selected phytoremediation technology results in
uncontaminated biomass, it might be possible to harvest
the vegetation as a cash crop to offset some of the reme-
dial costs. Examples include the harvest of grasses or al-
falfa for animal feed, or lumber from poplars. It must be
verified, however, that the plant materials do not contain
hazardous substances.
4.5 Treatment Trains
Phytoremediation could be part of a treatment train at a
site. Pretreatment of soil or water might be necessary be-
fore application of phytoremediation, such as with the ad-
justment of inflow water chemistry into an engineered
rhizofiltration system. Phytoremediation might also be used
as a finishing step to decrease contaminant concentra-
tions below what is achieved by a different initial remedial
technology, such as land treatment biodegradation. The
disposal or treatment of plant matterthat contains the con-
taminants will be the final step in a treatment train. In gen-
eral, however, research has focused on phytoremediation
as a stand-alone technology, with little or no integration of
phytoremediation with other remedial technologies.
Phytoremediation could be a partial solution at a site.
For example, excavation of highly-contaminated soil and
treatment by other remedial technologies could be followed
by a phytoremediation technology. A backup remedial tech-
nology might also be necessary for times when the
phytoremediation system is not working effectively, such
as during winter when plant growth has stopped or when
the vegetation is damaged by pests or weather.
4.6 Additional Information Sources
Successful phytoremediation requires a multidisciplinary
approach. This approach will call forthe knowledge, input,
and/or participation of a wide range of professionals and
practitioners. Many of these fields have conducted research
on topics relevant to phytoremediation before the applied
concept of phytoremediation was developed. In addition,
valuable information on potentially useful local plants or
cultural practices can be obtained from less-commonly-
used resources such as farmers, agricultural extension
services, and even local garden clubs and nurseries.
Relevant disciplines, resources, and information sources
are described below. Since a typical phytoremediation sys-
tem is not likely to be a research project, all of these infor-
mation sources do not need to be fully utilized. As more
experience is gained in researching and applying
phytoremediation, the most relevant disciplines will likely
be identified and more specific information can be pro-
vided as to how the following professionals can assist in
phytoremediation.
Agricultural extension agents or state university agricul-
tural departments can provide invaluable information on
the particular plants that grow in the local region, the cul-
tural practices for these plants, and the local soils. Most
information is likely to be about commodity crops grown in
the region and the weeds that affect these crops.
Agricultural engineers are more likely to have experience
with soil properties, drainage, tilling equipment, and irriga-
tion than other engineers.
Agronomists can provide assistance in working with soil
and crops.
Botanists can provide critical information on the identifi-
cation, growth, properties, and behavior of plants.
Ecologists will be crucial when hazardous waste site
remediation is also part of a longer-term ecosystem resto-
ration project.
Environmental/civil engineers have significant experience
using many technologies to characterize and remediate
hazardous waste sites.
Food scientists, vegetable crop specialists, and pomolo-
gistscan provide information on contaminants in food crops
and fruits; this information can provide clues as to which
plants are useful in the uptake of contaminants.
Foresters can provide tree propagation and culture in-
formation.
Hydrogeologists can evaluate the contaminated media
in the system and can evaluate the interactions with sur-
face water, groundwater, and soil water.
Land reclamation specialists have knowledge of the
plants and techniques used to restore degraded land; some
of the contaminants at hazardous waste sites, however,
have not been encountered in most cases of land recla-
mation.
Landscape architects can advise on the selection and
placement of plants.
Nurseries and seed companies can provide advice on
the selection and care of seeds and plants under local
conditions.
Soil scientists are specialists in understanding soil prop-
erties.
Soil microbiologists will be particularly useful for work
involving rhizodegradation, and as an aid in explaining how
soil microorganisms will interact with plant exudates, con-
taminants, and any amendments to the soil.
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Chapter 5
Remedial Objectives, Treatability, and Evaluation
5.1 Remedial Objectives
The remedial objectives that will be appropriate for a par-
ticular site are determined by site-specific conditions and
the requirements ofthe Federal or state program underwhich
the cleanup action will be conducted. Such cleanup pro-
grams include the RCRA Corrective Action and Underground
Storage Tank Remediation programs, which are Federal pro-
grams typically implemented by the states; the Federal
Superfund program; and state cleanup programs. These
cleanup programs generally require that remedial measures
be taken to:
• Prevent contaminants from reaching human or environ-
mental receptors above acceptable risk levels (prevent
exposure);
• Control further migration of contaminants from source
materials to groundwater or surface water (source con-
trol); and
• Control further migration of contaminated groundwa-
ter (plume control).
Some programs have additional requirements. For ex-
ample, the Superfund program generally requires that treat-
ment remedies be used for source materials that are de-
termined to be "principle threat" wastes, and generally al-
lows containment remedies for source materials determined
to be "low level threat" wastes. Also, the Superfund and
RCRA Corrective Action programs generally require that
contaminated groundwater be restored to cleanup levels
appropriate for current or future beneficial uses (e.g., drink-
ing water). Remedies that involve treatment of source
materials and restoration of groundwater will also set
cleanup levels to be attained by the remedy.
Two factors must be considered to determine the reme-
dial objectives for phytoremediation projects: 1) the target
concentration for each contaminant in each type of media
(soil, water, etc.) and 2) the desired fate of each contami-
nant, i.e., containment, uptake and removal, destruction,
or a combination of these options.
Ecosystem restoration could be a primary or secondary
objective in combination with soil or groundwater
remediation. Although remediation might be a secondary
goal as opposed to reestablishment of vegetation and habi-
tat, destruction ofthe contaminant is preferred over con-
taminant containment. Processes that transfer the contami-
nant to another location or phase are also less desirable.
5.1.1 Cleanup Levels
The target concentration for each contaminant may be
driven by environmental regulations such as RCRA,
CERCLA, the Clean Water Act, or state-specific cleanup
requirements. For example, surface water discharges, if
any, from the site may be required to meet National Pollut-
ant Discharge Elimination System (NPDES) limitations. If
soil or water is removed from the site for treatment or dis-
posal, RCRA standards are applicable.
A specified contaminant concentration is often a goal in
soil orgroundwater remediation. Because phytoremediation
is an emerging technology, there is still uncertainty regard-
ing the contaminant concentrations that are achievable by
the various types of phytoremediation. The information com-
piled in Chapter 3 provides a rough guide as to the con-
taminant concentrations that have been achieved in re-
search studies.
5.1.2 Fate of the Contaminant
Destruction of each contaminant is the preferred reme-
dial objective. However, depending on the phytoremediation
technology selected, contaminants may be contained and
left in place, or extracted or taken up by the plant into the
plant tissue and then left in place, removed, or volatilized.
Table 5-1 summarizes the methods of contaminant control
for each phytoremediation technology.
Table 5-1. Summary of Phytoremediation Technologies and Method
of Contaminant Control
Method
Destruction Extraction/Uptake Containment
Phytoextraction
(concentration)
Rhizofiltration
Phytostabilization
Rhizodegradation
Phytodegradation
Phytovolatilization
Plume control
Vegetative cover
Riparian corridors
V
V
Va
V
V
V
V
V
V
V"
V
a Phytoremediation cover.
b Evapotranspiration cover.
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5.2 Treatability Studies
Phytoremediation techniques are almost by definition in-
novative. Most have not been applied very often. In spite
of the body of information concerning applications of
phytoremediation to contaminated soils, groundwater, and
surface waters, there is still a need to determine a priori if
the specific plant(s) and treatment procedures indicated
for cleanup will work to remediate the contaminant(s) in
the soil or water at a specific site. Many factors will influ-
ence the success of phytoremediation at a given site, in-
cluding contaminant concentration, availability of nutrients,
daily maximum and minimum temperature, rainfall or pos-
sibility of irrigation, grade on site, aesthetic considerations,
daily illumination level, relative humidity, wind patterns, and/
or the presence of growth-suppressing contaminants. The
desired level of cleanup and the desired rate of decon-
tamination also need to be considered. All of these factors
need to be evaluated prior to a substantial expenditure of
time and money on a large-scale phytoremediation effort.
5.2.1 Optimization Studies
The phytoremediation system should be optimized prior
to the actual field application. There may be a need to
modify the soil or water pH or to add soil amendments
such as chelating agents (to make metals more
bioavailable), nutrients (to increase rate of plant growth),
and/or organic matter (to facilitate the growth of the de-
sired plant species or to improve soil microbial viability).
Organic amendments used in phytoremediation studies
have included leaf mulch, ground corn stalks, peanut shells,
cotton gin debris, and ground bark. Caution should be ex-
ercised in the use of plant-derived amendments because
some plant materials have been shown to possess phyto-
toxic properties. Canola leaf and root residues have been
shown to suppress the growth of corn, barley, and wheat
(Wanniarachchi and Voroney 1997). Such naturally occur-
ring phytotoxins probably have an evolutionary advantage
by suppressing competition for nutrients. Prior to full-scale
implementation, candidate amendments should be tested
in small-scale studies for their ability to suppress the growth
of the desired phytoremediating species.
Dibakar (1997) recommends groundwater monitoring if
amendments might mobilize contaminants. An interesting
example of a pilot study using amendments is the work by
Blaylock et al. (1997) concerning the use of several che-
lates at multiple concentrations. In this study, the
phytoremediation potential of Indian mustard (Brassica
juncea) was tested for removing metals from soils. This
study and other studies are discussed in section 5.2.5.
Amendments have also been used to adsorb contami-
nants so that they could later be available to plants or de-
graded by soil microbes. Cunningham et al. (1995b) as-
sessed the stabilization of lead in soil by adding an alkaliz-
ing agent, phosphates, mineral oxides, organic matter, or
biosolids.
Phytoremediation might also be enhanced by the addi-
tion of microbial innocula that would increase rhizosphere
degradation or uptake. Successful enhancements to
phytoremediation have been noted such as the inocula-
tion of wheat seeds with TCE-degrading bacteria (Yee et
al. 1998).
5.2.2 Other Considerations
Treatability studies could also provide information relat-
ing to disposal of contaminated biomass. Such disposal is
a major consideration in the cleanup of metal-containing
soils. Depending on regulations and plant concentrations
of metals, plants may need to be landfilled, or the metals
reclaimed through smelting, pyrolysis of biomass, or ex-
traction. In a discussion of the reclamation of metal-con-
taminated plant tissue by smelters, Dibakar (1997) stated
that plant tissue with a dry-weight concentration of over
one percent metal was amenable to reclamation.
Insecticides or herbicides might be used at the field treat-
ment site to preserve the plant species selected for
phytoremediation or to prevent the overgrowth of hardy
indigenous species. The site survey should consider the
prevalence of insect pests and invasive native species. Sub-
sequent laboratory trials may need to evaluate pesticide
usage to ensure that it does not interfere with
phytoremediation.
Treatability studies often use radiolabeled contaminant
preparations to assess the toxicity of plant-generated me-
tabolites of the contaminant(s) of interest or to assess the
possibility of volatilization orsolubilization of toxic contami-
nants. This use of radiolabels allows for a much greater
sensitivity in the analysis for contaminant or metabolites
(much lower detection limit), thereby facilitating the track-
ing of metabolic transformation of the contaminant in the
phytoremediation system. Studies using radiolabeled con-
taminants are usually performed in greenhouses or growth
chambers, although limited studies have been done in the
field using nonvolatile radiolabeled contaminants and en-
casing the plant roots in an impermeable barrel-like con-
tainerto prevent migration of radiolabels into the surround-
ing soil or into the groundwater. If volatility of contaminant
or metabolites is a concern, then studies should be per-
formed in a greenhouse. Agas-tight barrier can be installed
between the soil surface and the air so that evaporation
from the soil can be differentiated from plant uptake and
subsequent volatilization. Several studies are underway
using Populus species in an attempt to discern the
mechanism(s) by which poplars remove TCE from con-
taminated groundwater.
As a special case, phytoremediation studies that deal
with contaminant removal from aqueous media (ground-
water, waste water, wetlands) might use a radiolabeled
contaminant to address the comparative rates of transpi-
ration, bioconcentration, and/or degradation. Pilot studies
have been performed with radiolabeled contaminants to
evaluate phytoremediation treatment of groundwater to
remove persistent herbicides (Burken and Schnoor 1997)
and metals (Salt et al. 1997).
Treatability studies often also use hydroponic systems
in initial, proof-of-concept trials. Although hydroponic sys-
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terns should not be used to infer the rate of uptake or deg-
radation for soil systems, such studies can determine if a
particular plant can be used with a particular contaminant.
Hydroponic systems are often used for screening several
options (e.g., several concentrations of a contaminant),
since these systems are inexpensive and allow rapid growth
of plant tissue.
The issue of available time should be considered in the
design of treatability studies. The candidate plant species
must have sufficient time to develop roots and biomass
(and possibly metabolic enzymes) to perform the
phytoremediation. The ability of a plant to degrade or to
take up a contaminant varies with the age and metabolic
status of the plant (water content, diurnal cycles, tempera-
ture). Factors to consider include growth rate, period of
dormancy (deciduous plants), and any other known fac-
tors, such as the development of metabolizing enzymes,
that change with the age of the plant. Table 5-2 shows
experimental factors to consider in conducting treatability
studies.
Table 5-2. Experimental Factors for Testing in Treatability Studies
Essential Optional
Contaminant reduction
Phytotoxicity of contaminant(s)
levels
at site
Growth of plant species on site
(soil type, nutrients, temperature, rainfall,
illumination)
Rate of cleanup
Level of cleanup
Different soil types on site
Different contaminant
on site
Allelopathy
Aesthetic considerations
Soil amendments
Microbial innocula
Pesticide usage
Disposal options for plant
materials (metals,
radioisotopes)
5.2.3 Experimental Design for Plant
Selection
If a contaminant has not been studied for
phytoremediation, but a chemically similar contaminant was
successfully treated, a trial might be conducted to deter-
mine if the species used to treat the chemically similar
contaminant would work. If initial trials with one species
are unsuccessful, then different cultivars/strains, or related
species in the same genus, or related genera in the same
plant family might be assessed.
A survey of the site vegetation should be undertaken to
determine what species of plants are able to grow on that
site. Plants from the site can be assessed for uptake of the
contaminant, bioconcentration, and/or biodegradation.
The species chosen for a field study must be suited to
the soil, terrain, and climate at the site. If a plant species
previously used elsewhere for phytoremediation cannot be
successfully grown at a specific field site, then (as is the
case for chemically similar compounds) similar strains/cul-
tivars, species, or genera should be assessed in a pilot
study.
Soil samples should be taken in conjunction with plant
samples from the site to assess the concentration of con-
taminants in the immediate soils around the plants at the
site; soil contaminants have been shown to be degraded
by the microbial population found around the roots of plants
growing in contaminated soils (Anderson and Walton 1995).
These soil samples might also serve as sources of innocula
for microbial seeding of soils, seeds, or roots during sub-
sequent remediation studies.
The greenhouse or laboratory trial should use soil or
water from the site, if possible. This allows for assessment
of soil toxicity and assessment of the possibility of migra-
tion of the contaminant in the soil column (leachability).
Soil toxicity should be assessed using Standard Practice
E 1598-94 from the American Society for Testing and Ma-
terials (ASTM). The soil-dwelling microflora that aid in
phytoremediation are affected by the type and level of con-
tamination in the soil on site. These microflora are esti-
mated to take 2 to 16 weeks to recover from pesticide
treatment, more than 10 years to recover from toxic insult
due to oil spills on soil, and 50 to 100 years to recover from
metal contamination (Shimp et al. 1993). If soil from the
site cannot be secured, then a soil of the same USDAtype
should be used and adjusted for pH at the site. Commer-
cial suppliers of standard USDAsoil types should be con-
tacted as needed. In most studies, soils are screened
through a coarse mesh to remove rocks and large biom-
asses; however, this process may perturb the soil sample
(Nwosu 1991).
The trial should duplicate the illumination and moisture
conditions at the site as closely as possible since these
factors often have a significant influence on the rate of
remediation. Temperature and relative humidity have also
been shown to affect the rate of uptake of contaminants
(Dibakar 1997). If the site has several soil types, samples
of each soil type should be collected to assess growth in
each type. If uncontaminated areas are within the site, then
soils should be collected from these areas also for use as
experimental controls and for use in assessing the maxi-
mum tolerable level for a plant species to a given contami-
nant through addition of the contaminant of interest to the
uncontaminated soil. Table 5-3 summarizes factors to con-
sider in designing a phytoremediation trial.
