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RESEARCH: PHYTOREMEDIATION

BACK TO RESEARCH

Phytoremediation is the use of plants to clean contaminated soil and ground water. The Crop Physiology Lab at Utah State University is involved with several phytoremediation projects, including cooperative research programs with the Utah Water Research Lab, Phytokinetics, Inc., and Idaho National Engineering and Environmental Laboratory.

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ABSTRACT Sulfolane (tetrahydrothiophene 1,1-dioxide, C₄H₈O₂S) and diisopropanolamine (DIPA) are highly water soluble organic compounds used in the Sulfinol^TM process to remove hydrogen sulfide from natural gas and have been found in samples of wetland vegetation collected near a sour gas processing facility in Alberta, Canada.  Concentrations within individual plants and between plants at different locations within the wetland varied greatly but were generally higher than expected, based on exposure concentrations and plant uptake predictions using octanol-water partition coefficients.  To better understand the uptake of these highly water-soluble compounds by wetland plants and to substantiate the field findings, the uptake of sulfolane and DIPA by cattails (Typha latifolia) was investigated in a greenhouse microcosm study.  Cattails were grown hydroponically in aqueous solutions containing sulfolane and DIPA for a period of 50 days. Non-planted and non-planted poisoned hydroponic systems were run simultaneously as controls.  Sulfolane and DIPA concentrations in the hydroponic solution and plant tissues were monitored throughout the study.  Uptake and translocation of sulfolane and DIPA by cattails was found to be a function of exposure concentration and water transpired.  However, the neutral sulfolane was translocated into the foliar portion of the cattails to a significantly greater extent than the protonated DIPA.  Sulfolane concentrations were consistently greatest in the leaf tips with concentrations as high as 33000 mg/kg dry weight for the 200 mg/L exposure.  DIPA leaf concentrations were more uniform but much lower than sulfolane.  The highest DIPA concentration observed was 1014 mg/kg dry weight for the 100 mg/L exposure.  The average leaf to root tissue concentration ratio for sulfolane was 53 (152 for leaf tips) while for DIPA the ratio was 0.6.  Normalizing the leaf concentration in each system to the amount of water transpired during exposure and dividing it by the average exposure concentration yielded approximate transpiration stream concentration factors (TSCF) that ranged from 0.1 (entire leaf) to 0.9 (leaf tip) for sulfolane and <0.01 to 0.02 for DIPA.  Overall, the laboratory uptake trends matched those observed in the limited field sampling and suggest that the uptake of non-ionizable, highly water soluble organics such as sulfolane may not be well predicted using existing relationships between TSCF and log K_ow.  In addition, the relatively high concentrations observed in the foliar tissue suggests that wetland plants could play an important role in the natural attenuation of sulfolane, provided the sulfolane is not released by the plants during winter dormancy.

Trichloroethylene (TCE) contaminated groundwater originating from Hill Air Force Base (HAFB) in northern Utah has migrated into surrounding communities. Concern among base officials and local residents regarding the potential for TCE uptake and translocation into edible fruits prompted an initial field survey in fall 2001.  Approximately 200 tree core and fruit samples were collected and TCE was identified using headspace gas chromatography with electron capture detection (GC/ECD) in several samples from fruit trees growing within the plume.  The identity of TCE was confirmed on a subset of samples using headspace gas chromatography/mass spectrometry (GC/MS) operated in selected ion monitoring (SIM) mode.  A more rigorous follow-up study was conducted in fall 2002 to determine if the TCE identified in trees and fruit during the previous year was representative of a continuing problem or was a one-time occurrence.  Prior to the follow-up study, the headspace GC/MS method was validated specifically for apples, peaches, tomatoes and carrots.  In fall 2002, over 400 samples, including replicates, were collected from six communities surrounding HAFB.  No TCE was found in any of the fruit or vegetable samples above the method detection limit (approximately 0.1 mg/kg fresh weight, depending on sample size) but TCE was again detected in several fruit tree core samples.  The apparent difference between the 2001 and 2002 results may be from an improvement in data quality or from changes in the environmental conditions associated with transfer of TCE into fruit.  Results of continued monitoring at select sites throughout the 2003-growing season will also be presented.

