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Technology Overview as part of a Web-based Technical and Regulatory Guidance

Phytotechnologies

1. Introduction
Click Here to view case study table at the end of this document.
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Phytotechnologies use plants to remediate various media impacted with different types of contaminants. Phytotechnologies can be applied to address certain issues associated with mining solid wastes and mining-impacted waters. Phytotechnologies can also stabilize tailings and act as a hydraulic control for drainage, thereby decreasing exposure of contaminants to humans and the ecological environment. Implementation of phytotechnologies is a common component of mining reclamation and restoration projects by the establishment of a plant cover as a final remedy. However, in certain cases, application of phytotechnologies can be used for removal of metals from contaminated media. Establishing phytotechnologies requires careful plant species selection and soil amendments, which equates to an investment of time up-front; however, these systems, once established, can be maintained with minimal effort. This document provides an overview of the ITRC document Phytotechnology Technical and Regulatory Guidance and Decision Trees, Revised (ITRC 2009), highlighting the key concepts relevant to using phytotechnologies specifically for mining solid waste and mining-impacted waters. Please refer to the full document for more thorough guidance on the use of phytotechnologies.

2. Applicability
There are six basic phytoremediation mechanisms that can be used to clean up mining-contaminated sites: phytosequestration, rhizodegradation, phytohydraulics, phytoextraction, phytodegradation, and phytovolatilization (Table 1-1). The particular phytotechnology mechanisms used to address contaminants depend not only on the type of contaminant and the media affected, but also on the cleanup goals. Typical goals include containment through stabilization or sequestration; remediation through assimilation, reduction, detoxification, degradation, metabolization, or mineralization; or both. To achieve these goals, the proper phytotechnology system must be selected, designed, developed, implemented, and operated using detailed knowledge of the site layout, soil characteristics, hydrology, climate conditions, analytical needs, operation and maintenance (O&M) requirements, economics, public perspective, and regulatory protection of the environment.

Table 1-1. Summary of phytotechnology mechanisms (ITRC 2009)

Mechanism
Description
Cleanup goal

Phytosequestration

The ability of plants to sequester certain contaminants in the rhizosphere through exudation of phytochemicals and on the root through transport proteins and cellular processes. Containment
Rhizodegradation Exuded phytochemicals can enhance microbial biodegradation of contaminants in the rhizosphere. Remediation by
destruction
Phytohydraulics The ability of plants to capture and evaporate water off the plant and take up and transpire water through the plant. Containment by
controlling hydrology
Phytoextraction The ability of plants to take up contaminants into the plant with the transpiration stream. Remediation by
removal of plants
Phytodegradation The ability of plants to take up and break down contaminants in the transpiration stream through internal enzymatic activity and photosynthetic oxidation/reduction. Remediation by
destruction
Phytovolatilization The ability of plants to take up, translocate, and subsequently transpire volatile contaminants in the transpiration stream. Remediation by
removal through plants


Phytotechnologies are applicable to the following:

Phytotechnologies are applicable where the following cleanup goals are required:

Due to the complexity of contaminants associated with the mining waste process, potentially all of the above phytotechnology mechanisms could be applied as sole remediation technology or as part of suite of remediation technologies. However, the phytotechnologies that may be most useful for mining wastes are likely those that relate to remediation of inorganic contaminants and hydraulic control: phytosequestration, phytohydraulics, and phytoextraction. More details on the types of applications are provided in the following sections. All tests have been conducted at pilot scale or smaller.

2.1 Phytosequestration
Phytosequestration reduces the mobility of the contaminant and prevents migration to soil, water, and air are as follows: Case studies using phytosequestration include the following:

2.2 Phytohydraulics
Plants significantly affect local hydrology. Phytohydraulics is the ability of vegetation to transpire sources of surface water and groundwater. The vertical migration of water from the surface downward can be limited by the water interception capacity of the aboveground canopy and subsequent evapotranspiration through the root system. If water infiltrating from the surface is able to percolate below the root zone, it can recharge groundwater. However, the rate of recharge depends not only on the rooting depth of the species, but on the soil characteristics as well (ITRC 2009). The horizontal migration of groundwater can be contained or controlled (USEPA 2000) using deep-rooted species such as prairie plants and trees to intercept, take up, and transpire the water. One class of trees that has been widely studied in phytotechnologies are the phreatophytes, which are deep-rooted, high-transpiring, water-loving trees that send their roots into regions of high moisture and that can survive in conditions of temporary saturation (Gatliff 1994). Typical phreatophytes include species within the Salicaceae family, such as cottonwoods, poplars, and willows.