5.2.4 Experimental Design
Most published phytoremediation pilot studies have uti-
lized a block design, with a first-level assessment of all
possible combinations of several key factors (e.g., several
plant species/several pH levels/several chelators). A sec-
ond series of tests could then evaluate the best combina-
tion of first-level factors (e.g.,the best combination of plant/
pH/chelator) for another set of factors (e.g., plant-toler-
ated level of contaminant and rate of cleanup). Such an
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Table 5-3. Information Needed for a Pilot Treatability Study
Identification of contaminant(s)
Level (concentration) of contaminant(s)
pH
Average monthly temperature, plus expected maximum
and minimum temperatures
Soil nutrient levels (P, K, N) and organic matter levels
Average monthly rainfall
Grade on site
Aesthetic considerations (proximity to commercial or
residential properties or recreational areas)
Daily illumination
Average relative humidity
Wind patterns (prevailing direction and velocity)
Presence of growth-suppressing contaminants
experimental design allows forthe efficient use of time and
funds, with assessment of multiple factors concurrently.
Information can be collected on interactions between fac-
tors affecting phytoremediation, and an optimal field
phytoremediation plan can be developed. The analysis of
data from such studies can be complex. Analysis of vari-
ance methods (ANOVA) can be used if certain conditions
are met. Standard statistics texts present a discussion of
ANOVA procedures. The best approach, however, is to
consult a statistician experienced in experimental design.
5.2.5 Completed Pilot-Scale Studies
Several pilot studies of phytoremediation options have
been published. The following paragraphs include brief
notes on the methods used, the experimental design, and
other information relevant to the design of a pilot study.
Rhizosphere soils (soils around the plant roots) were
collected from several plant species growing in an area
with prior herbicide application. These soils were dosed
with 14C-labeled metolachlor and incubated in biometer
flasks under controlled conditions. Evolution of 14CO2was
measured as an indication of herbicide degradation (Ander-
son and Coats 1995).
A greenhouse study evaluated the efficacy of the
remediation of PAH-contaminated soil by eight species of
prairie grasses. Reaction units were PVC pipe, capped at
one end. The eight reaction units were dosed with a mix-
ture of four PAHs; of these units, four were seeded and
four were not seeded. Additional controls were four undosed
units (two seeded and two unseeded). The concentration
of the PAHs was measured in the soils and leachates (Aprill
and Sims 1990).
Soil additives (KH2PO4, limestone, gypsum, sulfur, vari-
ous iron compounds, and various organic carbon sources)
were used in soils taken from three industrial sites to im-
mobilize lead for subsequent phytoremediation. A test for
immobilization/leaching potential was described (Berti and
Cunningham 1997).
Studies in growth chambers used a USDA standard soil
amended with lime and fertilizer and dosed with metal salt
solutions (Cd, Cu, Pb, orZn). Five chelating agents were
evaluated at four concentrations to determine their ability
to enhance uptake of the metals by Indian mustard, Bras-
sicajuncea (Blaylock et al. 1997).
Hybrid poplar cuttings were rooted in aqueous medium
and planted in 1-liter bioreactors filled with uncontaminated
soil or sand. Each bioreactor system was dosed with 14C-
labeled atrazine. After controlled incubation, uptake and
degradation were measured and metabolites were identi-
fied, where possible. 14C was rinsed from some sand-con-
taining bioreactors, and it was demonstrated that degra-
dation of atrazine could be accomplished in plant tissue. A
mathematical model was developed to describe atrazine
uptake, distribution, and metabolism (Burken and Schnoor
1997).
In a review paper, Cunningham et al. (1995a) discuss
soil amendments and their use in phytoremediation.
A greenhouse study evaluated three conditions (nutri-
ent-amended soil/ryegrass, nutrient amended soil/no
plants, and unamended soil/no plants) for remediation of
soils contaminated with pentachlorophenol or a mixture of
polycyclic aromatic hydrocarbons. Replicate soil columns,
subjected to the three treatments, were analyzed overtime
for contaminant concentration (Ferro et al. 1997).
In a small-scale field trial, a series of self-contained plots
was established with or without poplar trees and with/with-
out trichloroethylene (TCE), which was added to the water
supply during the growing season. The poplar/TCE and
the poplar/no TCE plots were replicated, while the other
conditions were not. Removal of TCE from the water and
metabolite formation were measured (Newman et al.
1997c).
Axenic tumor plant cell cultures were utilized to demon-
strate metabolism of TCE in plant tissue, as contrasted
with metabolism by rhizosphere microbes (Newman et al.
1997a).
Soil was collected at a contaminated site, and the germi-
nation of cucumber and wheat seeds was assessed in con-
tainers incubated on site. Seed germination was evaluated
against matched controls composed of the same seeds
planted in clean sand and incubated on site (Nwosu et al.
1991).
Astatic renewal bioassay was developed in which plants,
grown several weeks in uncontaminated soil in a green-
house, were then exposed to several different concentra-
tions of a solution of nutrients and the contaminant of in-
terest. Growth was evaluated through measurement of dry
weight, visual observation, and chlorophyll assay. This pro-
cess was used to establish tolerance levels for a contami-
nant-plant system (Powell et al. 1996).
After a preliminary seed germination study, tall fescue
grass was chosen as the best grass species to use in this
study. Triplicate microcosms of vegetated and unvegetated
soils were dosed with benzo(a)pyrene or hexachlorobiphenyl,
both of which were 14C labeled. Offgassing of radiolabeled
metabolite was monitored. Degradation of the contaminants
55
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in soil and binding to soil were also measured. A complete
randomized block design was used and data were ana-
lyzed by two-way ANOVA (Qiu 1995).
Four species of aquatic plants grown in glass tanks un-
der controlled conditions were evaluated to determine their
ability to accumulate either HgCI2 or CH3HgCI. Tests were
run in duplicate with two water sources and three sedi-
ment types. Uptake of Hg and plant growth were mea-
sured (Ribeyre and Boudou 1994).
Indian mustard seedlings were grown in aerated water
under controlled conditions and dosed with 210Pb, 85Sr, 109Cd,
63Ni, 51Cr, or 134Cs. Bioaccumulation of metal-associated
radiolabel was measured. Effects of competing ions (Ca++,
Mg++, K+, SO4= and NO3~) were also assessed (Salt et al.
1997).
Ten plant species were evaluated for remediation of TNT
and other explosives in groundwater. By-product forma-
tion, plant density, and rhizosphere interactions were evalu-
ated. Time-course studies were performed and kinetics
were described (Saunders 1996).
Phytoremediation of diesel-fuel-contaminated soils was
studied in this small-scale field study. Fourtreatments were
used: clover, fescue, bermuda grass, and no plants. Six
replicates were made of each treatment, and each repli-
cate plot contained four sampling sites. Decreases in total
petroleum hydrocarbons and plant growth were measured
over a 1-year period (Schwab and Banks 1995).
Shimp et al. (1993) published a review paper containing
an overview of factors affecting the phytoremediation of
soils and groundwater containing organic contaminants.
The paper's emphasis is on rhizosphere mechanisms along
with a list of plants/contaminants that have been studied.
5.3 Monitoring for Performance Evaluation
The phytoremediation system must be monitored and pe-
riodically evaluated to measure progress toward the reme-
dial objective.
5.3.1 Performance Evaluation
To evaluate the performance of soil remediation, contami-
nant and degradation product concentrations in the soil must
be measured. In rhizodegradation, the microbial populations
could be counted and/or identified to confirm biodegrada-
tion. In collecting and analyzing the soil, samples should
be collected from the root zone because the proximity and
influence of the root zone, as well as the density of the
roots, may affect how much rhizodegradation or
phytostabilization is measured.
To evaluate the performance of groundwater remediation,
the contaminant and degradation product concentrations
should be measured. The depth, flow rate, and volume of
groundwater should be monitored to evaluate the success
of hydraulic control. Periodic water content measurements
should be made, or tensiometers used to measure soil
moisture tension, which then could be related to water con-
tent through site-specific calibration. To evaluate processes
designed to impact water movement, the transpiration rate
should be determined.
Because phytoremediation is an emerging technology,
standard performance criteria for phytoremediation sys-
tems have not yet been completely developed. Data are
being gathered and assessed to develop performance
measures that can be used to assess the function of an
individual system.
Long-term monitoring may be needed for
phytoremediation systems that require long time periods
to demonstrate their effectiveness. Monitoring may be con-
tinued after short-term cleanup goals have been met in
order to determine the impact of the phytoremediation sys-
tem on the ecosystem.
5.3.2 Monitoring Plan
A monitoring plan forthe phytoremediation system should
be prepared to collect data to:
• Optimize operation of the phytoremediation system.
• Monitor potential adverse impacts to the ecosystem.
• Measure progress toward the remedial objectives, i.e.,
destruction, extraction, or containment of contami-
nants.
The monitoring plan should contain the following ele-
ments:
• Constituents, parameters, or items to be monitored
• Frequency and duration of monitoring
• Monitoring/sampling methods
• Analytical methods
• Monitoring locations
• Quality assurance/quality control (QA/QC) require-
ments.
Table 5-4 lists common parameters monitored in a
phytoremediation system. This list is not all-inclusive and
is dependent upon the individual phytoremediation system.
Modeling may be necessary to optimize the
phytoremediation system or to predict behavior. Modeling
may be especially relevant to evapotranspirative covers
where the water balance is critical to the success of the
system. Plant uptake models may be used to predict the
rate at which a contaminant will be degraded within a plant.
Monitoring of the ecosystem for potential adverse effects
may be necessary for some phytoremediation systems. If
the system uses phytovolatilization, air sampling might be
necessary to address concerns about contaminants ordeg-
56
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Table 5-4. Summary of Monitoring Parameters
Monitoring Parameter
Reason for Monitoring
Climatic data
•Temperature
•Precipitation
•Relative humidity
•Solar radiation
•Wind speed and direction
Plants
•Visual characteristics (viability, signs of stress, damage from insects or
animals, growth, leaf mass, etc.)
•Tissue composition (roots, shoots, stems, leaves, etc.)
•Transpiration gases
•Transpiration rate
•Root density
Soil
•Geochemical parameters (pH, nutrient concentrations, water content,
oxygen content, etc.)
•Microbial populations
•Contaminant and breakdown product levels
Groundwater
•Aquifer information (direction and rate of flow, depth to groundwater,
specific yield, etc.)
•Contaminant and breakdown product levels
•Maintenance requirements (irrigation)
•Determine water balance and evapotranspiration rates
•Maintenance (plant replacement, fertilizer, pesticide application, etc.)
•Quantify contaminants and byproducts
•Quantify and/or predict system operation
•Optimize vegetative, root, or microbial growth
•Determine water balance and evapotranspiration rates
•Quantify contaminants and byproducts
•Quantify and/or predict system operation
•Quantify contaminants and byproducts
•Quantify and/or predict system operation
radation products that may be released to the environment. If
soil additives are used to enhance the bioavailability of met-
als in soil, monitoring may be required to ensure the metals
are not migrating to groundwater. Extraction of contaminants
by plants with uptake to edible portions of the plant such as
leaves and seeds may require monitoring of the food chain for
bioaccumulation of the contaminant.
The monitoring plan should include QA/QC procedures
for sample collection, analysis, and data interpretation.
Since remedial site and analytical personnel may not be
experienced in sampling, preservation, or analytical meth-
ods for plant matter, properly developed and validated meth-
ods must be used to ensure conclusions are valid.
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Chapter 6
Case Studies
The six case studies presented in this chapter illustrate
specific field applications of phytoremediation. Site descrip-
tions, design considerations, monitoring recommendations,
status, and cost of various phytoremediation processes are
presented. The completeness of the information provided
varies based on the status of the project (i.e., complete
costs or degree of contaminant removal may not be fully
defined because the project is ongoing).
6.1 Edgewood Area J-Field Toxic Pits Site
Aberdeen Proving Grounds
Edgewood, Maryland
Site name:
Location:
Media:
Primary
contaminants
and maximum
concentration:
Type of plant:
Area of planting:
Date of planting:
Edgewood Area J-Field Toxic Pits Site
Aberdeen Proving Grounds,
Edgewood, Maryland
Groundwater (8 ft bgs)
1,1,2,2-tetrachloroethane (1122-TCA),
170ppm
Trichloroethylene (TCE), 61 ppm
Populus tricocarpaxdeltoides
(Hybrid poplar)
1 acre
March/April 1996
6.1.1 Site Description
The Aberdeen Proving Grounds (APG) in Maryland be-
gan serving as a U.S. Army weapons testing facility in 1918.
Military weapons testing and past disposal activities over
the years have caused extensive pollution throughout the
soil and groundwater of the Proving Grounds. As a result,
the entire Edgewood area of Aberdeen appears on the Su-
perfund National Priority List (NPL). The Department of
Defense (DOD) and the U.S. Environmental Protection
Agency (EPA) are jointly funding field-scale applications of
innovative treatment technologies around the facility. At the
J-Field Site in the Edgewood Area, EPAs Environmental
Response Team (ERT) coordinated the planting of hybrid
poplars over a shallow plume of chlorinated solvents in an
effort to hydraulically contain the contaminants and treat
the groundwater.
J-Field is located at the tip of Gunpowder Neck, in the
Edgewood Area of APG. Two pits measuring 10 x 15 x 200
feet were used forthe disposal of chemical warfare agents,
munitions, and industrial chemicals from the 1940s to the
1980s. Disposal methods included open burning of waste
material such as high explosives, nerve agents, mustard
agents, and smoke-producing materials. Wood and fuel were
used to feed the fire. Decontaminating agents used in the
operations were solvent-based. During this burning process,
large volumes of various chlorinated solvents were dis-
charged. As a result, a plume of chlorinated solvents formed
in the aquifer below the burning pits. The predominant sol-
vents in the groundwater are 1,1,2,2-tetrachloroethane (1122-
TCA) and trichloroethylene (TCE), with maximum concen-
trations in the groundwater of 170 ppm and 61 ppm, respec-
tively. Total volatile organic compound (VOC) concentrations
in the groundwater range up to 260 ppm.
6.1.2 Design, Goals, and Monitoring
Approaches
Several technologies were considered for cleaning the soil
and groundwater at the site. Soil washing, vapor extraction,
and capping were considered for cleaning up soils, while
pump-and-treat and air sparging were considered for
remediating the groundwater. These technologies were elimi-
nated from consideration for a number of reasons. Tech-
nologies that involved a rigid installation design were elimi-
nated because of the potential for unexploded bombs bur-
ied on site. Pumping and treating the water would be diffi-
cult because of the high concentrations of contaminants
and strict discharge regulations. Thus, the pump-and-treat
system would need to remove high concentrations of con-
taminants from large volumes of groundwater, and then dis-
charge the groundwater after it had been treated. Soil exca-
vation was eliminated from consideration due to the pres-
ence of unexploded ordnance (UXO) and its high cost. After
eliminating the other possibilities, project managers decided
the J-Field site was a candidate for a pilot-scale
phytoremediation system.
Based on site conditions and the possible presence of
UXO at the J-Field Toxic Pits Site of APG, phytoremediation
was deemed a viable remedial alternative to hydraulically
contain the contaminants and treat the groundwater. Ap-
plied Natural Sciences, Inc., was subcontracted to design and
install the phytoremediation system. The phytoremediation strat-
egy employed at the J-Field site began in September 1995
with a phytotoxicity assessment of on-site pollutants to
58
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determine any nutrient deficiencies that would hinder tree
growth. Four planting areas were designated at the J-Field
site, totaling approximately 1-acre. Holes were augered to a
depth of 8 feet to allow homogenization of soil layers. Soil
samples were collected and analyzed for VOCs, metals,
and chloride. The design was based on the location of the
toxic pits, various wells which would be utilized in monitor-
ing the system, and the flow of contaminated groundwater.
