THE use of plant-based systems to stabilize or reduce the

Transcription

1 Plant System Technologies for Environmental Management of Metals in Soils: Educational Materials Gary Pierzynski,* Peter Kulakow, Larry Erickson, and Lucinda Jackson ABSTRACT The use of plants to remediate soil and shallow ground water contaminated by inorganic and organic substances is gaining acceptance worldwide. Various terms and phrases have been used to describe the general approach; we choose plant system technologies to avoid some of the confusion with previously used terms. Phytoremediation is synonymous with plant system technologies in common usage; however, many people mistakenly assume a more narrow definition of phytoremediation. Little information is available for curriculum development for the use of plant system technologies for remediation of contaminated sites. The purpose of this paper is to present a primer on plant system technologies for instructors who may wish to incorporate this material into their courses, and to provide instructional materials for courses that may already discuss plant system technologies. Many colleges and universities have courses in introductory environmental science. In those courses, the use of plants for remediation is a popular topic because it is viewed as a natural or environmentally friendly means of dealing with contaminated soil or shallow ground water. The information presented in this paper serves as a useful introduction to the topic. As plant system technologies gain acceptance, they have also become a topic for more advanced environmental science or engineering courses. In this case it is likely that the students have had little training in basic plant and soil science, and the material presented here addresses that deficiency. THE use of plant-based systems to stabilize or reduce the concentration of inorganic contaminants in soils and shallow ground water is becoming more widespread. Much of the available information on plant system technologies is highly technical and not suitable for the lay person or as introductory material for undergraduate students. The purpose of this paper is to provide introductory information on the use of plantbased systems for remediation of soils and shallow ground water contaminated with inorganic substances. G. Pierzynski and P. Kulakow, Dep. of Agronomy, 2004 Throckmorton Plant Science Center, Kansas State Univ., Manhattan, KS ; L. Erickson, Dep. of Chemical Engineering, Kansas State Univ., Manhattan, KS 66506; and L. Jackson, Chevron Research and Technology Co., 100 Chevron Way, Richmond, CA Contribution no J from the Kansas Agric. Exp. Stn. Received 4 June *Corresponding author (gmp@ksu. edu). Published in J. Nat. Resour. Life Sci. Educ. 31:31 37 (2002). WHAT ARE PLANT SYSTEM TECHNOLOGIES? Plant system technologies are an emerging group of plantbased technologies that, if properly and professionally managed for a contaminated site, can help restore or stabilize land impacted by inorganic or organic contaminants. This material will focus on metal contaminants such as arsenic (As), cadmium (Cd), lead (Pb), nickel (Ni), and vanadium (V). The purpose is to provide basic information on plant system technologies to help assess the use of this technology for a contaminated site, and not necessarily to promote the use of any one technology. There are a variety of terms and phrases that are used to describe plant system technologies. Common names include phytotechnologies, phytoremediation, and vegetative remediation. Phytoremediation implies using plants for remediation and has two subcategories that are important for metal contaminants: phytoextraction and phytostabilization (Pierzynski et al., 2000). Phytoextraction, also called phytoaccumulation, is the use of plants to remove metals from soils and is still considered experimental. Phytostabilization is the use of plants to stabilize contaminated soils to minimize leaching and wind or water erosion, and has been used extensively for managing metal-contaminated sites. In promotional literature, mass media, etc., it is important to understand the context in which the terms are used. Unfortunately, there is little uniformity and, for example, phytoremediation may be used to describe phytoextraction or phytostabilization, even though the distinction is very significant. The application of plant system technologies for metal contaminants is different than for organic contaminants. Since metals cannot be broken down into simpler substances by microorganisms, plant system technologies for metals are limited to phytoextraction or phytostabilization. A series of steps outlined in this paper will help assess use of this technology for a metal-contaminated site. Success with plant systems is different than walking away from a site and letting nature take its course. Plant systems can fail if basic agronomic requirements are ignored. Proper risk assessment and site management are needed to achieve goals. Since