Case study using phytohydraulics:

2.3 Phytoextraction
Phytoextraction refers to the ability of plants to take up contaminants into the roots and translocate them to the aboveground shoots or leaves. For contaminants to be extracted by plants, the constituent must be dissolved in the soil water and come into contact with the plant roots through the transpiration stream. Once a chemical is taken up, the plant may store the chemical and/or its by-products in the plant biomass via lignification (covalent bonding of the chemical or its by-products into the lignin of the plant) or sequester it into the cell vacuoles of aboveground tissues (as opposed to in root cells as part of phytosequestration, see above). Alternatively, the contaminant may be neutralized through phytochemical reactions and/or phytovolatilized in the transpiration stream exiting the plant. Specifically, tobacco plants have been modified to be able to take up the highly toxic methyl-mercury, alter the chemical speciation, and phytovolatilize relatively safe levels of the less toxic elemental mercury into the atmosphere (Heaton et al. 1998).

2.4 Applying Phytotechnologies
Applying phytotechnologies to environmentally impacted sites entails selecting, designing, installing, operating, maintaining, and monitoring planted systems that use the various mechanisms described above. The goal of the system can be broadly based on the remedial objectives of containment, remediation, or both. Furthermore, the target media can be soil/sediment, surface water, or groundwater, and these can be either clean or impacted. In some cases, groundwater transitioning to surface water (daylighting seep) can be addressed as a riparian situation where target media are combined. The possible combinations of treatment goal, target media, and applicable mechanisms are summarized in Tables 2-1 and 2-3 for each application. However, specific applications can be designed such that a particular mechanism is emphasized as the primary means of treatment either through plant selection, engineering and design, or method of installation or construction.

Table 2-1. Summary of phytotechnology applications and potential mechanisms for containment treatment goals
(applications covered in ITRC 2009 in bold)

Media
Application
Potential Mechanisms
Comments
Soil/Sediment (impacted) Phytostabilization Cover
(soil/sediment stabilization)
Phytosequestration
Phytoextraction (no harvesting)
Adsorption (abiotic)
Precipitation (abiotic)
Settling/Sedimentation (abiotic)
Also controls soil erosion
by wind/water
ITRC WTLND-1, 2003 for sediment aspects
Surface Water (clean) Phytostabilization Cover (infiltration control) Phytohydraulics (evapotranspiration)
Runoff (abiotic)
Vertical infiltration control
See ITRC ALT-1 (2003), ALT-2 (2003), ALT-3 (2006), ALT-4 (2006) for alternative (evapotranspiration) covers
Surface Water (impacted) Pond/Lagoon/Basin
Riparian Buffer
Phytosequestration
Phytohydraulics (evapotranspiration)
Phytoextraction (no harvesting)
Evaporation (abiotic)
Infiltration (abiotic)
See ITRC WTLND-1 (2003)
Includes wastewater
Also controls soil erosion by water run off
Groundwater (clean) Tree Hydraulic Barrier
Riparian Buffer
Phytohydraulics (evapotranspiration) Lateral migration control
Groundwater (impacted) Tree Hydraulic Barrier
Riparian Buffer
Phytosequestration
Phytohydraulics (evapotranspiration)
Phytoextraction (no harvesting)
Lateral migration control
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Adapted from ITRC 2009, Tables 1-5a and 1-5b.


Table 2-2. Summary of phytotechnology applications and potential mechanisms for remediation treatment goals
(applications covered in ITRC 2008 in bold)

Media
Application
Potential Mechanisms
Comments
Soil/Sediment (impacted) Phytoremediation Groundcover Rhizodegradation
Phytoextraction (with harvesting)
Phytodegradation
Phytovolatilization
Biodegradation (microbial)
Oxidation/Reduction (abiotic)
Volatilization (abiotic)
Phytohydraulics (evapotranspiration) assumed for phytoextraction, phytodegradation, and phytovolatilization
Surface Water (impacted) Pond/Lagoon/Basin
Riparian Buffer
Constructed Treatment Wetland
Rhizodegradation
Phytoextraction (with harvesting)
Phytodegradation
Phytovolatilization
Biodegradation (microbial)
Oxidation/Reduction (abiotic)
Volatilization (abiotic)
See ITRC WTLND-1, 2003
Includes wastewater and extracted groundwater
Phytohydraulics (evapotranspiration) assumed for phytoextraction, phytodegradation, and phytovolatilization
Groundwater (impacted) Phytoremediation Tree Stand Riparian Buffer Rhizodegradation
Phytoextraction (with harvesting)
Phytodegradation
Phytovolatilization
Oxidation/Reduction (abiotic)
Biodegradation (microbial)
Phytohydraulics (evapotranspiration) assumed for phytoextraction, phytodegradation, and phytovolatilization
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Adapted from ITRC 2009, Tables 1-5a and 1-5b.