In March and April 1996,184 bare-root hybrid poplars (P.
trichocarpa x deltoides [HP-510]) were purchased from a
tree farm in Pennsylvania and planted 2 to 6 feet below
ground surface (bgs) in the areas of highest pollutant con-
centration around the leading edge of the plume. These trees
were planted in an attempt to intercept groundwater, thus
preventing further contamination of the nearby marsh. The
phytoremediation planting area covers approximately 1 acre
southeast of the toxic pits, and is surrounded by wooded
areas and scattered thickets. Groundwater flows from the
toxic pit area to the south and southeast. Perched ground-
water in the planting area varies throughout the year from 2
to 8 feet below ground surface (bgs). To promote growth
down to the saturated zone, each tree was planted with a
plastic pipe around its upper roots. A long piece of rubber
tubing was also added from the surface to the deeper roots
in orderto provide oxygen. Adrainage system was installed
in May 1996 to remove rainwater and thus encourage the
plants' roots to seek groundwater. A sweetgum tree growing
on site prior to installation of the phytoremediation system
was left standing. It will be monitored along with the pop-
lars.
Since the Aberdeen project involves a new treatment strat-
egy, extensive monitoring is taking place to determine the
fates of the pollutants, the transpiration rates of the trees,
and the best methods for monitoring phytoremediation sites.
Groundwater contaminant levels, water levels, tree growth,
tree transpiration rates, tree transpirational gas and con-
densate water contaminant levels, soil community, and tree
tissue contaminant levels were monitored overthe second
yeargrowing season to determine the effectiveness of this
emerging technology. The monitoring approaches are sum-
marized in Table 6-1. The sampling design of the site in-
volves collecting soils, transpiration gases, and tree tissues
from the roots, shoots, stems, and leaves. Results will help
determine the concentrations of contaminants and their me-
tabolites along each step of the translocation pathway.
Nine wells were located in the surficial aquifer near the
study area at the time of tree planting. To determine the
effects of the phytoremediation study on groundwater, an
additional five wells and four lysimeters were installed in
November 1996. Monitoring wells were screened from 4 to
14 feet bgs. Two sets of two lysimeters were installed near
the new monitoring wells. The lysimeters were placed in
pairs and set at depths of 4 and 8 feet bgs. These depths
allow for coverage of the capillary zone during seasonal
highs and lows. Groundwater and lysimeters were monitored
on a quarterly basis for VOCs, metals, and chloride. The
data obtained from the lysimeters are currently being corn-
Table 6-1. Monitoring Approaches at the J-Field Site.
Type of Analysis or Observation Parameters Tested or Methods
Used
Plant growth measurements and
visual observations
Groundwater and vadose zone
sampling and analysis
Soil sampling and analysis
Tissue sampling and analysis
Plant sap flow measurements
Transpirational gas sampling
and analysis
Diameter, height, health, pruning,
replacement
14 wells and 4 lysimeters to
sample for VOCs, metals, and
nutrients
Biodegradation activity, VOCs,
metals
Degradation products, VOCs
Correlate sap flow data to
meteorological data
Explore various methods
Source: Tobia and Compton (1997)
pared with the surrounding tree tissue and transpirational
gas data to determine the degree of success of the study.
6.1.3 Results and Status
Plant tissue samples were taken from certain trees and
analyzed for VOCs and metals. Results have shown parent
compounds and degradation products increasing in concen-
tration through mid-growing season and waning in the fall.
Weather parameters were measured by an on-site me-
teorological station, correlated with tree data, and were uti-
lized to estimate seasonal, daily, and yearly water uptake.
These parameters included precipitation, incident solar ra-
diation, temperature, humidity, and wind speed. All of these
factors play a role in transpiration rates and sap flow.
Tree sap flow rates are being monitored in orderto deter-
mine the pumping rates of the trees. A noninvasive tech-
nique was used to measure sap flow on certain trees during
the various sampling seasons. The Dynamax Flow32™ Sap
Flow System was used to measure the water flux of the
trees in grams of water/hour/tree by utilizing the heat bal-
ance method.
Transpiration gas sampling was performed by placing a
100-LTedlar™ bag over a section of branch on each of the
selected trees. Air was drawn from the sealed branch by
using a carbon Tenaxtube, summa canister, Sciex Trace
Atmospheric Gas Analyzer, and on-site Viking Gas Chro-
matograph/Mass Spectrometer (GC/MS). The most reliable
results were obtained by collection into the summa canis-
ter, then analysis by laboratory GC/MS. The results show
similar patterns to those found in the leaf tissue. The parent
compounds of 1122-TCAand TCE were detected at increas-
ing levels through the mid-growing season (maximum 2,000
ppb), with subsequent decreasing concentrations in the fall.
Condensate water was collected from the bags and ana-
lyzed for VOCs. There was a strong correlation (0.92) be-
tween condensate water and transpiration gas, with a maxi-
59
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mum concentration of 640 ppb of 1122-TCA in the con-
densate water.
Soil samples were collected from the rhizosphere of the
selected trees. These samples were analyzed for VOCs,
chloride, and metals, and utilized for soil community com-
parisons. There were noteworthy changes to the nematode
functional group populations. The nematode community ap-
pears to have increased, both in diversity and concentra-
tion, from previous samples taken from the area before the
trees were planted. Studies are planned to assess chlori-
nated solvent degradation by soil microbes.
Growth measurements, visual observations, and mainte-
nance were performed on all trees during planting and at the
end of each growing season to monitor tree growth and
health. Tree diameter and height were measured and tree
health observed, including monitoring of insect damage,
chlorosis, and wilting. Approximately 10% of the trees have
been killed by frost, deer rub during rutting season, and
insect predation.
Sap flow rate data indicate that on a daily scale, maxi-
mum flow occurs in the morning hours. In addition, increas-
ing amounts of solar radiation seem to increase sap flow
rates, as would be expected in a tree. Groundwater monitor-
ing data from May 1997 indicate that the trees are pumping
large amounts of groundwater. Data indicate that there is
roughly a 2-foot depression in the water table beneath the
trees in comparison to data from April 1996. Tree tissue
samples indicate the presence of trichloroacetic acid
(TCAA), a breakdown product of TCE. These data correlate
with the results from University of Washington greenhouse
scale studies that also found TCAA in plant tissues in both
axenic poplars cell cultures and hybrid poplar tissues. Site
managers at Aberdeen are also finding that chlorinated sol-
vents (TCE and 1,1,2,2-tetrachloroethane) are being
evapotranspirated by the trees. To date, no mass balance
studies have been performed to quantitatively determine
the different fates of chlorinated solvents in this treatment
system. Future monitoring of the site will hopefully answer
some of the questions about solvent fate. To accomplish
this, additional types of monitoring will be employed, such
as on-site infrared spectrometry and on-site GC/MS.
Based on tree containment measurements, the results of
the second growing season show that the trees are remov-
ing contaminants from the groundwater and transpiring par-
ent compounds and their degradation products. The ground-
watertable has been lowered by tenths of feet in the plant-
ing area at the end of the growing season, indicating pos-
sible groundwater withdrawal by the trees for containment
of the contaminated groundwater in future growing seasons.
The trees are utilizing the groundwater at rates of 2 to 10
gpd/tree.
6.1.4 Costs
Tree installation cost is about $80/tree, or approximately
$15,000 for the installation of 184 trees. The costs of moni-
toring are highly varied due to the numerous monitoring tech-
niques employed at the site.
6.1.5 Conclusions
Phytoremediation using trees to clean up groundwater
contaminated with volatile organic compounds may be an
ideal choice for this site and others due to the low cost,
low maintenance, and low impact associated with the tech-
nology. Much more work needs to be performed to further
confirm: (1) the correlation between transpiration gas and
condensate water; (2) soil community contaminant degra-
dation rate; (3) soil flux rate of VOCs; (4) contaminant ex-
posure to the root zone versus sap and condensate water;
(5) leaf litter exposure pathway; and (6) microwells to de-
termine the zone of contamination.
The environment benefits from the presence of trees re-
gardless of whether or not the technology is effective in
removing contamination. These environmental benefits in-
clude habitat for wildlife, protection of the soil against wind
and water erosion, reduction of rainwater infiltration and flush-
ing, an increase in organic matter, and an increase in soil
aeration and microbial activity.
6.1.6 Contacts
Harry Compton
US EPA Environmental Response Team
Edison, NJ 00837
(732)321-6751
Steve Hirsh
US EPA Region 3
Philadelphia, PA 19103
(215)814-3352
6.2 CarswellSite
Fort Worth, Texas
Site name:
Location:
Media:
Primary contaminant
and maximum
concentration:
Type of plant:
Area of planting:
Date of planting:
Former Carswell Air Force Plant
Fort Worth, Texas
Groundwater (12 ft bgs)
Trichloroethylene (TCE),
<1,000 ppb
Populus deltoides
(Eastern Cottonwood)
1 acre
April 1996
6.2.1 Site Description
The efficacy and cost of phytoremediation with respect
to the cleanup of shallow trichloroethylene (TCE) contami-
nated groundwater are being evaluated at the field scale in
a multiagency demonstration project in Fort Worth, Texas.
This U.S. Air Force project, which is being conducted as
part of the Department of Defense's (DOD) Environmental
Security Technology Certification Program (ESTCP), as well
as the U.S. Environmental Protection Agency's (EPA) Su-
perfund Innovative Technology Evaluation (SITE) Program,
60
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entails the planting and cultivation of Eastern Cottonwood
(Populus deltoides) trees above a dissolved TCE (<1,000
ppb maximum concentration) plume in a shallow aerobic
aquifer to investigate the ability of these trees to control
and degrade the plume. The plume is located near Air Force
Plant 4 at the Naval Air Station Ft. Worth, also known as
the Carswell Air Force Base. Data are being collected to
determine the ability of trees planted as short-rotation woody
crops to perform as a natural pump-and-treat system.
6.2.2 Design, Goals, and Monitoring
Approaches
The U.S. Air Force (USAF) Acquisition and Environmen-
tal Management Restoration Division and the EPA National
Risk Management Research Laboratory (NRMRL) carried
out the design and implementation of the phytoremediation
strategy at Carswell Site. In April 1996, the USAF planted
660 cottonwoods in an effort to contain and remediate a
plume of dissolved TCE located in a shallow alluvial aquifer
(<12 feet below ground surface). The species P. deltoides
was chosen over a hybridized species of poplar because it
is indigenous to the region. Therefore it has proven its abil-
ity to withstand the Texas climate, local pathogens, and
other localized variables that may affect tree growth and
health.
Two sizes of trees were planted: whips and 5-gallon buck-
ets. The whips were approximately 3/4 inch in diameter and
were about 18 inches long at planting. The whips were planted
so that about 2 inches remained above ground and the rest
of the tree was below ground to take root. The 5-gallon bucket
trees were about 1 inch in diameter and 7 feet tall when
planted. The 5-gallon bucket trees were estimated to have
about twice as much leaf mass as the whips when planted,
and thus they were expected to have higher evapotranspira-
tion rates.
The layout forthe project (see Figure 6-1) involved plant-
ing a separate plot of trees forthe whips and the 5-gallon
buckets, with both plots perpendicular to the contaminant
plume. The plume is moving to the southeast, so the plots
were laid out on a northeast axis. The whips section was
planted to the northwest of the 5-gallon buckets, so that the
plume would first travel through the root zone of the whips
and then through the root zone of the 5-gallon buckets. A
control area with monitoring wells was placed to the north-
west of the whips, and another in between the whips and
the 5-gallon buckets, along with monitoring wells through-
out the treatment site. These control areas enable data to
be collected on the amount of contaminant that enters each
of the treatment areas (whips and 5-gallon buckets), so that
\
Experimental Design
Legend
Monitoring Well
Monitoring Well
with Data Logger
o Nested Wei Is
Figure 6-1. Experimental Design
61
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a comparison of the performance of each type of tree can
be made.
One unique aspect of Carswell Site is the 19-year-old ma-
ture cottonwood growing on the site. This 70-foot-tall tree is
located just southeast of the planting area on the other side
of a cart path. Groundwater monitoring wells were installed
around this tree, and it has been sampled in a similar man-
ner to the planted cottonwoods to see how well a mature
tree functions in this phytoremediation system.
6.2.3 Results and Status
Seventeen months afterthe trees were planted (summer
1997), several trenches were dug adjacentto selected trees,
and it was determined that the tree roots had reached the
water table (Hendrick 1997). Although the trees are pump-
ing water from the contaminated aquifer, they have not yet
begun to hydraulically control the plume. Combined results
of a transpiration model and a groundwaterflow model will
be used to determine when hydraulic control of the plume
might occur. Transpiration measurements indicate that the
largest planted trees pumped approximately 3.75 gpd dur-
ing summer 1997; the mature 19-year-old cottonwood tree
near the planted trees was determined to pump approxi-
mately 350 gpd (Vose 1997).
Some analytical work has been done on the tree tissues
at the site, but this type of information is still in the early
stages of collection. Data from November 1996 indicated
TCE in the whips that were planted over an area where the
groundwater was the shallowest. This indicates that the
young trees were capable of evapotranspirating TCE after
just one growing season. Qualitatively, both types of trees
were capable of evapotranspiring TCE, and the 5-gallon trees
are evapotranspiring more waterthan the whips. This was
to be expected because of the greatertotal surface area of
the leaves of the 5-gallon trees. In addition, the transpira-
tion rates were generally higher in June than May, which is
likely due to a combination of warmer weather and more
fully developed leaves. There also appeared to be a midday
decline in transpiration during June, indicating that the plants
were experiencing water stress during the hottest part of
the day in the summer months. Thus, the water demand for
the tree exceeded the supply during that time. There was
also a notable difference in transpiration rates between days
in June, with cloudier days resulting in lower transpiration
rates. In addition to evapotranspiration information, some
tree growth data have also been collected. In 16 months the
whips grew about 20 feet, and the 5-gallon bucket trees
have grown faster than the whips. Now that the trees have
been on site for over an entire growing season, site manag-
ers at Carswell Site have increased monitoring at the site to
include a whole suite of water, soil, air, and tree tissue sample
analysis. Some of the more unique data they are collecting
(in relation to the other case study sites) are analyses of
microbial populations and assays of TCE-degrading en-
zymes in the trees.
Laboratory experiments conducted on root samples from
the site show the disappearance of perchloroethylene (PCE)
in the presence of roots from the cottonwood trees. The
products of degradation are anaerobic in the rhizosphere
and aerobic (haloacetic acids and carbon dioxide) in the
canopy. Increased amounts of vinyl chloride and a trace of
TCE as well as iron- and sulfur-reducing conditions in the
rhizosphere were detected at the end of these experiments
(Harvey 1998). The disappearance of PCE in the presence
of roots from a willow tree near the site was even more
remarkable (Wolfe 1997). These experiments indicate that
cottonwoods and willows produce enzymes that can de-
grade PCE and TCE. Researchers trying to determine how
the trees change the geochemistry of an aerobic aquifer
contaminated with TCE and its breakdown product found
that labile organic matter from the cottonwoods and several
otherspecies of trees is promoting reducing conditions con-
ducive to the degradation of TCE (Harvey 1998).
Groundwater samples had been collected from the 29
monitoring wells and analyzed on three occasions as of
August 1997. Concentrations of TCE, cis-DCE, trans-DCE,
and vinyl chloride were determined from these samples.
They ranged from 2 to 930 ppb TCE in the groundwater, with
most samples falling in the 500- to 600-ppb range (see Fig-
ure 6-2). Average concentrations of the contaminants on
the three sampling dates are provided in Table 6-2, with the
exception of vinyl chloride. Vinyl chloride was only detect-
able in a handful of samples and generally at low levels;
thus, an average concentration was not determined.
TCE concentrations in groundwatersamples collected be-
neath the 19-year-old cottonwood tree during summer 1997
were about 80% less than concentrations in groundwater
beneath the planted trees, and cis-1,2 DCE (byproduct of
TCE degradation) concentrations were about 100% greater.