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SUMMARY OF PRACTICAL IMPLICATIONS  Field studies in Florida and Utah showed that indigenous vegetation contributed to the natural attenuation of trichloroethylene (TCE) in shallow groundwater plums.  However, despite similar exposure to TCE, plant uptake appeared greater at the Utah site where summer precipitation is minimal.  Trichloroethylene was detected in all exposed plants at both sites, but plant concentrations were 10 to 100 times higher at the Utah site.  Plant metabolites of TCE were also detected.  Using a novel sampling technique, TCE was identified in transpiration samples collected at the Utah site, but not at the Florida site.  The lack of phytovolatilization and significantly lower TCE plant concentration at the Florida site are most likely due to the smaller fraction of contaminated groundwater used by plants for transpiration because of the frequent precipitation.  However, additional studies are necessary before definite conclusion can be drawn regarding the influence of climate on plant uptake.  The impact of plants relative to other attenuation mechanisms was not directly evaluated at these sites and should be addressed in future studies.

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ABSTRACT  The highly water-soluble chemicals sulfolane and diisopropanolamine (DIPA) are used to remove hydrogen sulfide from natural gas and have been identified in groundwater, soil and wetland vegetation near sour gas processing facilities. Cattails (Typha latifolia) were grown in aqueous solutions containing known concentrations of sulfolane (40 or 200 mg/L) and DIPA (20 or 100 mg/L) for a period of 50 days. The concentrations of sulfolane and DIPA in the hydroponic solution and plant tissues were monitored periodically during and at the completion of the study using gas chromatography, high performance liquid chromatography and/or electrospray liquid chromatography mass spectrometry methods. Both sulfolane and DIPA were taken up and translocated into the foliar portion of the cattails. Howev of exposure concentration and water transpired. At the end of the study, sulfolane tissue concentrations ranged from 4000 to 6000 mg/kg dry weight for the 40 ppm exposure concentration and 12,000 to 32,000 mg/kg dry weight for the 200 ppm exposure concentration. DIPA concentrations in the foliar tissue ranged from 6 to 20 mg/kg dry weight for the 20 ppm exposure and 100 to 250 mg/kg dry weight for the 100 ppm exposure. The plant tissue concentrations of sulfolane were highest in the upper leaves while for DIPA the concentrations were highest in the roots. This followed the general trend observed at several field sites.

ABSTRACT  A greenhouse study was conducted to determine the potential for uptake of trichloroethylene (TCE) by fruit trees.  The study was initiated following a preliminary field survey that found the presence of TCE in fruit trees from residential areas in and around Hill Air Force Base (HAFB) in northern Utah in the fall of 2001.  Two species of trees were used in the study, peach and apple, and they were grown in a heterogeneous mix of soil, sand, and organic matter.  Dosing levels of approximately 5 and 500 ml/L (low and high dose respectively) bracketed the levels of TCE that was found in the shallow groundwater in the residential areas where TCE was detected in fruit samples.  Control trees were grown within the canopy of the dosed trees to account for air deposited TCE.  Additional control trees were grown in a separate greenhouse that did not contain TCE.  The greenhouse study was conducted for two years using radiolabeled TCE as a tracer to detect the presence and movement of TCE within the trees.  Analysis for parent TCE was conducted using a headspace gas chromatography mass spectrometry (HS/GC/MS) method developed at Utah State University that follows the U.S. Environmental Protection Agencies method for soil.  Analysis for ¹⁴C TCE was conducted by a liquid scintillation counter (LSC) for aqueous samples, and biological oxidation of tissue with the resultant evolved ¹⁴CO₂ captured and counted by LSC.  All plant tissue (leaves, stems, fruit, trunk, roots) contained significant levels of ¹⁴C, but parent TCE was not detected in any tissue samples with the exception of the lower portion of the trunk and the roots.  Apple and peach fruit were sampled on a regular basis beginning with the first fruit set.  Total ¹⁴C concentrations appeared to reach a maximum midway through the fruiting season and then leveled off and decreased with age and size.  This may indicate that ¹⁴C (TCE equivalents) moves into fruit early in the season when the fruit receive the majority of their water volume from xylem before the flow mechanism switches to phloem flow.  A mass balance of total ¹⁴C showed that the majority was bound to organic matter in the soil with minimal ¹⁴C distributed throughout the trees.  TCE was not detected in any leaves or fruits using HS/GC/MS implying that the ¹⁴C found is associated with non-volatile TCE transformation products and/or TCE strongly bound to the plant tissue. While the identification of the metabolites trichloroacetic acid (TCAA) and dichloroacetic acid (DCAA) in leaves from a high dose apple tree support this hypothesis, the differences in analytical methodologies make this comparison tenuous.