some contaminants can enter plants, careful risk assessment is required where there are mechanisms for transfer to humans and wildlife. Plant systems should be used in concert with other remediation strategies and should involve expertise in agronomy, soil science, hydrology, plant biology, environmental engineering, cost analysis, risk assessment, and toxicology. Where applicable and successful, plant-based systems can achieve cost savings from 20 to 80% compared with conventional cleanup methods such as soil removal (Schnoor, 1998). However, limited cost data are available for plant system technologies. HOW DO WE MANAGE METALS IN SOILS? The term bioavailability is often used for metals. In a general sense, bioavailability simply means the availability of a substance for uptake by an organism. For our purposes, we are interested in the availability of metals in soils to plants. The same type of plant growing in two different soils with the same metal concentration might remove very different amounts of Abbreviations: EDTA, ethylenediaminetetraacetic acid; RTDF, Remediation Technologies Development Forum. J. Nat. Resour. Life Sci. Educ., Vol. 31,

2 Fig. 1. Possible interactions and fate of metal ions in the soil solution illustrating how we can manage metals in soils by understanding how they behave in the soil system. the metal, meaning that the bioavailability of that metal is different in each soil. In addition, some metals, such as Cd, are readily taken up by plants whereas others, such as Pb or As, are not. Thus, Cd is thought to have higher bioavailability to plants than As or Pb. It may be desirable to increase or decrease soil metal bioavailability in management of plant system technologies. It is relatively easy to understand how metals can be managed in soils if we consider how they behave in the soil solution. Soils are comprised of solids, water, and gases. The soil solution represents the water in the soil that plants use to meet their water requirements. The soil solution is also where plants obtain their nutrients and many other substances that are taken into the plant roots and possibly transported to the portions of the plant that are above the soil surface. In the case of phytoextraction, the soil should be managed to maximize the amount of metals in the soil solution to maximize the amount available for plant uptake. The potential for ground water contamination by metals must be carefully considered in this case, since movement below the rooting zone may be possible. Conversely, for phytostabilization, soils are managed to minimize the amount of metals in the soil solution to minimize plant uptake. Figure 1 illustrates how some of these interactions occur and how to manage them. The center of the diagram represents the soil solution or soil water. Plants can remove certain forms of metals from the soil solution. For metals, it is generally the free ionic form (e.g., Ni 2+ ) that is taken up by the plants, so of primary interest would be factors that influence the concentration or activity of the free ionic form, and hence the bioavailability. For management, we can most easily influence the amounts held by the soil particles, the formation of insoluble compounds, and the formation of soluble complexes. Any management practice that increases the amount of metals held by the soil particles or present as insoluble compounds will reduce the soil water concentration, while practices that increase the formation of soluble complexes will increase the soil solution concentration. Examples include increasing soil ph by the addition of calcium carbonate, which will increase the amount of metals held by soil particles, and decrease plant uptake. Similarly, the addition of phosphorus may promote the formation of insoluble compounds and decrease plant uptake. To increase plant uptake, chelators can be added to release metals held by soil particles or to dissolve metals present as insoluble compounds and bring them into the soil solution. This strategy is sometimes employed with phytoextraction. There are many chelators available. One that is commonly used is ethylenediaminetetraacetic acid (EDTA). These changes in metal bioavailability to plants are done without changing the total amount of metals in the soil. The oxidation reduction status (aerobic vs. anaerobic) of the soil can also influence metal bioavailability, both directly and indirectly. These changes are complex, and a qualified soil chemist should be consulted if a site is flooded or cycles between wet and dry conditions on a regular basis. Table 1 provides some basic characteristics of metals that are commonly found at contaminated sites that might be candidates for phytoextraction or phytostabilization. Metals that occur as cations in the soil solution generally become less available to plants as soil ph increases. Average concentrations for metals in soils that are not considered to be contaminated are provided for comparison. HOW DO PLANT