These data, along with additional geochemistry data from
the site, are consistent with microbial degradation of TCE
beneath the mature tree (Lee 1997). Microbes with the abil-
ity to readily degrade TCE require an environment that is
low in dissolved oxygen and high in an appropriate source
of organic carbon. These conditions, which are often lack-
ing at sites contaminated with TCE, exist in the aquifer un-
derthe mature tree and are likely due to the introduction of
organic matter from tree-root activity. Once the planted cot-
tonwood trees have established more mature root systems,
an environment could develop in the aquifer beneath the
trees that would promote biodegradation and result in an
additional mechanism for attenuation of TCE. The effect of
other mature trees such as willows, oaks, junipers, mes-
quite, ashes, and sycamores on the geochemistry of the
groundwater in the winter and spring is also being explored.
6.2.4 Costs
Some rough estimates of cost for the Carswell Site have
been provided by site managers. These estimates can be
found in Table 6-3. Since this site involves an innovative
treatment technology, these costs are substantially inflated
due to the heavy monitoring taking place at the site. Also,
long-term projected costs and/ortotal project costs are not
available because the time involved in remediating the site
is uncertain. In addition to the costs in the table, $200,000
62
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TCE Concentrations (PPB) from Site Characterization Work
Figure 6-2. TCE Concentrations
Table 6-2. Average Concentrations of TCE, cis-DCE, and trans-DCE
at Carswell Site.
Contaminant Average Concentration (ppb)
December 1996 May 1997 July 1997
TCE
cis-DCE
trans-DCE
610
130
4
570
140
2
550
170
4
Table 6-3. Estimated Cost of Phytoremediation at the Carswell Site.
Activity Estimated Cost
Wholesale cost of trees (does not $8/tree for 5-gallon bucket
tree
include delivery or installation costs) $0.20/tree for whips
29 wells (including surveying, drilling $200,000
and testing)
Subsurface fine biomass study (the
vertical and lateral extent of tree
roots less than 2 mm in diameter)
$60,000
will be spent for extensive site monitoring that would not
normally be associated with a phytoremediation system;
thus, this amount was not included in the cost estimates.
More extensive cost and performance data from the dem-
onstration are being compiled to assist others in selecting
phytoremediation as a treatment technology. The subsur-
face fine biomass study will also define the volume of soil
exploited by the trees at any given point in time. A typical
poplar plantation grown as a short rotation woody crop can
produce up to 50,000 to 75,000 miles of fine roots per acre.
Also, a groundwaterflow and transport model of the site is
planned to help determine the relative importance of vari-
ous attenuation processes in the aquifer to guide data col-
lection at future sites. The model will also be used to help
predict the fate of TCE at the demonstration site in an effort
to gain regulatory acceptance of this remedial action.
6.2.5 Conclusions
There are over 900 Air Force sites with TCE contamina-
tion within 20 feet of land surface that could be reviewed for
potential application of phytoremediation by use of poplar
trees (Giamonna 1997). Costs may be 10 to 20% of those
for mechanical treatments. Scale-up costs for large scale
applications of phytoremediation can be minimized by ex-
ploiting the body of data developed for the Department of
Energy on the planting and cultivation of poplartreesforthe
purpose of biomass production.
63
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6.2.6 Contacts
Greg Harvey
US Air Force ASC/EMR
WPAFB, OH 45433
(937) 255-7716, Ext. 302
Steven Rock
US EPA National Risk Management Research Laboratory
Cincinnati, OH 45268
(513)569-7149
6.3 Edward Sears Properties Site
New Gretna, New Jersey
Site name:
Location:
Media:
Primary contaminant
and maximum
concentration:
Type of plant:
<\rea of planting:
Date of planting:
Edward Sears Properties Site
New Gretna, New Jersey
Groundwater (9 ft bgs)
Trichloroethylene (TCE),
<400 ppb
Populus charkowiieensisxincmssata
(Hybrid Poplar)
1/3 aere
December 1996
6.3.1 Site Description
From the mid-1960's to the early 1990's, Edward Sears
repackaged and sold expired paints, adhesives, paint
thinners, and various military surplus materials out of his
backyard in New Gretna, NJ (see Figure 6-3). As a result,
toxic materials were stored in leaking drums and containers
on his property for many years. The soil and groundwater
were contaminated with numerous hazardous wastes, in-
cluding methylene chloride, tetrachloroethylene, trichloro-
ethylene, trimethylbenzene, and xylene. After his death, no
one could be found responsible for the site or its cleanup;
thus, On-Scene Coordinators (OSC) from EPA's Region 2
Removal Action Branch were called in to remove the leak-
ing drums of hazardous materials, including off-specifica-
tion paints and solvents. Soil sampling indicated that two
areas, 35 x 40 feet and 15 x 20 feet, were very heavily
contaminated with solvents. These soils were removed to 8
feet below ground surface (bgs) (just above the watertable).
Further excavation could not be achieved without pumping
and treating large volumes of groundwater. The excavated
areas were backfilled with clean sand and the OSC acti-
vated the EPA's Environmental Response Team (ERT) of
Edison, NJ to determine the extent of groundwater and deep
soil contamination.
Using innovative hydraulic-push groundwater sampling
techniques, the ERT investigation revealed localized, highly
contaminated groundwater. Based on this information, a lim-
ited number of monitoring wells (see Figure 6-4) were in-
stalled to determine vertical contaminant migration and to
conduct aquifertests necessary to evaluate pump-and-treat
options. A pilot test for a pump-and-treat system with air
stripping and activated carbon was then conducted. The
aquifer tests revealed a high yield aquifer, which would
require severe over pumping to create any substantial cone
of influence around the pumping wells. Contaminants trapped
in the silty-clay lens beneath the site would be difficult to
extract in this manner because the transfer rate of contami-
nants into the groundwater is slow. As a result, large vol-
umes of groundwater would need to be pumped to the sur-
face for treatment, and this water would contain low con-
centrations of contaminant. Also, neighbors of the property
would be disturbed by the noise created by a pump-and-
treat system.
Based on these results, a pump-and-treat option would
be expensive and inefficient for the Edward Sears site. Site
managers then moved to consider a phytoremediation op-
tion. This site was judged as a potential candidate for a
phytoremediation system due to the nature of the soils and
groundwater. There is a highly permeable sand layer about
4 to 5 feet bgs, but below that exists a much-less-perme-
able layer of sand, silt, and clay from 5 to 18 feet bgs. This
silt, sand, and clay layer acts as a semiconfining unit for
water and contaminants percolating down toward an uncon-
fined aquifer from 18 to 80 feet bgs. This unconfined aquifer
is composed primarily of sand and is highly permeable. The
top of the aquifer is about 9 feet bgs, which lies in the less-
permeable sand, silt, and clay layer. Most of the contami-
nation is confined from 5 to 18 feet bgs; thus, site manag-
ers decided to plant hybrid poplars in order to prevent fur-
ther migration of the contaminants and ultimately remove
contaminants from the groundwater.
Samples were taken from temporary well points through-
out the site. Data from these sampling efforts indicated
trichloroethylene (TCE) concentrations in the groundwater
ranged from 0 to 390 ppb. Most of the TCE was concen-
trated in a small area on the site. Seven monitoring wells
were installed based on the information obtained from the
temporary well points. Monitoring Well 1 was installed in the
area of highest TCE contamination. Little or undetectable
TCE was found in the groundwater samples from the other
six wells.
6.3.2 Design, Goals, and Monitoring
Approaches
Underthe Response Engineering and Analytical Contract
(REAC), a pilot phytoremediation test was conducted at the
Sears site to determine whether hybrid poplar trees can be
used to reduce soil and groundwater VOC contamination
levels in the planted area and to prevent further offsite mi-
gration of contaminated groundwater. In October and No-
vember 1996, the site was cleared of debris and a 4-inch
clay layer was placed approximately 1 foot bgs to prevent
penetration of rainwater into the upper root zone, thus pro-
moting root growth into the underlying aquifer. This was fol-
lowed by the replacement and grading of the native surface
soil.
Thomas Consultants, Inc. of Cincinnati, OH were sub-
contracted to lay out the phytoremediation design. In De-
cember 1996, 118 hybrid poplar saplings (Populus
64
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Delaportes
roperty - Occupied
Legend
Tree Line
Wetland
Roads
Fence
Residence
110
Graphic Scale
0 55 110
Site Map
Edward Sears Property Site
New Gretna, NJ
March 1997
U.S. EPA Environmental Response Team Center
Figure 6-3. Site Map
charkowiieensis x incrassata, NE 308) were planted by
ERT, REAC, and Thomas Consultants personnel in a 1/3-
acre plot. The trees were planted 10 feet apart on the axis
running from north to south and 12.5 feet apart on the east-
west axis.
A process called deep rooting was used to plant the trees.
In deep rooting, the roughly 12-foot trees were buried 9 feet
so that only about 2 to 3 feet remained on the surface. Deep
rooting the trees involved drilling 12-inch-diameter holes to
a depth of 13 feet. These holes were then back filled to 5
feet below ground surface with amendments such as peat
moss, sand, limestone, and phosphate fertilizer. This back-
fill was installed to provide nutrients to the roots as they
penetrated down through the soils. Waxed cardboard cylin-
ders 12 inches in diameter and 4 feet long were installed
above the backfill to promote root growth down into the
groundwater. These barriers settled about 1 foot into the
planting holes; therefore, 5-gallon buckets with the bottoms
cut out were placed on top of the cylinders to create a 5-
foot bgs root barrier. The trees were placed in the cylinders
and the remaining 5 feet to surface was filled with clays
removed during the boring process.
About 90 poplars still remained after the deep rooting
was completed. These extra trees were planted along the
boundary to the north, west, and east sides of the site.
These trees were only planted to a depth of 3 feet, or shal-
low rooted. The shallow-rooted trees were added to pre-
vent rainwater infiltration from off site and to replace any
loss of deep-rooted trees. These trees were planted very
close together (about 3 feet apart) under the assumption
that natural thinning would take place over subsequent
growing seasons. A surface water control system was then
installed by planting grasses over the entire site. These
grasses came from commercially available seeds pur-
chased from a lawn and garden store.
ERT is conducting an ongoing maintenance and moni-
toring program at Edward Sears. Monitoring of the site in-
cludes periodic sampling of groundwater, soils, soil gas,
plant tissue, and evapotranspiration gas. Continued growth
65
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Legend
Clean Backfill
Deep Planted Tree
Shallow Planted Tree
Soil oring
Monitor Well
Property Boundary
Tree
Geoprobe Sampling
Locations
DCW Contamination
TCE Contamination
PCE Contamination
\_LJnoccupied
Gas Station
Delaportes
•operty - Occupied
Edward Sears
Residence Unoccupied
70
U.S. Environmental Response Team Center
Response Engineering and Analytical Contract
Sampling Grid with EOC
Outlines-9/96 Data
Edward Sears Property
New Gretna, NJ
October 1997
Figure 6-4. Sampling Grid
measurements will also be made as the trees mature. In
the fall of 1997, the surface water control system was re-
placed due to a summer drought that killed much of the
grass. Site maintenance also involves the prevention of
deer and insect damage. Bars of soap were hung from the
trees to deter deer from rubbing their antlers on the trees.
Some damage was inflicted by an insect larva known as
the poplar leaf caterpillar. This caterpillar lives on poplar
trees and makes its cocoon by rolling itself in a poplar leaf.
A spray containing Bacillus thuringesis, a bacteria that pro-
duces toxins specific to various insects, was applied to the
site. This spray has been effective in killing most of the
caterpillars living on the trees.
6.3.3 Results and Status
Because the trees had only one full growing season,
very little performance data are available; however more
data are expected in the next growing season. Evapotrans-
piration gas was sampled by placing Tedlar™ bags over
entire trees. Data from these air samples suggest that the
trees are evapotranspirating some of the VOC's. However,
the VOC concentration in the Tedlar™ bags matches the
background concentrations of VOCs in control samples.
This could be due to VOCs volatilizing from the soils, or it
could be due to evapotranspirated VOCs that may have
gotten into the control samples. Future sampling designs
will attempt to determine accurate background VOC con-
centrations. The trees have grown about 30 inches since
planting. Site managers plan to sacrifice one tree either
after or during the next growing season to determine the
extent of root growth.
6.3.4 Costs
The total cost for the installation of 118 deep-rooted and
90 shallow-rooted trees was approximately $25,000. Ad-
ditionally, installation of the surface water control system
and one year of on-site maintenance totaled about $15,000.
66
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6.3.5 Contacts
George Prince
US EPA Environmental Response Team
Edison, NJ 00837
(732)321-6649
6.4 Bioengineering Management: U.S.
Nuclear Regulatory Commission
Beltsville, MD
Site name:
Location:
Media:
Primary contaminant
and maximum
concentration:
Type of plant:
Area of planting:
Date of planting:
Bioengineering Management
U.S. NuclearRegulatory Commission
Beltsville, Maryland
Landfill cover
Cover was installed over a lysimeter
Pfitzer juniper and fescue
70 m x 45 m test plots
1987-present
6.4.1 Site Description
Three distinct landfill cover concepts were investigated
at the University of Maryland Agricultural Experiment Sta-
tion, sponsored by the Office of Nuclear Regulatory Re-
search, in Beltsville, MD. The purpose of the full-scale dem-
onstration was to examine and demonstrate various ap-
proaches for minimizing water infiltration through landfill
covers. The study, initiated in 1987, evaluated the following
type of design concepts: resistive layer barrier (compacted
clay design), conductive layer barrier (capillary design), and
bioengineering management surface barrier (runoff control/
evapotranspiration design).
Among the three basic design types in the study, the
bioengineering management design demonstrated the great-
est potential for preventing water infiltration and for manag-
ing subsidence conditions. The bioengineering management
surface barrier utilizes the evapotranspiration processes of
vegetation and enhancements to runoff to prevent water in-
filtration to underlying waste. By diverting enough annual
precipitation to runoff and by removing moisture from the
soil profile using evapotranspiration processes, the design
can potentially prevent deep percolation on a yearly basis.
The bioengineering management design is based on a
similar cover design applied at Maxey Flats, KY. In the Maxey
Flats project, a bioengineered cover was constructed by
partially covering fescue grass with an engineered cover of
stainless steel that resulted in no measured percolation of
waterthrough the cover.
During the Beltsville, MD 9-year study, the bioengineer-
ing management surface layer also prevented deep perco-
lation. In addition, this type of design is easily repairable
and involves a minimum amount of materials, equipment,
and labor for construction. Thus, this system provides a
potentially effective approach for addressing damages from
active subsidence conditions.
6.4.2 Goals
The goal of the project was to assess several landfill
cover designs for controlling water infiltration in a humid
region. Results of the demonstration could be applicable
to various types of disposal materials such as radioactive
waste, uranium mill tailings, hazardous waste, and sani-
tary waste.
6.4.3 Design
Cover performance was demonstrated in six large-scale
lysimeters with dimensions of 70 ft x 45 ft, a slope grade of
5%, and the bottom of the lysimeters at 10 ft below grade.
For each lysimeter, underlying waste conditions were simu-
lated by applying the contents of 55-gallon steel drums one-
third filled with gravel and by tilling the remaining area with
native soil. Table 6-4 summarizes the design type for each
lysimeter.
The bioengineering management surface barrier in Lysim-
eters 1 and 2 was installed in May 1987. The bioengineering
management technique utilizes a combination of engineered
enhanced runoff and vegetation to minimize water infiltra-
tion. The covers consisted of 4-ft-wide rows of alternative
aluminum and fiberglass panels with Pfitzer Junipers planted
between the panels at 4-inch widths. The alternating alumi-
num and fiberglass panels covered over 90% of the surface
layer of the cap. Pfitzer Juniper was chosen in part because
of its drought-resistant characteristics and the success that
the Maxey Flats project encountered using this type of veg-
etation. The water levels for Plots 1 and 2 were approxi-
mately 35 and 75 inches above the bottom of the lysim-
eters, respectively. The water levels were used to simulate
the watertable in the flooded disposal cell.
In May 1987, reference Lysimeters 3 and 4 were con-
structed alongside Lysimeters 1 and 2. The reference lysim-
eters are similar in design to Plots 1 and 2. However, the
cover designs for the reference plots contain only fescue
grass; no impermeable cap was installed. Additionally, Plot
4 was discontinued as a reference lysimeter in February
1988 and converted to a rip-rap surface layer and gravel
drainage layer over a compacted clay layer cover. The cap
design in Lysimeter 5 was a vegetated soil surface layer,
Table 6-4. Design Type and Completion Dates for the Experimental
Covers.