3. Advantages
One of the main advantages of phytotechnologies, as compared to alternative cleanup technologies, is that most phytotechnologies can be applied to both organic and inorganic contaminants and to soil/sediment, surface water, and groundwater. Furthermore, in some cases, it can be applied to various combinations of contaminant types and impacted media simultaneously. In most other remedial approaches, these combinations would have to be addressed using a treatment train. Other advantages are listed below:

4. Limitations
Phytotechnologies are appropriate only under certain conditions. The major limitations are depth, area, and time. The physical constraints of depth and area depend on the plant species suitable to the site (i.e., root penetration) as well as the site layout and soil characteristics. Phytotechnologies typically require larger tracts of land than many alternatives. Time can be a constraint since phytotechnologies generally take longer than other alternatives and are susceptible to seasonal and diurnal changes. These limitations should be considered along with several other decision factors when evaluating a phytotechnology as a potential remedy (ITRC 2009).

Other limitations include the following:

Many of the limitations can be overcome by proper plant selection. All plant selections must be made based on site-specific conditions. Climate, altitude, soil salinity, nutrient content, fertility, location, depth, concentration of contaminant, commercial availability, plantability, and plant hardiness are some of the determining elements. A variety of approaches and information resources can be used, including databases, site-specific vegetation surveys, and specifically designed tests to evaluate species (ITRC 2009). In addition to selecting species for the remediation, end-use considerations can be included in the initial plant selection. Typically, 10%–15% climax species might be included in the initial design.

5. Performance
In some cases, the application of phytotechnologies can have an immediate effect on contaminant concentrations upon planting. In other cases, it may require several seasons before the plant can interact with a contaminated zone at depth. Furthermore, it may depend on whether the plant itself is directly or indirectly involved with remediating the contaminant.

The time it takes for cleanup to be achieved depends on the criteria set forth in defining the cleanup objectives for the site. Furthermore, it depends on the type, extent, and concentration of contamination, continuing sources, obstructions, soil conditions, hydrologic/groundwater conditions, and other site characteristics, the plant species, growth rate, and climate conditions. Complete restoration will depend on the type of phytotechnology applied at the site (ITRC 2009).

For assessing results, phytotechnology systems should be monitored using the same primary lines of evidence as any other alternative (i.e., concentration trends, hydrology, soil effects, etc.). That information may need to be supported by secondary lines of evidence, which generally entail analyzing the plants in some manner (ITRC 2009).

6. Costs
The benefits of using the phytotechnology-based techniques are the relative lower costs, labor requirements, and safer operations compared to the more intensive and invasive conventional techniques. Establishment of phytotechnology systems include various expenditures, such as earthwork, labor, planting stock, planting method, field equipment, heavy machinery (typically farming or forestry equipment), soil amendments, permits, water control infrastructure, utility infrastructure, fencing, security, etc.

Phytotechnologies require significant operation, maintenance, and monitoring for several years after planting. Costs can include labor, sampling, analytical, materials, field equipment, utilities, waste handling, and disposal. Once the plantation becomes established, however, the operation and maintenance (O&M) costs tend to diminish. Furthermore, additional sampling and monitoring will typically be required during the initial phases compared to subsequent years. Phytotechnologies are generally long-term remedial solutions.

In addition, phytotechnology plantations may require irrigation, fertilization, weed control (mowing, mulching, or spraying), and pest control. At the onset of a planting, which too may be a reoccurring O&M event, some percentage of replanting may be required due to the lack of establishment. As a general rule of thumb, 10%–15% of the initial capital costs should be added as a contingency for replanting.

7. Regulatory Considerations
When selecting a phytotechnology as the remedy for the site, one of the absolute requirements is to demonstrate to regulators that the contaminants of concern (COCs) can be contained and/or remediated using the phytotechnology. This is often demonstrated in feasibility studies conducted specifically for the site or extrapolated from literature results that are sufficiently similar to the site conditions. Furthermore, the proposed remedy must ensure that the fate and transport of the contaminant(s) and/or by-products are acceptable through all potential exposure pathways.

Once feasibility is demonstrated, the ability of the phytotechnology system to reasonably and in high confidence achieve cleanup goals in a satisfactory time frame must also be demonstrated for regulatory acceptance to be granted. This is often demonstrated in treatability studies, which can often be planned and conducted in concert with feasibility studies, including using the same experimental setup (scale, materials, duration, techniques, etc.). The primary difference between treatability and feasibility is the level of quantitative evaluation included in the study. For example, a feasibility study examines whether a specific plant species is capable of treating the contaminant regardless of the time or rate of concentration or mass reduction, whereas a treatability study compares the effectiveness of the treatment in relation to the remedial objectives and applicable or relevant and appropriate requirement (ARARs) set forth for the site. Treatability results are often compared to other remedial alternatives to ultimately select the technology that can best meet the site remedial objectives. In many cases, contingency conditions must be established that either trigger a continuation of the phytotechnology solution or initiate one of these other alternative remedies. Furthermore, these contingencies can be addressed if there is an existing system in place that the phytotechnology solution is meant to supplement or eventually supplant at the site.