SYSTEMS WORK? Phytoextraction The goal of phytoextraction is to transfer metals from the soil to a portion of a plant that can be easily harvested and removed from the site (Fig. 2). Generally, this involves moving metals to the aboveground portion of a plant, but in theory could involve transfer to a belowground tuber that could also be harvested. The living plant material produced by plants, aboveground or belowground, is called biomass. While all plants accumulate metals to some degree, and would eventually deplete the soil of metals if biomass were continually removed, the amounts removed are normally very small compared with the total amount of metals in the soil. The key to successful phytoextraction is to have a combination of high metal concentrations in the biomass and high biomass production, such that the rate of metal removal is great enough to have a measurable effect on soil metal concentrations within a reasonable amount of time, or the length of time the site managers are willing to manage the site for phytoextraction. The answers to a few key questions can quickly provide Table 1. Basic characteristics and phytoextraction potential of common soil metal contaminants. Compiled in part from information in Kabata-Pendias and Pendias (1992), Pierzynski et al. (2000), and Adriano (2001). Change in metal Common availability Known chemical to plants Avg. soil hyperac- Phytoexform(s) in soil as soil ph conc., cumulator traction Element solution increases mg kg -1 species? potential Arsenic (As) AsO 3-4, AsO 3-3 varies 5 yes medium Cadmium (Cd) Cd 2+ decreases 0.06 yes high Cobalt (Co) Co 2+,Co 3+ decreases 8 yes low Chromium (Cr) CrO 2-4,Cr 3+ varies 100 yes low Copper (Cu) Cu 2+ decreases 30 yes low Nickel (Ni) Ni 2+ decreases 40 yes high Lead (Pb) Pb 2+ decreases 10 yes high Selenium (Se) SeO 2-4, SeO2-3 increases 0.3 yes medium Vanadium (V) VO 2+,VO - 3,VO3-4 varies 100 no none Zinc (Zn) Zn 2+ decreases 50 yes high These are typical values for soils that are not considered to be metal contaminated. Phytoextraction is still experimental. The potential reflects the relative possibility that a reliable technology for phytoextraction of this element might be developed in the future. 32 J. Nat. Resour. Life Sci. Educ., Vol. 31, 2002

3 valuable information on the suitability of phytoextraction for a given site: 1. How much metal needs to be removed from the soil? 2. How much metal can the plants remove? 3. How long will it take to reach the cleanup objectives? Some simple calculations can illustrate these points. The total amount of metal in a soil can be estimated by calculating the mass of soil in a given volume, using assumptions of soil bulk density, and then multiplying by the desired reduction in soil metal concentration (Eq. [1]). Typically, such calculations are based on an area 1 ha in size ( m 2 ) and assume a soil bulk density of 1.35 Mg m -3. Soil depth is typically expressed in units of cm, and soil metal concentrations are often in units of mg kg -1 (ppm). Using this information, the total amount of metal that needs to be removed can be estimated by: Amount of metal to be removed (kg ha -1 ) = Depth (cm) Reduction in concentration (mg kg -1 ) [1] where the factor incorporates the assumption of bulk density and the necessary unit conversions. As an example, if a 250 mg kg -1 reduction in soil Pb concentration were desired for a 1-ha area with 15 cm of contaminated soil, 506 kg ha -1 of Pb would need to be removed. The total amount of metal removed in biomass can be estimated by multiplying the amount of biomass produced by the metal concentration in the biomass (Eq. [2]). Typically, such calculations are based on an area 1-ha in size, and metal concentrations in biomass and biomass production are expressed on a dry-weight basis. The amount of metal removed in biomass can be estimated by: Amount of metal removed (kg ha -1 ) = Biomass production (kg ha -1 ) Metal concentration (mg kg -1 ) 10-6 [2] where the factor 10-6 incorporates the necessary unit conversions. For example, if a crop produces 5000 kg ha -1 of biomass containing 1000 mg kg -1 Pb, the removal would be 5 kg ha - 1. The number of cropping cycles per year will depend on the crop species and the climate. It is not difficult to see that the length of time required to obtain the desired reduction in soil metal concentrations can be estimated from the total amount of metal to be removed and the removal rate for metals in the biomass. For example, if one crop per year were produced, it would take >100 yr to remove 506 kg Pb ha -1 if only 5 kg Pb ha -1 were removed with each crop. The actual length of time may be less than estimated from such mass balance calculations because of dispersion of metals in the soil, which also may cause soil metal concentrations to decrease. Such calculations can be useful to easily determine if phytoextraction is appropriate for a given site. Phytoextraction has been used for remediation of metal-contaminated