Lysimeter
1
2
3
4
4
5
6
Description of Date of Completed
Design Construction
Bioengineering Management
Bioengineering Management
Vegetated Crowned Soil Cover
(reference plot)
Vegetated Crowned Soil Cover
(reference plot)
Rip-Rap over Resistive Layer
Barrier
Resistive Layer over Conductive
Layer Barrier
Vegetation over Resistive Layer
Barrier
May 1 987
May 1 987
May 1 987
May 1 987
October 1988
January 1990
April 1989
67
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gravel drainage layer, and compacted clay layer over a
gravel capillary barrier. In Plot 6, the cap design consisted
of a vegetated surface layer and gravel drainage layer over
a compacted clay layer.
6.4.4 Monitoring Approaches
For the 9-year study, several water balance parameters
were measured, such as annual precipitation and runoff.
Additionally, neutron probe measurements of soil moisture
in all six lysimeters were taken continuously for 8 years. To
increase the accuracy of the measurements, the neutron
probe apparatus was calibrated with the native soil used for
each of the lysimeters or plots. Instruments were placed
between liners to measure leakage at each lysimeter. Four
liners were used in each plot to create a closed system for
a complete water balance system.
6.4.5 Results and Status
During the 9-year period (1988-1996), the bioengineered
covers in Lysimeters 1 and 2 experienced no deep percola-
tion. Additionally, the watertables for both plots were elimi-
nated by July 1989. Table 6-5 illustrates the percentage of
rainfall managed by runoff, evapotranspiration, and deep per-
colation at the two bioengineering management surface bar-
riers. The percent of precipitation associated with evapo-
transpiration increased annually because of the greaterveg-
etative canopy.
The moisture content of the soil profiles for Lysimeters 1
and 2 decreased annually. Thus, afterthe watertable was
eliminated from both lysimeters, the soil profiles continu-
ally "dried out." However, the soil moisture content for both
lysimeters, although much lower in the soil profile than at
the beginning of the study, still increased with depth. In ad-
dition, seasonal cyclical variations in moisture content oc-
curred throughout the study. For example, high moisture
peaks in the soil profile were observed during high incidents
of rainfall events and periods of low evapotranspiration.
A majority of precipitation for reference Lysimeters 3 and
4, with only fescue grass, was managed by evapotranspira-
tion. However, this process was not adequate to prevent the
rise of the watertable. Subsequently, both lysimeters were
pumped to prevent the water from overflowing. As a result,
Table 6-5. Summary of Run-off, Evapotranspiration, and Deep
Percolation From the Bioengineered Plots.
Year
Runoff Evapotranspiration Deep Percolation
Percent of Precipitation
1988
1989
1990
1991
1992
1993
1994
1995
1996
80
74
70
67
63
61
61
58
57
20
26
30
33
37
39
39
39
43
0
0
0
0
0
0
0
0
0
a rip-rap surface layer cover was installed on Lysimeter 4
in February 1988. Lysimeter 3 continued as a reference
plot throughout the study, although results continuously
indicated deep percolation. For example, deep percola-
tion accounted for 40% of the fate of total precipitation
during 1993-1994.
The bioengineering management surface barrier has
been implemented at two sites in Hawaii and New York. At
the Marine Corps Base in Kaneohe Bay, Hawaii, a 14-
month study was conducted to demonstrate diversion and
removal of annual precipitation by runoff control and evapo-
transpiration. The Kaneohe Bay study evaluated two types
of designs: 20% enhancement of runoff and 40% enhance-
ment of runoff, along with a conventional soil cover de-
sign. The covers with the runoff enhancements used rain
gutters and several native types of vegetation such as
grasses and shrubs (primarily of the genera Acacia and
Panicum). Results of the study demonstrated that the two
design types increased runoff by a factor of 2 to 3 over the
conventional soil design cover. Additionally, the data indi-
cated a reduction in percolation by a factor of 2 to 3 from
the two infiltration control covers over the soil cover, al-
though these differences were not statistically significant.
Finally, statistical tests indicated no advantage of using
the 40% enhancement of runoff over the 20% enhance-
ment of runoff: both produced the same amount of runoff
and percolation.
In 1993, the New York State Energy Research and De-
velopment Authority implemented a bioengineering man-
agement system at a low-level waste facility in West Val-
ley, NY. The bioengineered cover was only installed over
one trench at the site, 550 ft x 35 ft, for a total cost of
$70,000. Soil moisture and trench leachate data have
shown no vertical infiltration to date.
6.4.6 Contacts
Edward O' Donnell
NRC Project Manager
U.S. Nuclear Regulatory Commission
Washington, DC 20555-0001
(301)415-6265
6.5 Lakeside Reclamation Landfill
Beaverton, Oregon
Site name:
Location:
Media:
Primary contaminant
and maximum
concentration:
Type of plant:
Area of planting:
Date of planting:
Lakeside Reclamation Landfill
Beaverton, Oregon
Landfill Cover
NA
Hybrid Poplars and Rye Grass
8 acres
1990-present
68
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6.5.1 Site Description
The Lakeside Reclamation Landfill (LRL), located near
Beaverton, OR, is actively receiving nonrecyclable con-
struction demolition debris for disposal on the 60-acre site.
As the waste cells fill to capacity, the owner/operator of
the landfill is required to install covers to fulfill final closure
requirements. In April 1990, a 0.6-acre prototype cap con-
sisting of hybrid poplar trees, primarily cottonwoods, was
installed on a recently filled waste cell. The demonstration
cover was an alternative to using a conventional cover con-
sisting of a geosynthetic membrane and several soil lay-
ers. The objective of the initial project was to demonstrate
the application of hybrid poplar trees to effectively prevent
infiltration of water to underlying waste. The prototype plot
was designed to provide the data required to meet regula-
tory compliance and to provide enough information to close
the entire landfill using this capping technique. The migra-
tion of contaminated leachate is a concern to the Oregon
Department of Environmental Quality (DEQ) because the
Tualatin River is located adjacent to the LRL.
The waste buried at Lakeside Reclamation Landfill is
restricted primarily to construction demolition debris, tree
stumps, yard wastes, and packaging and crating materi-
als. The contaminants of concern are metals, nitrates, and
phosphorous leachate.
Based on the results of the initial prototype cap, the dem-
onstration was expanded to a 2-acre area in 1991. To date,
the cap has expanded to a total of 8 acres and has dem-
onstrated no deep percolation. For the final closure of the
landfill, the vegetative cover will enclose an area of ap-
proximately 40 acres at the Lakeside Reclamation Land-
fill.
For the LRL landfill, the Oregon DEQ has issued a per-
mit for final closure requirements under Condition 3 of
Schedule C, Solid Waste Disposal Permit No. 214. The
final cover must meet or exceed the mandatory minimum
groundwater quality protection requirements.
6.5.2 Goals
The goal of the initial project was to demonstrate an ef-
fective cover design for preventing water infiltration and ac-
quire enough data from the prototype cap to satisfy the
Oregon DEQ. Concurrently, the project must meet or ex-
ceed the regulatory requirements for groundwater quality
protection and establish a capping technique forthe entire
landfill. Other objectives of installing this cover design were
to provide a low-cost manageable coverthat provides a wild-
life habitat, stable soils, and a sustainable ecosystem.
6.5.3 Design
The cover design forthe prototype cap is composed of
hybrid poplars (primarily cottonwood trees) and silt loam
soils. The waste cells of the landfill were initially covered
with two layers of silty loam soil at a thickness of roughly 5
ft and graded at a 3% slope. The layer installed over the
waste is approximately 1 ft of compacted silt loam soil that
has a high-clay content. The surface layer consists of
loosely placed loam soil at a depth of 4 ft.
The hybrid poplar tree cuttings and cool-season grasses
were selected as the type of vegetation to be planted on the
initial demonstration cell cover. Hybrid poplars were selected
primarily because of the research being conducted at the
University of Iowa using these trees for buffer systems. Sec-
ondly, the landfill borders the Tualatin River riparian area
populated by both deciduous and conifertrees, thus provid-
ing evidence that the site is capable of growing hybrid pop-
lars. Thirdly, the hybrid poplars had a relatively long growing
season, extending from mid-March through November. Fi-
nally, hybrid poplars offered the potential for dense tree popu-
lation, deep root placement, and large quantities of waterto
be transpired pertree.
In April 1990, approximately 7,455 tree cuttings were
planted on the 0.6-acre site (60 ft x 600 ft), for an average
plant density of 3.4 ft2 per tree. The rows are 42 inches
apart with roughly 1 ft of spacing between the trees in each
row. Three different hybrid poplar varieties were planted in
the prototype cover: DO-1 variety trees from Dula Nurser-
ies, Canby, OR; Imperial Carolina variety from Ecolotree,
Inc., Iowa City, IA; and NE-19 variety from Hramoor Nurser-
ies, Manistee, Ml. All tree varieties were available in 5-ft
cutting lengths and were planted at a depth of 40 inches.
The Imperial Carolina and DO-1 variety were also available
in 2-ft cutting lengths and planted at 15-inch depths. No soil
stabilizers, fertilizers, orpre-emergentweed herbicides were
used to plant the tree cuttings.
The area of the demonstration was expanded by an addi-
tional 1.3 acres in 1991 and a new planting density of 5.2 ft2
pertree. To diversify the cultivar base, 18 new tree varieties
were planted. The new tree cuttings planted were composed
of varieties that had the capability to grow in the Pacific
Northwest region. Forthe 1992 growing season, a cool-sea-
son grass crop, rye grass, had been established on a por-
tion of the demonstration site. The addition of grasses to
the design improved the cover by increasing the evapotrans-
piration process during the tree's dormant period, control-
ling weed growth, and increasing the overall soil stability
during the early periods of tree development. In addition, as
an alternative to grass, another portion of the cover was
mulched with bark chips.
6.5.4 Monitoring Approaches
Lysimeters, piezometers, and tensiometers were installed
in May 1990 to collect water samples and measure mois-
ture content in the soil cover. Four instrument "nests" were
placed on the landfill cover. Each instrument nest contained
three suction lysimeters and three ceramic cup tensiom-
eters installed at 1-, 3-, and 5-ft depths. Another instrument
nest was installed off the prototype cap to provide back-
ground measurements.
6.5.5 Results and Status
The survival rate forthe hybrid poplar trees planted in
April 1990 was greater than 85% and there were no ob-
served losses forthe 1991 growing season. The survival
rates for the Imperial Carolina tree variety and the DO-1
tree variety were greater than 90%. However, the NE-19
69
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variety had only a 54% survival rate. The low survival rate
was likely due to a December freeze that damaged the cut-
tings. Despite this relatively low survival rate, the NE-19
variety is desirable to plant because of its longer growing
season. The base diameter of the tree stem and terminal
bud height was measured to estimate tree growth and vigor.
The extent of tree growth was illustrated by comparing mea-
surements in August 1990 to those in March 1992, which
demonstrated a mean height of 6.8 ft and 12.7 ft, respec-
tively.
Weekly tensiometer data were collected to determine the
moisture content of soils at various locations both on and
off the 0.6 acre cover. The moisture content is described in
general terms, "wet" or "dry." The tensiometers were not
calibrated to the variable soil conditions because of the
mixture of repacked soils in the landfill material. From Sep-
temberto December 1990, the tensiometer indicated mois-
ture content, at the 1-ft horizon, fluctuated considerably from
saturated to very dry depending on precipitation, solar in-
tensity, airtemperature, relative humidity, ground cover, and
shade at the soil surface. In addition, during the growing
season, there was no apparent impact on the tensiometer
reading at the 3- and 5-ft depths, implying no change in the
moisture content. Significantly less water was present at
the 5-ft soil profile in 1991, than in 1990. In addition, mois-
ture content observed in December 1991 indicated the soil
profile had more storage capacity available than was avail-
able in measurements taken the previous year.
Samples of the soil water were acquired through suction
lysimeters as part of the instrumental nest monitoring. Only
one set of samples was screened in May 1990 for nutrients.
Nitrate concentrations in this set of lysimeter samples did
not measure above the EPA's Maximum Contaminant Level
(MCL).
During 1993 and 1994, the Oregon Graduate Institute of
Science and Technology conducted a field experiment at
LRL to investigate leachate production potential under a
grass cap and a hybrid poplar cap. The soil moisture was
monitored in 25 wells (20 in the hybrid poplar plot and 5 in
the grass plot) using a neutron probe and later a capaci-
tance probe. The depths of the wells varied from 32.5 to 59
inches. Annual precipitation was 37 inches for 1993 and 59
inches for 1994. In general, the results of the 2-year study
indicate that the average soil moisture underthe grass cover
was higher than that under the hybrid poplar cover. Addi-
tionally, the vertical soil moisture profile study revealed that
soil moisture varied less underthe poplartrees than under
the grasses.
Currently, the cap has been expanded to 8 acres and con-
sists of 25 varieties of hybrid poplars (cottonwoods). The
diversity in tree species prevents a single disease from de-
stroying the entire tree population and allows for a broader
growing season forthe cap. In addition, another25 varieties
of hybrid poplars are being evaluated in a greenhouse at
LRL for incorporation into the cap. To date, monitoring data
has indicated that moisture has only penetrated to a maxi-
mum depth of 4 feet.
6.5.6 Contacts
Howard Grabhorn
Owner/Operator
Lakeside Reclamation, Grabhorn, Inc.
Beaverton, OR 97006
(503)628-1866
Wesley M. Jarrell
Professor
Oregon Graduate Institute of Science & Technology
P.O. Box 9100
Portland, OR 97291
(503)690-1183
Louis A. Licht
President
Ecolotree, Inc.
102AOakdaleHall
Iowa City, IA52319
(319)358-9753
MarkF. Madison
Agricultural Engineer
CH2MHHI
Portland, OR
(503) 235-5002 Ext. 4453
6.6 Alternative Landfill Cover
DemonstrationSandia National
Laboratories Albuquerque, NM
Site name:
Location:
Media:
Primary contaminant
and maximum
concentration:
Type of plant:
Area of planting:
Date of planting:
Alternative Landfill Cover Demonstration
Sandia National Laboratories
Albuquerque, New Mexico
Landfill Cover
Covers were installed over lysimeters
with no waste or contaminants
Wheat Grass, Indian Ricegrass, Alkali
Sacaton, Sand Dropseed and Four-
Wing Saltbrush
10 m X 10 m test plots
1995-1996
6.6.1 Site Description
The Alternative Landfill Cover Demonstration (ALCD) is
a large-scale field test comparing final landfill cover de-
signs at Sandia National Laboratories, located on Kirtland
Air Force Base in Albuquerque, New Mexico. The demon-
stration is testing innovative landfill covers using currently
70
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accepted EPA cover designs as baselines. Two conven-
tional and four alternative cover designs were constructed
in 1995 and 1996 and are currently being monitored. The
two traditional cover designs are a RCRA Subtitle D Soil
Cover and a RCRA Subtitle C Compacted Clay Cover, and
the four alternatives are Geosynthetic Clay Cover, Capil-
lary Barrier, Anisotropic Barrier, and Evapotranspiration
Cover. Of the four alternatives, the Evapotranspiration
Cover utilizes vegetation, along with soil texture and depth,
as the primary mechanism to minimize infiltration of water
into underlying waste. To a lesser degree, the Capillary
Barrier and the Anisotropic Barrier are also designed to
enhance evapotranspiration using vegetation and encour-
age water storage for the prevention of water infiltration.
In addition, the ALCD includes a side study to assess eight
different enhancement techniques to the vegetation planted
on several of the covers.
6.6.2 Goals
The purpose of the demonstration is to evaluate the vari-
ous cover designs based on their respective water balance
performance, reliability and ease of construction, and cost
for arid and semiarid environments. Also, the data from the
demonstration will be available for validation of EPAs HELP
(Hydrologic Evaluation of Landfill Performance) model.
6.6.3 Design
All of the test covers are installed and instrumented in a
side-by-side demonstration. Each of the plots are 100 m x
13 m, crowned in the middle with a constructed 5% slope
for each layer. Hence, the slope lengths are 50 m each and
sloping to the west and east. The western slope of each
cover is monitored under ambient or passive conditions.