ABSTRACT The soil-plant transfer of an element is determined not only by the soil solution concentration of the element, but also by the concentration of competing elements. For example, ammonium (NH4+) displaces strontium (Sr) on soil surfaces, leading to an increase in Sr concentration in the soil solution. Fertilization with ammonium should therefore increase plant uptake of Sr. However, ammonium strongly inhibits calcium uptake and therefore should also inhibit Sr uptake as plants do not differentiate between Sr and Ca in nutrient acquisition. While similarities in uptake between Sr and Ca are well documented, the effect of ammonium on plant uptake of Sr is poorly characterized. The literature has focused on displacement of Sr, Cs and other cations from the soil surface and has not addressed the competitive effect of ammonium on plant uptake. We conducted Sr plant uptake studies both in hydroponics and in soil. The uptake rate, distribution, and phytotoxicity of Sr in crested wheatgrass (Agropyron cristatum) was determined in hydroponics with treatments of 0, 3, 10, 100, and 1000 µM Sr in two separate trials. For the soil studies, our goal was to quantify the effect of ammonium on Sr uptake in crested wheatgrass. Two soils with similar native Sr content but varying native Ca and Mg contents were fertilized either with nitrate or ammonium. The results of the soil study are compared to the hydroponic study to separate the differential effects of soil chemistry and plant nutrition on plant uptake of Sr.

ABSTRACT Root exudates include important chelating compounds and can change the rhizosphere pH by several units. These changes are essential for nutrient uptake and can also alter solubility of soil contaminants and increase plant uptake. Mild root-zone stress may increase exudation and more severe stress can damage membranes and increase root turnover, all of which increase root-zone carbon. Increased carbon from this rhizodeposition can increase microbial activity, which might help degrade contaminants.  We studied the effect of three types of stress on root exudation of crested wheatgrass (Agropyron cristatum low K⁺, drought, and flooding. These stresses were compared to two types of controls: 100% NO₃^- and high NH₄⁺:NO₃^- ratio. We developed an improved axenic system to keep plants microbe-free for 70 days while analyzing exudates for total organic carbon (TOC) and organic acids. Axenic conditions were confirmed by plate counts of the leachate and microscopic observations of the leachate and roots. Optimal conditions for plant growth were maintained by monitoring temperature, light, humidity, water, O₂, CO₂, nutrient availability, and root-zone pH.  Plants were grown in Ottawa sand that was layered by size to optimize water availability. Total organic carbon released over the 70-day growth period in mg per gram dry plant was 2.6 in the control, 2.3 in the NH₄⁺ treatment, 3.7 in the flood and K⁺ stress treatments, and 4.4 in the drought treatment, which was the only treatment significantly higher than controls (p = 0.05). TOC and organic acid levels in the exudates peaked before the end of the study. The peak TOC levels, expressed as mg TOC per gram new dry plant mass, were 1.9 in the control, 3.0 in the NH₄⁺ treatment, 2.9 in the flood, and 5.8 in the drought and K⁺ stress treatments. Organic acids were measured by gas chromatography-mass spectrometry (GC-MS). Malic acid was the predominant organic acid, and accounted for the majority of the TOC in the drought treatment. Oxalic, succinic, fumaric, and malonic acids accounted for less than 10% of the TOC. These data indicate that stress may enhance phytoremediation by changing root-zone exudate composition.

ABSTRACT Nonylphenol ethoxylates (NPEs) are widely used surfactants that are commonly disposed of in wastewater collection systems. NPEs are subject to biological treatment within wastewater treatment facilities, but measurable amounts of untreated NPE and biodegradation intermediates such as nonylphenol (NP) have been identified in digested sewage biosolids (sludges).  These biosolids are often disposed of by applying them to agricultural soils where they can be used for their nutrient value.  Further biodegradation of NPE and NP within the soil/biosolids system has been observed but there is little known about the impact of plants on the biodegradation of these compounds and if plants take up these compounds or their metabolites.  This information is needed to determine the potential for food chain contamination as part of the overall terrestrial safety evaluation of applying biosolids containing NPE and NP.

The focus of this research is to determine the extent to which NPE and NP, introduced into the environment during the application of biosolids to soils, are taken up by plants.  Hydroponic studies were used to determine the uptake of NPE and NP as a function of exposure concentration and water transpired by the plant.  For fundamental plant uptake studies, the use of hydroponics greatly reduces the exposure variability, the influence of soil desorption kinetics and cost associated with soil-based studies using ¹⁴C-labeled chemicals.  Plant tissue containing ¹⁴C-labeled compounds can be combusted and analyzed for ¹⁴CO₂ in order to determine if any unextractable NPE and NP or their degradation products remain.