sites only on an experimental basis. There are two general approaches to phytoextraction. The first utilizes plants that are hyperaccumulators of metals. A metal hyperaccumulator is defined as a plant having >1000 mg kg -1 of copper (Cu), cobalt (Co) or Pb; > mg kg -1 of zinc (Zn) or manganese (Mn); or >100 mg kg -1 of cadmium (Cd). For many metals, naturally occurring hyperaccumulating plants have been identified (Table 1); however, because a hyperaccumulator exists does not mean that a phytoextraction technology exists. The presence of hyperaccumulators has been used as a prospecting tool for locating metal deposits. The Fig. 2. In phytoextraction, metals move into the plant biomass, and the biomass is harvested. J. Nat. Resour. Life Sci. Educ., Vol. 31,

4 primary shortcoming of the natural hyperaccumulators is their tendency to produce low amounts of biomass, which limits their phytoextraction potential. There are plant breeding research projects addressing this issue. The second approach is known as induced hyperaccumulation. Here, plants that are not natural hyperaccumulators, but can produce considerable biomass, are induced to take up metals by modifying the soil. Typically, chelating or acidifying agents are added to the soil to bring the metals into the soil water and make them available for plant uptake. Since chelating agents bring more of the metal into the soil water, they also increase the risk of leaching of metals below the treatment zone and perhaps to ground water if the plants cannot take up the metals in solution. Because of increased metal transport in the soil water, the metal concentration in the highly contaminated soil decreases and the volume of soil with metal concentration above the background level increases. Therefore, it is important to be able to track the fate of the metals in a phytoextraction project to ensure we know how much of the metal was removed in the plants and the amount dispersed in the soil. Phytostabilization The goal of phytostabilization is to contain metals by stabilizing the soil through a permanent vegetative cover (Fig. 3). The vegetative cover reduces erosion of soil by wind and water, and will influence the water budget such that both leaching and runoff water losses are reduced compared with an unvegetated state. The vegetation may be native or introduced grasses, broadleaf plants, or trees. Phytostabilization has a much longer history than phytoextraction and has been used extensively for remediation of metal-contaminated sites, particularly ecological restorations where intimate contact between the soil and humans is minimal. In some phytostabilization situations, plant growth can be limited by various factors. The most common limiting factors are soil physical or chemical properties that inhibit plant growth or phytotoxicity from one or more metals in the soil. Evidence of poor soil physical properties could include limited water availability or a high bulk density that restricts root growth. Poor soil chemical properties could include extreme ph conditions (too high or too low), low amounts of plant nutrients, or high salt concentrations. Phytotoxicity problems are more likely for metals such as Zn, Cu, and Ni and less likely for metals such as Pb, As, or V. It is imperative that the factors limiting plant growth be properly identified so that corrective measures can be taken. Corrective measures can include covering contaminated soil with sufficient clean soil to establish a vegetative cover, or amending the contaminated soil with the intent of addressing the factors that are limiting plant growth. Covering with clean soil is often used in small areas but is not practical for large areas, as the volume of clean soil required soon becomes cumbersome and expensive. Amendments can be selected to improve soil physical and chemical properties. Organic waste products (e.g., biosolids, composts, animal manures) and/or liming materials are commonly used amendments that are mixed with the soil. These materials will typically reduce phytotoxicity problems as well. Biosolids (the new term used for sewage sludge) are thought to be particularly effective for phytostabilization because they add organic matter and nutrients, similar to animal manures and composts, and they also contain a significant inorganic fraction that helps to further reduce metal bioavailability to plants and other organisms. In Fig. 3. In phytostabilization, plants and soil amendments are used to reduce erosion of soil by wind and water and to reduce metal bioavailability. 34 J. Nat. Resour. Life Sci. Educ., Vol. 31, 2002