For the eastern slope, a sprinkler system is utilized to fa-
cilitate additional precipitation to the test plots, providing
hydrological stress to the various covers. This system rep-
resents peak or worse case precipitation events.
The two conventional cover designs were installed to pro-
vide a baseline for comparison among the four alternatives.
The Soil Cover design satisfies the minimum requirements
set forth for
RCRA Subtitle D landfills, which are typically municipal
solid waste landfills. The second baseline cover is a Com-
pacted Clay Cover designed to meet the RCRA Subtitle C
requirements for hazardous waste landfills. Among the four
alternative cover designs, the Geosynthetic Clay Cover is
the most similar in design and function to the traditional
compacted clay cover. The Geosynthetic Clay Cover de-
sign is identical to the compacted clay cover except forthe
clay barrier layerwhich consists of a manufactured sheet, a
geosynthetic clay liner (GCL).
The Evapotranspiration (ET) Cover consists of a single,
vegetative soil layer constructed with an optimum mix of
soil texture, soil thickness, and vegetation cover. The cover
is engineered to increase water storage and enhance evapo-
transpiration to minimize the infiltration of water. The design
of the cover was based on the results of a computer model
and the climate conditions of the area. The Albuquerque
climate is an arid/semiarid environment with average an-
nual rainfall of 20.6 cm/yr. Based on the results, a 90-cm-
thick monolithic soil cap was installed. The origin of soil
was from on-site cut excavations. The bottom 75 cm was
placed in 15-cm-deep lifts and compacted, while the top
15 cm was loosely placed topsoil. The type of vegetation
used in the design was based on the premise of extending
the evapotranspiration process of the plants over as much
of the growing season as possible. Therefore, the vegeta-
tion consisted of an optimum mixture of species (such as
grasses, shrubs, or trees) and an optimum blend of cool
and warm weather plants. The vegetation drill-seeded for
the cover was composed of native species, primarily
grasses such as crested wheat grass, Indian ricegrass,
alkali sacaton, sand dropseed, and four-wing saltbrush.
A side study was established to assess enhancements
to induce the growth of vegetation seeded on the ET Cover.
Twenty-four 10-m x 10-m test plots were installed along-
side the cover. Accordingly, there are three sets of eight
different surface augmentations for statistical analysis. Soil
moisture is being sampled at a depth of 1.2 m to evaluate
the effect of the surface enhancements on evapotranspi-
ration.
The Capillary Barrier is designed to use the difference in
hydraulic conductivity of the two soil layers under unsatur-
ated flow conditions to cause water to be retained in the
upper soil layer. The cover design of this capillary barrier
was composed of four primary layers: surface layer, upper
drainage layer, barrier layer, and lower drainage layer. In
general, the cover design consisted of the barrier layer
and lower drainage layer forming the capillary barrier. The
upper drainage layer, composed of pea-gravel and sand,
served as both a filter to prevent clogging and allowed for
lateral water movement. The surface layer, 30 cm of top-
soil, is placed to provide a medium for growth of vegeta-
tion and enhance evapotranspiration. It also protects the
barrier soil layer from desiccation and protects against
surface erosion.
The Anisotropic Barrier is composed of layered capillary
barriers that function to limit downward movement of wa-
ter while enhancing lateral movement. The cover design
consists of four layers: top vegetation layer, soil layer, in-
terface layer, and sublayer. The top vegetation layer is 15
cm thick and is composed of topsoil and pea gravel (gravel
to soil mixture is 25% by weight). The vegetation is for
encouraging the evapotranspiration processes of the veg-
etation, while the pea gravel is primarily for minimizing ero-
sion effects. The soil layer is 60 cm of native soil and func-
tions for water storage and a rooting medium for vegeta-
tion. The interface layer serves as a drainage layer to lat-
erally divert water that has infiltrated the soil layer. Both
the interface layer and sublayer function as bio-barriers to
prevent roots and burrowing animals from intruding into
the underlying material.
6.6.4 Monitoring Approaches
Continuous water balance and meteorological data are
being collected for all six covers. In addition, periodic mea-
71
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surements are being taken to produce data on vegetation.
Continuous water balance data include soil moisture sta-
tus, soil temperature, runoff and erosion, and percolation
and interflow. At a weather station installed at the ALCD
site, meteorological data is being collected forthe following
parameters: precipitation, air temperature, relative humid-
ity, wind speed and direction, and solar radiation. Finally,
several attributes of vegetation are being measured sea-
sonally, such as biomass, leaf area index, and species com-
position.
6.6.5 Results and Status
Monitoring data will be collected for a minimum of 5 years
after construction of the covers. Additionally, the data will
be made available on a yearly basis. Table 6-6 displays the
results from the first year of collecting data forthe six cover
designs (May 1997 through March 1998).
Table 6-6. Summary of Percolation and Precipitation Rates From
May 1997 Through March 1998 for the Six Cover Designs.
Percolation/
Description Percolation (L) Precipitation (L) Precipitation (%)
RCRA Subtitle D 6,724 380,380
Soil Cover
RCRA Subtitle C 46 380,380
Compacted Clay
Cover
Geosynthetic Clay 572 380,380
Liner Cover
Evapotranspiration 80 380,380
Cover
Capillary Barrier 804 380,380
Anisotropic Barrier 63 380,380
1.77
0.01
0.15
0.02
0.21
0.02
According to Stephen Dwyer, the site manager, the
amount of percolation through the Capillary Barrier can be
attributed to the initial design of the cap. The fine layer of
the cover was installed below the surface to protect against
desiccation, freeze-thaw cycles, etc. Unfortunately, this
design specification does not allow for maximum enhance-
ment of evapotranspiration processes. This concept is fur-
ther illustrated by the Anisotropic Barrier, designed with the
fine layer at the surface, which seems to be performing ad-
equately to date.
6.6.6 Costs
The individual construction cost for each cover is pre-
sented in Table 6-7. These values only represent construc-
tion costs and do not include instrumentation equipment,
monitoring provisions, or other items associated with cover
testing.
6.6.7 Contact
Stephen F. Dwyer
Sandia National Laboratories (MS 0719)
P.O. Box 5800
Albuquerque, NM 87185
(505)844-0595
Table 6.7 Construction Costs for the Final Landfill Covers
Description Cost ($/
m2)
RCRA Subtitle D Soil Cover
RCRA Subtitle C Compacted Clay Cover
Geosynthetic Clay Liner Cover
Evapotranspiration Cover
Capillary Barrier
Anisotropic Barrier
52.42
157.58
92.89
72.66
96.45
74.92
72
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Appendix A
Glossary
Accumulation coefficient: The ratio of a contaminant
concentration in biomass to the contaminant concentra-
tion in soil.
Alpine pennycress (Thlaspi caerulescens): A poten-
tially useful, but slow-growing and low- biomass
hyperaccumulator plant.
Application(s): Something that is done by a
phytoremediation technology; e.g., remediation of TCE in
groundwater.
Bioaccumulation coefficient: The ratio of metal con-
centration in the plant (g metal/g dry weight tissue) to the
initial solution concentration of the metal (mg metal/L), for
rhizofiltration of metals (Salt et al. 1995).
Bioconcentration factor (Bv): The concentration in
aboveground plant parts (on a dry-weight basis) divided
by the concentration in the soil for organic compounds
(Paterson etal. 1990).
Biogeochemical prospecting: Exploration for mineral de-
posits through analysis of metal concentrations in plants
that might indicate underlying ore bodies.
Constructed wetlands: Artificial or engineered wetlands
used to remediate surface water or waste water.
Eastern cottonwood (Poplar) (Populus deltoides): A
widely studied tree with potential for hydraulic control,
phytodegradation, and phytovolatilization.
Ecosystem restoration: "The process of intentionally al-
tering a site to establish a defined, indigenous ecosystem.
The goal of this process is to emulate the structure, func-
tion, diversity, and dynamics of the specified ecosystem."
(Parks Canada, http://parkscanada.pch.gc.ca/natress).
Evapotranspiration cap (or cover): A cap composed of
soil and plants, engineered to maximize evaporation and
transpiration processes of plants and the available storage
capacity of soil to minimize infiltration of water. Synonym:
\AMer-balance Cover
Fibrous root: A root system that has numerous fine roots
dispersed throughout the soil.
Forensic ecology: Investigation of a site to determine
the history and causes of the current flora and fauna.
Forensic phytoremediation: Investigation of a naturally-
revegetated site to establish that remediation has occurred
or has begun to occur.
Geobotanical prospecting: The visual study of plants
as indicators of the underlying hydrogeologic and geologic
conditions.
Geobotany: The use of plants to investigate the under-
lying geology, especially related to metal ores.
Halophyte: A salt-resistant-plant; one that will grow in
saline soil. Salt cedar is an example.
Humification/fixation: The incorporation of contami-
nants into biomass in soil.
Hydrologic control: The use of plants to rapidly uptake
large volumes of water to contain or control the migration
of subsurface water. Synonym: Phytohydraulics.
Hyperaccumulators: Metallophytes that accumulate an
exceptionally high level of a metal, to a specified concen-
tration orto a specified multiple of the concentration found
in other nearby plants. Alpine pennycress is an example.
Indian mustard (Brassica juncea): A potentially use-
ful and relatively high biomass hyperaccumulator plant that
can accumulate metals and radionuclides.
Indicator plants: Plants with a metal concentration in
the aboveground biomass that reflects the soil concentra-
tion of the metal. Bladder campion is an example.
Land reclamation: The revegetation of eroded or
nonvegetated land to decrease erosion of the soil or to
increase the beneficial uses of the soil.
Metal-tolerant plants: Plants that can grow in metal-
rich soils without accumulating the metals.
Metallophytes: Plants that can only grow in metal-rich
soils.
Phreatophyte: A deep-rooted plant that obtains water
from the water table.
A-1
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Phytoaccumulation: The uptake and concentration of
contaminants (metals or organics) within the roots or
aboveground portion of plants.
Phytodegradation: The breakdown of contaminants
taken up by plants through metabolic processes within the
plant, or the breakdown of contaminants external to the
plant through the effect of compounds (such as enzymes)
produced by the plants. Synonym: Phytotransformation.
Phytoextraction: The uptake of contaminants by plant
roots and translocation into the aboveground portion of
the plants, where it is generally removed by harvesting the
plants. This technology is most often applied to metal-con-
taminated soil or water. See also: Phytoaccumulation.
Phytoextraction coefficient: The ratio of metal con-
centration in the plant (g metal/g dry weight tissue) to the
initial soil concentration of the metal (g metal/g dry weight
soil), for phytoextraction of metals (Nanda Kumar et al.
1995).
Phytoinvestigation: The examination of plants at a site
for information about contaminant presence, distribution,
and concentration.
Phytoremediation: The direct use of living green plants
for in situ risk reduction for contaminated soil, sludges, sedi-
ments, and groundwater, through contaminant removal, deg-
radation, or containment. Synonyms: Green remediation,
Botano-remediation.
Phytoremediation cap (or cover): A cap consisting of
soil and plants, designed to minimize infiltration of water
and to aid in the degradation of underlying waste.
Phytoremediation natural attenuation: Intrinsic
bioremediation processes (in soil orgroundwater) enhanced
by the presence of naturally-occurring plants.
Phytostabilization: Immobilization of a contaminant
through absorption and accumulation by roots, adsorption
onto roots, or precipitation within the root zone of plants.
Phytovolatilization: The uptake and transpiration of a
contaminant by a plant, with release of the contaminant or
a modified form of the contaminant to the atmosphere from
the plant.
Process, mechanism: Something that a plant does; e.g.,
uptake, transpiration.
Rhizodegradation: The breakdown of a contaminant in
soil through microbial activity that is enhanced by the pres-
ence of the root zone. Synonyms: Plant-assisted degrada-
tion, Plant-assisted bioremediation, Plant-aided in situ bio-
degradation, Enhanced rhizosphere biodegradation.
Rhizofiltration: The adsorption or precipitation onto plant
roots or absorption into the roots of contaminants that are
in solution surrounding the root zone.
Rhizosphere: The zone around plant roots that has sig-
nificantly higher microbial numbers and activity than in the
bulk soil.
Root concentration factor (RCF): The concentration
in the roots divided by concentration in external solution,
for non-ionized organic compounds taken up by plants with
nonwoody stems (Ryan et al. 1988; Paterson et al. 1990).
Root exudates: Chemical compounds such as sugars
or amino acids or that are released by roots.
Stem concentration factor (SCF): The concentration
in the stem divided by the concentration in the external
solution, for non-ionized organic compounds (Ryan et al.
1988; Paterson etal. 1990).
System: The overall picture; e.g., a constructed field plot
that uses mechanisms within technologies fora particular
application.
Tap root: A root system that has one main root.
Technology: A combination of processes and mecha-
nisms; e.g., phytoextraction, rhizodegradation (BP uses and
favors this terms).
Transpiration stream concentration factor (TSCF):
The concentration in the transpiration stream divided by
the concentration in the external solution, for organic com-
pounds (Paterson et al. 1990).
Vegetative cap (or cover): A long-term, self-sustaining
cap of plants growing in and/or over materials that pose
environmental risk; a vegetative cover reduces that risk to
an acceptable level and requires minimal maintenance. Two
specialized types of vegetative caps are the Evapotranspi-
ration Cap and the Phytoremediation Cap.
Vegetative soil stabilization: The holding together of
soil by plant roots to decrease wind or water erosion or
dispersion of soil.
A-2
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Appendix B
Phytoremediation Database
Project Name/
Location
Aberdeen Pesticide
Dumps Site - NC
Aberdeen Proving Grounds
J - Field Toxic Pits Site - MD
ACME landfill -NC
AGI-WA
Agricultural Cooperative - Wl
Agricultural Cooperative -Wl
Aluminum Manufacturing
Facility -SC
Aluminum Processing
Facility - NV
Amargosa Desert Research
Amarillo -TX
Amboer Road - OR
Anderson - SC
Annette Island Site -AK
Anonymous - KS
Army Ammunition
Plant -IL
Artemont, CA
DOE Facility -OH
Size
of Area
7 acres
1 acre
~ 2 acres (proposed)
~ 0.3 acres
0.9 acres
6 acres
6 acres
~ 5 acres
1 7 acres
Each plot 1 00
square ft.
~ 1 00 acres
Primary
Contaminant
VOCs, Pesticides/
Herbicides
Halogenated Volatiles
Phenols
Pesticides, herbicides;
VOCs
Ammonia
Heavy metals
Saline wastewater
from cooling tower
Radionuclides
Chlorinated Solvents
Heavy Metals
Semi-volatile Petroleum
Products
TPH
Municipal and hazardous
waste
Uranium, cesium,
strontium
Media and
Properties
Groundwater
Soil and groundwater
Groundwater
Surface water
Soil and groundwater
Soil
Soil and groundwater;
sandy clay
Dune sand
Groundwater and soil
Groundwater and soil
Native soil
Groundwater and
wastewater
Vegetative
Type
Hybrid Poplar
and ground cover
grasses
Hybrid Poplar
Poplar
Wetland plants
Hybrid poplar
Hybrid poplar
and grass
Hybrid poplar
Hybrid willows
Hybrid and native
poplar
Hybrid poplar,
Grasses
Fescue;, ryegrass,
and clover
Cottonwood, hybrid
poplar
Various trees
Sunflowers
Date
Planted
March 1996
Not planted
yet
May 1996
June 1997
May 1992
May 1997
April 1999
1993
Summer
1998
Spring
1998
November
1997
Barje Landfill - Slovenia
10 acre
Other, Heavy Metals
Subsurface soil
Hybrid poplar
1993,1994
B-1
-------
Project Name/
Location
Barrow Site -AK
Bayonne- NJ
Beltsville, MD
Bluestem Landfill #1 - IA
Bluestem Landfill #2 - IA
Bofors-Nobel - Ml
British Steel -
South Wales, UK
Bunker Hill -ID
C-H Plant Area -TX
Calhoun Park-SC
Campion Site-AK
Cantrall-IL
CarswellAFB (former) -TX
Brookhaven National
Labs - NY
Chernobyl -Ukraine
Chevron Facility
No. 1 29-0334 - UT
Chevron Site - CA
Chevron Station
No. 7-7992 -CO
Childerburg-AL
CityofGlendale
Landfill -AZ
City of Madras WWTP
Reuse - OR
WWTP Sludge Lagoon - OR
Natural Treatment
System (NTS) - OR
Size
of Area
Each plot is ~ 1 20
square ft.