Because phytotechnology systems use plants at a contaminated site, the potential ecological exposures posed by the species planted need to be considered. EPA guidance for the preparation of ecological risk assessments (USEPA 1999) should be used to evaluate any potential exposure pathways created or enhanced by using phytotechnologies. The level of detail required is site specific and varies with the application. Factors that should be incorporated into the risk assessment may include species-specific considerations of bioavailability (USEPA 2008), ecological exposures, and the transformation of the chemical composition or physical state.

Depending on the plant species chosen (e.g., invasive, or genetically modified organism [GMO]), other regulations may apply. On February 3, 1999, an Executive Order was signed that specifically addresses invasive species. It requires federal agencies to prevent the introduction of invasive species and to detect and respond rapidly to control established populations of invasive nonnative species. At this time, regulations on GMOs are unclear in the United States (possibly covered under a variety of statues). While EPA does not currently regulate GMO plants used for commercial bioremediation, it may have given them authority to do so under the Toxic Substances Control Act (TSCA). This authority could be invoked to regulate these plants if EPA believed such regulation necessary to prevent unreasonable risk to human health and the environment.

8. Stakeholder Considerations
The general perception is that “green” technologies are natural, environmentally friendly, and less intrusive. Phytotechnologies create sustainable greenspace and can also provide visual screening, reduce noise, and require less intense human interaction to install and operate in the long term. Furthermore, phytotechnologies also create a barrier to odors, noise, and dust generated from other site activities. Therefore, the public perception of phytotechnologies can be quite favorable. However, a perception could be that phytotechnologies are merely beautification and not cleanup, particularly since phytotechnologies can take longer than other alternatives to meet objectives. In some cases, community members may also express opinions on certain species based on personal preferences or medical conditions including allergies, asthma, perceived nuisances (i.e., cotton-like seeds from cottonwoods, excessive leaf, branch, or seed drop), wildlife use, type of wildlife attracted, etc.

9. Lessons Learned
Establishment of vegetation can be enhanced by using native soil or other amendments to offset the often poor growing conditions offered by the tailings material. Some suggestions follow:

10. Case Studies

Table 10-1. Case studies including phytotechnologies

Ely Copper Mine, VT, phytosequestration
Kerramerican NPL, ME, phytohydraulics
Magmont Mine, MO, phytosequestration
Black Butte Mercury Mine, OR, phytosequestration
Gribbons Basin, MI, phytosequestration, phytohydraulics
Valzinco Mine, VA, phytosequestration
Copper Basin, TN, phytosequestration
Sequatchie Valley Coal Mine, TN, phytosequestration, phytohydraulics
Bark Camp, PA, phytosequestration, phytohydraulics
Annapolis Lead Mine Site, MO, phytosequestration
UP Mines, MI, phytosequestration


11. References
Gatliff, E. G. 1994. “Vegetative Remediation Process Offers Advantages over Traditional Pump and-Treat Technologies,” Remediation 4(3): 343–52.

Heaton, C. P., C. L. Rugh, N.-J. Wang, and R. B. Meagher. 1998. “Phytoremediation of Mercury- and Methylmercury-Polluted Soils Using Genetically Engineered Plants,” Journal of Soil Contamination 7: 497–509.

ITRC (Interstate Technology & Regulatory Council). 2009. Phytotechnology Technical and Regulatory Guidance and Decision Trees, Revised. PHYTO-3. Washington, D.C.: Interstate Technology & Regulatory Council, Phytotechnologies Team. www.itrcweb.org.

USEPA (U.S. Environmental Protection Agency). 1999. Screening Level Ecological Risk Assessment Protocol, Appendix C, “Media-to-Receptor Bioconcentration Factors (BCFs)” and Appendix D, “Bioconcentration Factors (BCFs) for Wildlife Measurement Receptors.”
www.epa.gov/osw/hazard/tsd/td/combust/ecorisk/volume3/appx-c.pdf.
www.epa.gov/osw/hazard/tsd/td/combust/eco-risk/volume3/appx-d.pdf.

USEPA. 2000. Introduction to Phytoremediation. EPA/600/R-99/107. Cincinnati: Office of Research and Development. www.cluin.org/download/remed/introphyto.pdf.

USEPA. 2008. “Assessing Relative Bioavailability in Soil at Superfund Sites.” www.epa.gov/superfund/health/contaminants/bioavailability/index.htm.

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