5 addition, some biosolids are stabilized with lime, which provides additional benefits if an increase in soil ph is desirable. A second key component for phytostabilization is the selection of plant species. Phytostabilization may employ a single species or a mixture of species. All must be adapted to the local climate and management conditions. Temperature and precipitation patterns are the most important factors. Irrigation may be employed to help establish vegetation, but must be carefully considered if it is necessary for the long-term survival of the selected plant species. Species may also be selected for their tolerance to high levels of metals in the soil. Broadleaf plants are generally more sensitive to metals than grasses, and certain species and cultivars of grasses are more tolerant than others. Another form of phytostabilization involves the management of ground water that is contaminated with heavy metals. Plants utilize water from the soil in the process called transpiration. Transpiration can influence the depth of ground water and the movement of water in the soil. If the ground water is contaminated with heavy metals, mature trees can be used to manage the movement of the contaminated ground water to achieve hydraulic control. WHEN CAN PLANT SYSTEMS BE USED? Generally, plants can be used to treat shallow soil or ground water that is within the range of plant rooting capability. Often this is within about 1mofthesoil surface, but can extend much deeper. The USEPA-sponsored Remediation Technologies Development Forum (RTDF) website listed 22 examples of phytoremediation applied to heavy metal contaminated sites (see Phytoremediation Site Profiles under Recommended Resources ). Ten sites utilized phytoextraction, six sites used phytostabilization of contaminated soil, and six sites were attempting to achieve stabilization through hydraulic control of ground water. Most of the field trials with metals management using plants have provided limited documentation of project success. Here, we will describe two field tests of plant system technologies to manage heavy metal contaminated soils. Phytoextraction at a Brownfields Site An abandoned factory site in New Jersey was used to recycle Pb batteries. Soil at the 1-ha site was contaminated with Pb, averaging 1400 mg kg -1 to a depth of 30 cm; soil Pb levels ranged from 200 to 1800 mg kg -1. Three crops of Indian mustard [Brassica juncea (L.) Czern. & Coss.] were grown to remove Pb from the soil by phytoextraction. Before harvesting each crop, a mixture of soil additives, including chelating agents, were applied to increase availability of Pb to the plants. The plant material with high Pb content was harvested and properly disposed as hazardous waste. Soil Pb concentrations on many parts of the property fell below the 400 mg kg -1 required by the regulatory agency. The project was considered to have successfully reduced risk at the site. Several issues encountered at the site were: (i) weather conditions limited the number of crops that could be grown each season, (ii) soil Pb concentrations were reduced in some areas, though results were not consistent across the site, and (iii) there was some concern about achieving reduced concentrations by dilution through tilling. Although not documented at this site, there were also concerns about the potential for increased leaching of metals after applying the chelating agents. A follow-up field study was conducted at a small arms firing range in Fort Dix, NJ. Lead-contaminated soil was placed 30 cm deep in a lined containment basin with a leachate collection system. Initial Pb levels averaged 516 mg kg -1. Three crops Indian mustard, sunflower (Helianthus annuus L.), and a rye (Secale cereale L.) barley (Hordeum vulgare L.) mixture were grown in 1 yr. The crop biomass was harvested about 1 wk after applying soil conditioners to increase plant Pb uptake. Lead concentrations were determined for the soil, the crop biomass, and for leachate leaving the basin. Soil Pb concentration decreased by an average of 226 mg kg -1 in 1 yr; however, concerns were raised about the fate of the Pb. Significant Pb concentrations were observed in the leachate water that may have been caused by the addition of solubilizing agents. Soil Pb levels were reduced, but the reduction was due to a combination of plant uptake of Pb, loss in the leachate, and other unaccounted losses. Using Eq. [1] and [2] to estimate potential Pb removal by plants, it appears that <10% of the reduction in soil Pb can be accounted for by removal in plant biomass. For the conditions at this site, the use of the Pb solubilizing agents produced unacceptable Pb levels in the leachate. This indicates careful assessment of site specific conditions, and treatability studies are needed before adopting phytoextraction to remove Pb from surface soil. Tri-State Mining Region, an Example of Phytostabilization Lead and Zn were mined and smelted extensively in portions of Kansas, Oklahoma, and Missouri, forming what is known as the Tri-State Mining Region. Large tracts of land have little or no vegetative cover because of mine wastes at the surface that have low water holding capacity, low levels of plant nutrients, and high levels of Zn that can cause phytotoxicity. Approximately 365 ha around the city of Galena, KS, were in need of