1 ,000 square ft.
70' x 45' x 10'
3 acres
5 acres (2 test plots)
18 hectares
1 ,050 acres
27 acres
0.5 acre
Each plot is ~ 375
square ft.
1 -3 acres
1 acre
~ 1 /4 acre
30 acres, 90 acres
300 x 15 ft. area
(120 poplar trees)
1 acre (2 test covers)
1257. 5 acres
4,200 acres
Primary
Contaminant
VOCsand Semi-volatiles
Heavy metals
Radioactive - low level
waste
Leachate
Pesticides, herbicides,
dyes
Coke oven effluent
Heavy metals
Salt, metals (possible
radionuclides
Non-halogenated
Semi-volatiles
Volatile and Semi-volatile
Petroleum Products
Pesticides/Herbicides
Halogenated volatiles
Radionuclides
Radionuclides
Volatile petroleum products
Volatile petroleum products
Volatile petroleum products
Explosives
Wastewater; nitrogen;
phosphorus
Heavy metals, PCB's
Land-applied wastewater
Media and
Properties
Soil. Soils marine beach
Soil to 15cm bgs
Subsurface soil
Subsurface soil
Soil, sediments
Effluent
Soil
Groundwater
Ground 1 to 5' bgs
Soils will be more fully
characterized fall 1998
Soil and groundwater
Groundwater
Landscaping soils to
6 inches in depth.
Soil, groundwater, water
Groundwater
Soil at root level,
groundwater, wastewater
Groundwater
Soil at root level
Silty sand soils
Old sewage sludge
applied to grazing land
Wastewater
Vegetative
Type
Mix of grasses and
legumes planned
for spring 1999
Indian Mustard
Pfitzers junipers
Hybrid poplar and
grass
Hybrid poplar and
grass
Trees and wetland
plants
Soil based reed
bed
Mix of herbaceous
species
Trees
Native trees and
shrubs
Mix of grasses
and legumes
Hybrid poplars
Eastern Cottonwood
Redroot Pigweed
Sunflowers, Indian
Mustard
Hybrid poplar trees
(DN 34)
Fescue, Cowpeas,
cattails
Hybrid poplar trees
Parrot Feather
Ryegrass, Bermuda
grass
Turf grass
Poplars
Date
Planted
Spring 1999
Spring 1997
1987
1994
1994
Spring 1999
1998-2001
Planned
1999
Summer
1998
1992
April 1996
May 1 998
April 1 996
April 1 995
Already
planted
1990
CityofWoodburnWWTP-OR 7 acres
Other (Wastewater reuse) Soil
Hybrid Poplar
1995
B-2
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Project Name/
Location
Closed Disposal Facility - IL
Closed Terminal - Rl
Coffin Butte Landfill - OR
Columbus -OH
Corvallis-OR
Craney Island Fuel
Terminal -VA
Bearing (Cherokee
County) - KS
Chevron Bulk Facility
100-1 838 -OR
Delaware Solid Waste
Authority - DE
Diashowa Paper Mill - WA
Dieners-CA
Dorchester - MA
Eagle Flat - TX
East Ravendale -
Yorkshire, UK
Edward Sears Site
(Superfund) - NJ
Farm Service Facility - MN
Farmer's Loop Site -AK
Fertilizer Plant Site -SD
Fly ash Landfill
Covercap-MO
Fly ash Landfill -MO
Forest Grove WWTP - OR
Chevron Terminal
No. 1 29-0350 -UT
Former Farm Market - Wl
Former Farm Market - IL
Size
of Area
26 acres
~ 1 acre
14.4 acre overland
polishing system
1 acre
~180'x100'(4
plots with replicates
1 acre
0.6 acres planted
with grass
5 hectares
~ 1,200 square ft.
240 square miles
1/3 acre, 0.5 acre
0.4 acres
Multiple plots, each
~10'x10',2-3'deep
6 acres
5 acres
~ 5 acres
1 acre
1 acre
Primary
Contaminant
Petroleum Hydrocarbons
PAHs
Landfill leachate
Volatile petroleum
Hydrocarbons
TPH, PAHs
Heavy metals
Volatile petroleum
products
Fuel Hydrocarbons
Heavy metals
Lead
Radionuclides- Low
level waste
BOD, suspended solids
Halogenated volatile
petroleum products
Ammonia
Semi-volatile petroleum
products
Nitrogen
Fly ash
Volatile petroleum products
Pesticides, nitrates, and
ammonium
Pesticides, nitrates, and
Media and
Properties
Surface water
Surface soils
Leachate
Soil
Soil
Soil
Surface soil
Soil
Soil, groundwater and
effluent are affected
Soil
WWTP Effluent
Soil and groundwater
Soil
Soil
Soil and groundwater
Soil
Groundwater and vadose
zone are contaminated
Soil, groundwater
Silt loam with clay
Vegetative
Type
Groundcover plants
in combination with
phreatophyte trees
Indigenous plants/
trees
Grass, hay, and
native trees
Hybrid poplars;
ground cover
Fescue, rye, clover
Hybrid poplars
Tall fescue
Variety of grasses
and clovers.
Brassica sp.
Indian Mustard
Semiarid prairie
Soil based reed
bed
Hybrid poplar,
willow
Hybrid poplar and
grass
Ryegrassand
red fescue
Hybrid poplars
Hybrid poplars
Hybrid poplars
Hybrid poplar and
grass
Hybrid poplars,
fescue, alfalfa
Hybrid poplars
Hybrid poplars
Date
Planted
Existing
1997
March 1995
April 1995
1995
June 1997
1991
December
1996
May 18,
1998
Summer
1995
May 1996
May 1994
Spring 1992
Spring 1992
ammonium
B-3
-------
Project Name/
Location
Former Farm Market - SC
Former Farm Market - IL
Former Fertilizer Facility - OK
Former Fertilizer Facility - NC
Former Fertilizer Facility - NJ
Former Municipal
Landfill -NY
Petroleum Processing
Facility - PA
Standard Oil Facility
No. 100-1 348 -WA
Former Truck Depot - LA
Fort Carson (Landfills
5 and 6) -CO
Fort Lewis Army Base - WA
Fort Richardson -AK
Fort Riley - KS
Fort Wainwright -AK
Gardner Avenue - CY
Great River Regional Waste
Authority - IA
Green II Landfill -OH
Greenbe It Project -WY
Greenhouse studies of
Phytoremediation
Grundy County Landfill - IA
GulfcoastSite
Hanford Barrier -WA
Hawaii
Size
of Area
1 acre
1 acre
4 acres
1 acre
1 acre
3 acres border area
2 acres
5 acres
0.5 acres
20-40 acres
~ 1 0 acres
4,800 square ft.
2.5 acres
6 acres
30 acres
40 acres
~ 20' x 4' per
experiment
5 acres, 2 acres
1 ,800 square ft.
1 acre
~ 1 acre
Primary
Contaminant
High salt concentrations
Pesticides, nitrates,
ammonium
Nitrates and ammonium
Ammonium and nitrate
nitrogen
Ammonium and nitrate
nitrogen
Heavy metals
TPH in fill soil;
BTEX
Volatile petroleum
products
TPH in fill soil; BTEX
Municipal/mixed waste
Chlorinated solvents
Semi-volatile petroleum
products
Pesticides/Herbicides, Other
Pesticides/Herbicides,
Heavy metals
VOCs and other organics
in leachate
Wood preservatives
Heavy metals, halogenated
volatiles
Leachate
Volatile petroleum products
No waste
Pesticides
Media and
Properties
Surface soil
Silt loam soils
Soil and groundwater
Groundwater
Soil and groundwater
Groundwater
Ash and cinder with soil
fill
Perched aquifer
Groundwater and soil
Groundwater
Soil
Contaminated sediments
Soil and groundwater
Soil, groundwater
Leachate and subsurface
soil
Leachate and soil
Soil
Soil to 1 5' bgs, groundwater
Subsurface soil
Soil to 6", groundwater
Groundwater
Vegetative
Type
Various trees and
grasses
Hybrid poplars
Prairie grasses
Hybrid poplar
Hybrid poplar
and Australian
willows
Hybrid poplars
Hybrid poplar and
hybrid willows
Hybrid poplars
(DN 34)
Hybrid poplar,
hybrid willows
Hybrid poplar
Grasses and
legumes
Felt Leafwillow
Hybrid poplar
and grass
Hybrid poplar
and hybrid willows
Poplar, herbaceous
Gamagrass, poplars
willows
Hybrid poplar
Sorghum, Cowpeas
Sweet clover, rye
Koa (native
plant)
Date
Planted
Spring 1993
Spring 1992
Irrigation
est. 1990
Spring 1992
Fall 1992;
spring 1992;
spring 1994
June-July
1998
June 1996
April 1 995
June 1995
Proposed for
Spring 2000
September
1997
1997-1998
Fall 1998
March
1995-1997
1993,1994
June 1998
Hill Air Force Base - UT
Hillsboro Landfill
Wetlands-OR
No waste
Groundwater
Vegetative cover
54 acres
B-4
-------
Project Name/
Location
Hollola Landfill -
Hollola, Finland
ICI-Billingham, UK
ICI Explosives Americas
Engineering -MO
IMC Global Limited - Ontario,
Canada
IMC, Port Maitland-
Canada
Indianapolis -IN
Industrial Landfill -TN
Iowa
Army Ammunition Depot - IA
Jackson Bottoms
Wetland - OR
Johnson County Landfill - IA
Juniper Utility Co. VWVTP
Effluent Reuse - OR
Kaiser Hill - CO
Kauffman and Minteer- NJ
Keyport Naval Warfare
Facility -WA
Klamath Falls site -OR
Kurdjaly - Bulgaria
Lakeside Reclamation
Landfill -OR
Lamb-Weston Food
Processing Reuse - OR
Lanti Landfill- Finland
Liquid Fertilizer Plant -ND
Los Banos-CA
Size
of Area
3 hectares
5 hectares
~ 3.2 acres
100 acres
1 00 acres
1 -1 .5 acres
3 acres
9 acres
64.5 acres
1 5 to 46 acres
1 ) 50' x 300' and 2)
30' x 30'
~ 8 acres
12 acres (1994)
0.6 acres; 8 acres
>5,000 acres
1 7 hectares
~ 0.25 acres
0.5 hectares
Primary
Contaminant
Ash, oily waste
COD
Explosives, fertilizers
Pesticides/Herbicides
Volatile organic compounds
and thallium
Heavy metals, pesticides/
herbicides
Explosives
Halogenated volatiles,
heavy metals, other
Wastewater; nitrogen;
phosphorus. Secondary
effluent biosolids
Radionuclides
TCE; DCE
Chlorinated solvents,
PCB's
Halogenated semi-volatiles
Heavy metals
Landfill leachate
Food processing
wastewater
Ash
Nitrogen
Heavy metals
Media and
Properties
Effluent
Surface soils, surface
water, and groundwater
Leachate
Groundwater
Groundwater
Soil 0 to 48 cm
Wastewater, soil and
pond water
Subsurface soil
Shallow groundwater
and surface streams
Soil and groundwater
Groundwater
Shallow soil
Soil
Soil and water
Soil and groundwater
Clay, loam
Vegetative
Type
Soil based reed bed
Willows, ninebark
and cypress
Poplar
Hybrid poplar
Hybrid poplar
Hybrid poplar with
grass
Knotweed,
crabgrass
Wetland and
terrestrial
Hybrid poplar and
grass
Ryegrassand
Kentucky
bluegrass
Native cottonwoods;
Hybrid poplar
Black willow; hybrid
poplar
Hybrid poplars
Hybrid poplar and
grass
Alpine pennycress
Hybrid poplar,
grass
Grass, wheat,
barley, corn, alfalfa
Hybrid poplar
Indian Mustard,
Fescue, Trefoil,
Brassica sp.
Date
Planted
1990
February
1996
Spring 1998
1995
Spring 1998
Spring 1997
1992,1993
Not yet.
April 1998
Site in
preparation
1994, June
1995
September
1997
April 1990
May 10,
1996
1991
Magic Marker Site- NJ
0.25 acre, 1 acre, Heavy metals
4500 square ft. study
Shallow soil
Indian Mustard June 1997
(Brassica juncea)
B-5
-------
Project Name/
Location
Magnesite Processing
Plant -WA
Manchester Site -UK
Manufacturing Facility - Ml
Manufacturing Facility - Wl
Matoon -IL
Maxey Flats - KY
Metal Plating Facility - OH
Milan Army Ammunition
Plant (MAAP)-TN
Military site
Mill Creek Correctional
Facility -OR
Mississippi Site
Monmouth Site- NJ
Montezuma West - OR
Monticello-UT
Moonachie- NJ
MS Service Station - NJ
Kennedy Space Center- FL
NCASI Test Cells -Ml
New Hampshire Landfill - NH
Nitrogen Contaminated
Site - MN
Nitrogen Products
Site - AK
Northeast Site
Nu-Glo Site - OH
Size
of Area
1 acre
2 pilot beds each
3m x5m
0.5 acres
2 acres
3 acres
Demonstration
scale
Feasibility study test
cells
3. 5 acres
~ 3 acres
~ 1 acre
2 acres - 46 trees
-0.3 acre
3 acres
2.3 acres
Six buffer areas
(total 5 acres)
2000 square ft.
0.5 acres
Primary
Contaminant
Ammonia
Starch factory effluent
Halogenatedvolatiles
TPH in fill soil
Nitrate nitrogen
Radioactive -low level
waste
Heavy metals, halogenated
volatiles
Explosives (TNT, RDX,
HMX,TNB);BOD5,
nutrients
Explosives
Nitrogen
Volatile and nonvolatile
petroleum products
Radionuclides
Chlorinated volatiles
Radionuclides
Volatile petroleum products;
Halogenated volatiles
GROs
Halogenatedvolatiles,
volatile petroleum products,
heavy metals
Halogenated volatiles
Nitrate and ammonia
Pesticides, herbicides
Volatile petroleum products
TCE, PCE
Media and
Properties
Soil
Effluent
Groundwater, silty
clay soil
Ash and cinder with
soil fill
soil
Soil
Groundwater
Soil
Groundwater and soil
Soil and groundwater
Groundwater
Groundwater
Groundwater
Soil and groundwater
Soil and groundwater
Soil
Soilto2'bgs,
groundwater
Soil and groundwater
Vegetative
Type
Hybrid poplar
(5 varieties)
Soil based reed
bed
Hybrid poplar
Hybrid willow
Hybrid poplar
Indian Mustard
Grass, sweetflag,
parrotfeather
Proprietary
Native black
cottonwood; Hybrid
poplar (total of
8,500 planted
perpendicular to
groundwater flow)
Under consideration
Hybrid poplars
Hybrid poplars;
DN34(Populus
deltoids x P. nigra)
Phreatophyte trees
Hybrid poplars
and grass
Hybrid poplars,
and grass
Hybrid poplar
Perennial Warm
Season Grass,
Sorghum
Hybrid poplar
and willows
Date
Planted
May 1 9-20,
1995
1995
June 1996
June 1996
1994
April-May
1996
May 1 997
Spring 1999
May 24,
1997
May 28,
1997
Spring 1999
March -April
1998
May 1993
March 5-10
1995
May 1993
Fall 1998
B-6
-------
Project Name/
Location
Oconee - IL
Ohio Location
Ohio Site
Oil Refinery - Perth,
Australia
Gas Station - OR
OREMET Titanium, Inc. - OR
Osage River Riparian
Buffer -MO
OshKosh-WI
Palmerton - PA
Palo Alto -CA
Paper Industry
Petroleum Company - KS
Phytoremediation of Soils
from Argonne West - ID
Piketon DOE Facility -OH
Pipeline Site OBJ -MO
Poppy Lane -AK
PortHueneme
Prineville Golf Course
Reuse - OR
Rail Tie Yard -TN
Red Mud Coastal
Restoration Project - LA
Refinery - CA
Reliable Plating Site - OH
Residual Petroleum Waste
Remediation
Reuben Gaunts - Leeds - UK
Riparian Buffer, Grande
Ronde Valley -OR
Size
of Area
1 -3 acres
~ 2 acres
25' x 1 00'
1 0-1 00 square meters
75 square ft.
5 acres
0.1 5 acres
9 square meters
25 square meters
1 acre
0.5 acres
Greenhouse
5 acres
1 .25 acres
1 60 acres
0.75 acres
Test area ~ 2 acres
7,000 square ft.
0.5 acres
0.8 acres
450 square meters
Various, most are a
few acres
Primary
Contaminant
Pesticides/Herbicides
Volatile petroleum products
Volatile petroleum products
Semi-volatile petroleum
products
VOCs and semi-volatiles
Heavy metals
Heavy metals
Semi-volatile petroleum
products
Petroleum Hydrocarbons
Cesium 137, Cr, Hg.Zn,
Ag.Se
Halogenatedvolatiles
Petroleum Hydrocarbons
Volatile petroleum products,
Heavy metals
BODandTSS
Semi-volatile petroleum
products
Iron Sesquioxides
Hydrocarbons
TPHinfillsoil;BTEXin
groundwater
Petroleum Hydrocarbons
Dyes, Residual OPs, COD
Agricultural runoff
Media and
Properties
Groundwater and soil
Soil and shallow
groundwater
Shallow groundwater
Soil
groundwater
Wastewater
Streambanksoil
Soil 0 to 30 cm
Soil at 12cm
Groundwater
Soil
Soil
Soil
Shallow and deep
groundwater
Soil and groundwater
Soil, root level
Groundwater
Wastewater
Soil
Sludge
Soil
Excavated soil
Soil
Effluent
Grande Ronde River
Vegetative
Type
Alfalfa, corn, and
Hybrid poplar
Hybrid poplars; rye
grass
Hybrid poplars
Rye, legumes,
fescue, sedges
Hybrid poplars
(DN-34)
Hybrid poplar
Hybrid poplar and
willow
Fungi,
chrysanthemum p.
sordida
Campion, alpine
pennycress
Tamarisk,
Eucalyptus
Hybrid poplar
Hybrid poplars
Hybrid willow,
canola, Brassica
Hybrid poplars; rye
grass
Alfalfa,
Phreatophyte trees
Willow, Elderberry,
Alder, Cottonweed
Eucalyptus
Turf grass
Hybrid poplar and
grass
Grass, alfalfa,
willow, black locust
Grasses
Hybrid poplars and
hybrid willows
Hybrid poplar and
alder
Soil based reed
bed
Poplar and other
Date
Planted
May 1997
April 1997
Sept 1998-
Dec1999
May 1997
1995
June 1997
Nov1997
April 1 998
April 1998
Spring 1999
(planned)
Sept 1998
1993
1997
Fall 1992
1995
June 1995
1999
(Proposed)
1997
1997-1998
B-7
-------
Project Name/ Size
Location of Area
Rocky Mountain Arsenal - OR 74 acres
SaginawMill-WA -5-8 acres
Sandia National Laboratories - 40' x 300' for each
NM cover
Savannah Ricer Site - SC
Sludge Lagoon -CT
SRSNE-CT 2 acres
Tama County Landfill - IA 12 acres, 1 4 acres
Tanfield Lea - Newcastle - UK 1 ,800 square meters
Tennessee Site 2 acres
Texaco - WA 1 8 acres
Texas Land Treatment 22 acres
Facility -TX
Texas Site 5-1 0 acres
Thiokol Corp.
Tippee Beef Facility -IA 1.5 acres
Trucking Terminal - NJ ~ 1 acre
Twin CitiesArmy Ammunition Two demo areas
Plant -MN 0.2 acres
Union Carbide -TX 1 acre
Unknown - NJ
Unknown - NJ
Unknown -MD
Unknown - ID
Upper Plant Area - NJ 0.1 acre
Upper Silesia, Poland
US Generating -OR >5, 000 acres
Primary
Contaminant
Radionuclides - low level
waste
Formaldehyde
No contaminant
Halogenated volatiles
Herbicides, metals
Halogenated volatiles
Leachate
Heavy metals, TDS, COD
Volatile and nonvolatile
petroleum products
Petroleum Hydrocarbons
PAHsandO&G
Volatile petroleum products
Halogenated Volatiles
Volatile petroleum products
Heavy metals for
both sites
RCRA K-waste,
semi-volatiles
Non-halogenated
Semi-volatiles
Heavy metals
Heavy metals,
Radionuclides
GROsand DROs
Heavy metals
Heat (cooling water)
Media and
Properties
Groundwater
Groundwater
Groundwater
Shallow soil
Leachate effluent
Soil and groundwater
Soil
Residuals in soils
Soil
Groundwater - 2-3' depth
Surface water, soil,
groundwater
Shallow soil and
groundwater
Soil
Sludge
Soil
Soil, rocky, root level
Sludge
Soil
Groundwater
Clay and silt, 0-20 cm
Vegetative
Type
Trees
Poplar, Alder, and
Native willow
Various
Loblolly Pine,
grasses
Hybrid poplar
Hybrid poplar
Hybrid poplar
Soil based reed
bed
Hybrid poplar
Grass and clover
Groundcover plants
(grasses)
Grasses
185 Willow Trees
Hybrid poplars
and grass
Hybrid poplars
Corn, white
mustard
Mulberry, grasses
hackberry
Alfalfa, switch and
bluestem grass
Ragweed, Hemp,
Dogbane, Musk,
Nodding, Thistle
Poplars
Alfalfa,
Phreatophyte trees
Cereals, Potatoes
Grass, wheat, barley
Date
Planted
March 1999
May 1 8,
1998
1993-1995
1997
May 1 997
Proposed
4th Qtr 1998
Spring 1 999
Planted
1996
May 1998
June 1998
2yr. demo
is May-Oct
1 998 & 1999
Sept 1998
USA Waste-Chambers
Development-VA
10 acres
Subsurface soil
corn, alfalfa
Hybrid poplar
1995
B-8
-------
Project Name/
Location
Size
of Area
Primary
Contaminant
Media and
Properties
Vegetative
Type
Date
Planted
USA Waste Riverbend
Landfill-OR
Vernon Brincks Site - IA
Volunteer Army Ammunition
Plan-TN
Whitewood Creek - SD
Whyalla Site -Australia
Widen -WV
Wilmington - NC
Wisconsin Site-WI
Woodlawn Landfill - MA
14.3 acres irrigated
300'buffer strip
8 pilot beds ~ 3m x
10m; field = 2 hectares
<1 acre
17 acres
~ 21 acres
VOCs, Heavy metals, Leachate
Other
Explosives Water
Heavy metals Soil
Coke oven effluent Effluent
Volatile petroleum products Soil and groundwater
Groundwater and soil
Nitrate-Nitrogen,
ammonium-nitrogen
BTEXandTPH
Soil
Halogenated volatiles and Groundwater
metals
YPLMO-Edinburg, UK 2,000 square meters Surfactants, petroleum, Groundwater
hydrocarbons, organics
Hybrid poplar,
Grass
Hybrid poplar
Hybrid poplar
Soil based reed
bed
Hybrid poplar
Hybrid poplar
Species under
consideration
Hybrid poplars
Soil based reed
bed
1992
1991
1991,1993
1993
1994
Spring 1999
Pending
approval
1997
B-9
-------
Appendix C
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C-6
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Appendix D
Common and Scientific Names of Referenced Plants
Common Name Listed First
Common Name
Alfalfa
Algae stonewort
Alyssum
Bean
Bean,bush
Bermuda grass
Birch, river
Black locust
Black willow tree
Bladder Campion
Bluestem, big (prairie grass)
Bluestem, little
Bluestem, little
Boxwood
Buffalo grass
Canada wild rye (prairie grass)
Canola
Cattail
Cherry bark oak
Clover
Colonial bentgrass
Colonial bentgrass
Cottonwood, Eastern, tree
Cottonwood (poplar)
Crab apple
Crested wheatgrass (Hycrest)
Cypress, bald
Duckweed
Eastern Cottonwood tree
European milfoil/yarrow
Felt leaf willow
Fescue, hard
Fescue, red
Fescue, tall
Four-wing saltbrush
Grama, side oats (prairie grass)
Grama, blue
Grass, cool season (colonial bentgrass)
Grass, warm season (Japanese lawngrass)
Horseradish (roots)
Hybrid poplar tree
Scientific Name
Medicago saliva
Nitella
Alyssum wulfenianum
Phaseolus coccineus L.
Phaseolus vulgarism. "Tender Green"
Cynodon dactylon
Betula nigra
Robin ia pseudoacacia
Salix nigra
Silene vulgaris
Andropogon gerardiVA.
Andropogon scoparius
Schizachyrium scoparius
Buxaceae
Buchloe dactyloides
Elymus canadensis
Brassica napus
Typha latifolia
Quercus falcata
genus Trifolium
Agrostis tenuis cv Goginan
Agrostis tenuis cv Parys
Populus deltoides
Populus
Malus fusca Raf. Schneid
Agropyron desertorum (Fisher ex Link) Schultes
Taxodium distichum
Lemna minor
Populus deltoides
Achillea millefolium
Salix alaxensis
Festuca ovina var. duriuscula
Festuca rubra cv Merlin
Festuca arundinacea Schreb.
Aicanescens
Bouteloua curtipendula
Bouteloua gracilis
Agrotis tenuis
Zoysiajaponica
Armoracia rusticana
Populus deltoidesX nigra DN-34, Imperial California;
Populus charkowiieensisxincrassata;
Populus tricocarpax deltoides
D-1
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Common Name
Scientific Name
Hycrest, crested wheatgrass
Indiangrass (prairie grass)
Indian mustard
Indian ricegrass
Japanese lawngrass
Jimsonweed
Kenaf
Koa haole
Kudzu
Lambsquarter
Legume
Little bluestem (prairie grass)
Loblolly pine
Mesquite
Millet, Proso
Mulberry, red
Mustard, Indian
Mustard weed
Oak, cherry bark
Oak, live
Osage orange
Parrot feather
Pennycress
Pennycress, Alpine
Pennyworth
Poplar, cottonwood
Poplar
Poplar, hybrid
Poplar, yellow
Red fescue
Reeds
Rice
Sacaton, alkali
Sea pink; wild thrift
Salt marsh plant
Sanddropseed
Soybean
Spearmint
Sugarcane
Sundangrass
Sunflower
Switchgrass (prairie grass)
Tall fescue
Thornapple (or jimson weed)
Thrift (wild); sea pink
Tobacco
Water hyacinth
Water milfoil
Water velvet
Wheat grass, slender
Wheat grass, western (prairie grass)
Willow tree, black
Willow tree, felt leaf
Agropyron desertorum (Fisher ex Link) Schultes
Sorghastrum nutans
Brassica juncea
Oryzasativasubsp. indica
Zoysiajaponica
Datura innoxia
HibiscuscannabinusL. cv. Indian
Leucaena leucocephala
Pueraria lobata
Chenopodium
Lespedeza cuneata (Dumont)
Schizachyrium scoparius
Pin us taeda (L.)
Prosopis
Panicum miliaceum L.
Moms rubra L.
Brassica juncea
Arabidopsis thaliana
Quercus falcata
Quercus virginiana
Madura pomifera (Raf.) Schneid
Myriophyllum aquaticum
Thlaspi rotundifolium
Thlaspi caerulescens
Hydrocotyle umbellata
Populus
Populus
Populus deltoidesX nigra DN-34, Imperial California;
Populus charkowiieensisxincrassata;
Populus tricocarpaxdeltoides Populus charkowiieensisx
incrassata
Liriodendron tulipifera
Festuca rubra cv Merlin
Phragmites
Oryza saliva L.
Sporobolus wrightii
Armeria maritima
Spartina altemiflora
Sporobolu cryptandrus
Glycine max(L.) Merr, cv Davis.
Mentha spicata
Saccharum officinarum
Sorghum vulgare L.
Helianthus annuus
Panicum virgatum
Festuca arundinacea Schreb.
Datura innoxia
Armeria maritima
Nicotiana tabacum
Eichhornia crassipes
Myriophyllum spicatum
Azolla pinnata
Agropyron trachycaulum
Agropyron smithii
Salix nigra
Salix alaxensis
D-2
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Scientific Name Listed First
Scientific Name
Achillea millefolium
Agropyron desertorum (Fisher ex Link) Schultes
Agropyron smithii
Agropyron trachycaulum
Agrostis tenuis cv Goginan
Agrostis tenuis cv Parys
Aicanescens
Alyssum wulfenianum
Andropogon gerardi
Andropogon scoparius
Arabidopsis thaliana
Armeria martima (var. halleri)
Armoracia rusticana
Azolla pinnata
Betula nigra
Bouteloua curtipendula
Bouteloua gracilis
Brassica juncea (B. Juncea)
Brassicajuncea (L.) Czern
Brassica napus
Buchloe dactyloides
Buxaceae
Chenopodium
Cynodon dactylon
Datura innoxia
Eichhornia crassipes
Elymus canadensis
Festuca arundinacea Schreb.
Festuca ovina var. duriuscula
Festuca rubra cv Merlin
Glycine max(L.) Merr.
Helianthus annuus
HibiscuscannabinusL. cv. Indian
Hydrocotyle umbellata
Lemna minor
Lespedeza cuneata (Dumont)
Leucaena leucocephala
Liriodendron tulipifera
Madura pomifera (Raf.) Schneid
Malus fusca Raf. Schneid
Medicago sativa
Mentha spicata
Moms rubra L.
Myriophyllum aquaticum
Myriophyllum spicatum
Nicotiana tabacum
Nitella
Oryza sativa L.
Oryza sativa subs p. indica
Panicum miliaceum L.
Panicum virgatum
Phaseolus coccineus L
Phaseolus vulgaris cv. Tender Green
Phragmites
Common Name
European milfoil; yarrow
Hycrest crested wheatgrass
western wheatgrass (prairie grass)
slenderwheatgrass
Colonial bentgrass
Colonial bentgrass
four-wing saltbrush
Alyssum
big bluestem
little bluestem prairie grass
mustard weed
Sea pink; wild thrift
horseradish
water velvet
river birch
side oats grama (prairie grass)
blue grama (prairie grass)
Indian mustard
Indian mustard
canola
buffalo grass
includes boxwood
lambsquarter
Bermuda grass
jimson weed orthornapple
water hyacinth
Canada wild rye (prairie grass)
tall fescue
hard fescue
red fescue
soybean
sunflower
Kenaf
pennyworth
duckweed
a legume
Koa haole
yellow poplar
osage orange
crab apple
alfalfa
spearmint
red mulberry
parrot feather
water milfoil
tobacco
algae stonewort
rice
Indian ricegrass
proso millet
switchgrass (prairie grass)
Bean
bush bean
reeds
D-3
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Scientific Name
Common Name
Pin us taeda (L.)
Populus
Populus charkowiieensisx'mcrassata
Populus deltoidesX nigra DN-34, Imperial California
Populus frv'cocarpaxdeltoides
Prosopis
Pueraria lobata
Quercus falcata
Quercus virginiana
Robinia pseudoacacia
Saccharum officinarum
Salix alaxensis
Salix nigra
Schizachyrium scoparius
Silene vulgaris
Sorghastrum nutans
Sorghum vulgare L.
Spartina altemiflora
Sporobolu crypandrus
Sporobolus wrightii
Taxodium distichum
Thlaspi caerulescens
Thlaspi rotundifolilum
Trifolium (genus)
Typha latifolia
Zoysiajaponica
Loblolly pine
Poplar, cottonwood
hybrid poplar
hybrid poplar tree (eastern cottonwood)
a hybrid poplar tree
mesquite
Kudzu
cherry bark oak
live oak
black locust
sugarcane
felt leaf willow
black willow tree
little bluestem prairie grass
bladder campion
indiangrass (prairie grass)
sudangrass (prairie grass)
salt marsh plant
sand dropseed
Sacaton, alkali
bald cypress
Alpine pennycress
Pennycress
clover
cattail
Japanese lawngrass (warm season grass)
D4
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