phytostabilization to reduce wind and water erosion and, ultimately, to reduce human exposure to Pb and Cd. A pilot study identified the proper type and amount of a soil amendment to use, which allowed vegetation to be established (Norland, 1993). This pilot study examined the influence of turkey litter, mushroom compost, and cattle manure as soil amendments at three levels of addition, ranging up to 90 Mg ha -1. Commercial fertilizers were also included in the comparison, and the plots were seeded with a mixture of native grasses and legumes. Changes in vegetative cover, species density, and species diversity were measured over 3 yr. The mushroom compost and cattle manure were found to be the best soil amendments for this site. Commercial fertilizers were not effective for establishing vegetation because they did not correct problems with soil physical properties. In the first year, the amount of amendment added did not influence the results. However, by the end of the third year, it was clear the vegetative cover consistently increased as the amount of amendment added increased. The legumes did not survive, which was not unexpected since legumes are generally more sensitive to metals than grasses. Based on the results of the pilot study, cattle manure was selected for the large-scale remediation because large quantities were readily available. Before seeding, a significant amount of site preparation was J. Nat. Resour. Life Sci. Educ., Vol. 31,

6 performed. This was primarily filling voids, leveling piles, and working with surface drainage to minimize erosion potential. A mixture of native grasses was seeded after 45 Mg ha -1 of composted cattle manure had been added and incorporated. The site recently passed its 5-yr review. Assessing New Developments The use of plant system technologies is a rapidly evolving area. Advancements in technology occur frequently and our understanding of the benefits of vegetation is constantly improving. One would like to take advantage of the latest technologies without expending time or resources on approaches that are destined to fail at a given location. A new development needs to be placed in the proper perspective relative to its readiness for use in the field. For example, the discovery of a hyperaccumulating plant for a particular metal does not mean that plant is ready to use for phytoextraction. Has the approach been used in the field, and what were the results? For all plants, the species in question should be adapted to local conditions. For phytoextraction, the amount of metal that needs to be removed and the removal rate of the extracting species should be compared to see if the remediation can be completed in a reasonable amount of time. Example calculations were provided earlier. In all cases, pilot-scale tests are a good idea. STEPS TO USING PLANT SYSTEMS To determine if a plant system is a viable option for a particular location and if it will be successful, it is important to assess a number of issues regarding site management, environmental contaminants, and plant growth. Step 1. Identify Environmental Management Objectives and Regulatory Requirements/Endpoints (Several important factors need to be considered here) Consider the time frame available for treatment. Plant systems will take longer to achieve the desired results than some other technologies. Most systems require a minimum of2yr. Determine the best management strategies for a site. This includes water management (drainage and irrigation), soil amendments, fertilization, harvest, and disposal of plant material. Options for disposal of heavy metal enriched plant material include placing directly into an approved landfill, burning to reduce the volume followed by disposal of the ash, and burning to recover the metals. Determine future land use. Identify ecological risk issues. Consider aesthetics is it important how the site looks? Obtain acceptance by site managers and regulators. Identify desired endpoints and any regulatory requirements. Step 2. Assess Contaminant and Risk Determine identities of metal contaminants and their concentrations. Plan how to conduct sampling and analysis of contaminants. Assess toxicity of contaminants and potential for movement to receptor organisms. Step 3. Understand Environmental Factors Successful use of plant systems depends on growing healthy plants with strong root systems. Horticultural skills needed include a combination of farming, landscaping, and ecological restoration. Soil quality at the beginning of a plantbased project is often less than optimal, which means careful attention is needed to prepare the soil to ensure long-term success. To establish healthy plant growth, it may be necessary to add soil amendments based on soil tests. Active management of the growth of vegetation and continued monitoring are required to achieve environmental goals. With proper planning and initial management, a vegetated site can become self-sustaining or require minimal management. Determine soil type and conduct agronomic analyses. Understand climate, rainfall, available water, and growing season. Determine plant species that can grow in the area. Consider use of native species. Step 4. Select Plant System Technology Select the technology suitable for your site. If needed, conduct smaller scale tests for germination of proposed plant species. Step 5. Prepare the Field Site Select appropriate plant species, planting methods, and time of planting. Prepare the soil and add soil amendments to create a firm, fine seedbed or suitable site for plants. Install necessary irrigation and monitoring equipment. Step 6. Plant It! Step 7. Monitor Manage plant health and growth. Continue proper sampling to meet environmental goals. HOW MUCH DOES IT COST? Plant Extraction of Lead-Contaminated Soil The estimated cost for plant decontamination of 1 ha of Pbcontaminated soil to a depth of 50 cm using plants was $ to $ , whereas excavating and land filling the same soil volume was $1 million to $4.2 million (Schnoor, 1998). However, due to the limited availability of information on completed projects, limited cost data is available. According to Edenspace Inc., cleanup costs, including treatment and disposal, can range from $26 to $165 m -3 of contaminated soil. Costs would increase with the number of years necessary to complete the treatment and with any potential added requirements, such as a liner system. Phytostabilization Phytostabilization costs should be much less than for phytoextraction, because costs are limited to revegetation and monitoring. Costs include labor, seed, soil amendments, shipping of soil amendments, mulch, and site work such as incorporating the soil amendments and seeding. The phytostabilization work in the Tri-State Mining Region cost approxi- 36 J. Nat. Resour. Life Sci. Educ., Vol. 31, 2002

7 mately $8645 ha -1 (corrected for inflation to 2001 dollars). The cost of soil amendments can vary tremendously. If the soil amendment is a waste product such as municipal solid waste compost, animal manure, or biosolids the owner of the waste may provide material at little or no cost, because other disposal options available to them might be restrictive or expensive. SUMMARY The purpose of this paper is to provide basic information on plant system technologies for metals management in soils to help assess the suitability of various options for a metal-contaminated site. Two categories of plant system technologies are discussed in detail: phytoextraction and phytostabilization. Phytoextraction is still an experimental approach that is designed to remove metals from soils via plant uptake of metals and with subsequent removal of plant biomass. Phytostabilization is an accepted approach that is designed to stabilize metals in soils with the goal of reducing metal movement through control of leaching, and wind or water erosion. Steps to using plant systems include identifying environmental management objectives and regulatory requirements, assessing contaminant and risk, understanding environmental factors, selecting the appropriate technology, preparing the field site, planting, and monitoring. Costs are generally lower than more traditional dig and treat approaches. Suggested additional resources are provided for more detailed information. REFERENCES Adriano, D.C Trace elements in terrestrial environments: Biogeochemistry, bioavailability, and risks of metals. Springer-Verlag, New York. Kabata-Pendias, A., and H. Pendias Trace elements in soils and plants. 2nd ed. CRC Press, Boca Raton, FL. Norland, M.R Reclamation of abandoned Tri-State Mining District lead zinc chat tailing. p In Proc. of the Assoc. of Abandoned Mine Land Programs, Jackson, WY Sept Wyoming Dep. of Environ. Quality, Cheyenne, WY. Pierzynski, G.M., J.T. Sims, and G.F. Vance Soils and environmental quality. 2nd ed. CRC Press, Boca Raton, FL. Schnoor, J.L Phytoremediation. Technology Evaluation Rep. TE Ground-Water Remediation Technology Analysis Center, Univ. of Pittsburgh, Pittsburgh, PA. RECOMMENDED RESOURCES Introduction to Phytoremediation, USEPA Rep. 600-R ; and Phytoremediation of Contaminated Soil and Ground Water at Hazardous Waste Sites, USEPA Rep. 540-S [Online]. [4 p.] You can search for and download these USEPA documents at: (accessed 25 Mar. 2002; verified 18 Apr. 2002). USEPA, Washington, DC. Interstate Technology and Regulatory Cooperation. Technical and Regulatory Guidelines. Phytotechnologies [Online]. [124 p.] Available at (accessed 25 Mar. 2002; verified 18 Apr. 2002). ITRC, Washington, DC. Phytoremediation Site Profiles [Online]. [1 p.] Available at (accessed 25 Mar. 2002; verified 18 Apr. 2002). Remediation Technologies Dev. Forum, USEPA, Washington, DC. Great Plains/Rocky Mountain Hazardous Substance Research Center [Online]. [2 p.] Available at (accessed 25 Mar. 2002; verified 18 Apr. 2002). Hazardous Substance Res. Ctr., Kansas State Univ., Manhattan, KS. J. Nat. Resour. Life Sci. Educ., Vol. 31,