Results from a preliminary plant uptake study with crested wheatgrass showed some translocation of NP, NPE4 and NPE9 into the foliar portion of the plant.  The mass recoveries obtained for the NP uptake studies ranged from 75 to 100%.  At the end of the study, most of the ¹⁴C was found in the roots or root zone solutions.  The sorption to the plant roots slowed the volatilization of NP from the root zone.  While a high concentration of NP was found in the roots, little ¹⁴C was translocated to the foliar portion of the plant.  We are currently working on comparisons between ¹⁴C concentrations determined by combustion/LSC and steam distillation/HPLC.  So far, no nonylphenol has been detected in the foliar tissue using the steam distillation/HPLC procedure suggesting that the low levels of ¹⁴C found in the foliar tissue are likely metabolites of the NP.

ABSTRACT The objective of this work was to determine the uptake rate, distribution, and phytotoxicity of strontium (Sr) in crested wheatgrass.  We also determined the potential to use spectral reflectance measurements to identify high levels of Sr in the leaves of crested wheatgrass.  Experiments were performed on crested wheatgrass using hydroponic root zone environments to control the level of Sr at the root surface.  Sr solution concentrations ranged from 0 to 1000 mM.  Solutions were sampled at regular intervals to monitor and control the Sr concentration in solution.  Plant samples were taken at weekly intervals to assess the uptake rates and distribution of Sr in roots, leaves, stems, and heads of crested wheatgrass. Several sensitive, non-destructive indicators of plant growth were used to determine the toxicity of Sr to crested wheatgrass.  Strontium was readily taken up by crested wheatgrass at all concentrations tested.  The distribution of Sr within the plant mirrored that of calcium, with Sr accumulating in older material, especially older leaves.  We do not believe that Sr was toxic to crested wheatgrass at any of the solution concentrations tested.  Multiple state-of-the-art instruments were used to measure the spectral transmission of crested wheatgrass leaves with and without Sr, but no spectral signature was found for Sr.

ABSTRACT Constructed wetlands have been successfully used to remove or reduce contaminants such as organic matter, suspended solids, nutrients, and trace metals from water and wastewater through a combination of physical, chemical and biological processes including sedimentation, precipitation, sorption, plant uptake and microbial transformation.  The use of constructed wetlands for the treatment of dissolved phase petroleum hydrocarbons is a relatively novel application.  To assess its potential as a lower cost, reduced maintenance alternative to air stripper treatment for volatile hydrocarbons, a pilot scale constructed wetland was implemented in 1997 at the Gulf Strachan Gas Plant located approximately 200 km northwest of Calgary, Alberta, Canada.  Hydrocarbons were effectively removed during the first year of operation but specific removal processes were difficult to quantify.

A laboratory-scale constructed wetland system, using ¹⁴C-labeled benzene as a model volatile hydrocarbon, was thought to provide the best approach to accurately determine the relative importance of volatilization, mineralization and plant uptake. Four systems consisting of a root zone column, three volatile organic compound (VOC) traps and two CO₂ traps were used.  Two systems were planted with Phragmites and two served as unplanted controls (one control was poisoned with sodium azide).  All systems were manually dosed with ¹⁴C-benzene to keep an average exposure concentration of 5 mg/L over the course of the 26-day experiments.  A low flow of nitrogen was introduced into the root-zone to ensure mixing and keep the dissolved oxygen levels similar to those at the pilot system (< 1 mg/L).  The headspace above the root zone chamber was continuously drawn through the VOC and CO₂ traps to quantify benzene volatilization and mineralization.  ¹⁴C concentrations in the traps and root zone were measured throughout the study. Plant tissue concentrations of ¹⁴C and benzene were determined at the end of the study using combustion/liquid scintillation counting and headspace/gas chromatography.

ABSTRACT Plant uptake, metabolism, and volatilization of trichloroethylene (TCE) were quantified using a unique laboratory system.  Hybrid poplar trees were exposed to 1 or 10 mg/L TCE over a 43-d period.  [¹⁴C]TCE was added to four high-flow, aerated, hydroponic plant growth chamber systems designed to provide high mass recoveries, an optimal plant environment, and complete separation between foliar and root uptake.  The transpiration stream concentration factor (TSCF) is a useful measure of plant uptake.  A TSCF of zero indicates complete exclusion of the compound by the plant while a TSCF of 1.0 indicates unrestricted, passive uptake.  TSCFs for TCE, calculated from total [¹⁴C]TCE in shoot tissues plus phytovolatilized ¹⁴C, were 0.11 for two 1 mg/L treatments and 0.16 for a 10 mg/L treatment with roughly 30% attributed to phytovolatilization in all cases.  Though extended duration resulted in accumulation of more mass of ¹⁴C in plant tissues, it had no effect on TSCF.  These TSCF values are lower than other published experimental values and values predicted by theoretical relationships between TSCF and octanol-water partition coefficient.  The TCE metabolites trichloroethanol (TCEt), trichloroacetic acid (TCAA), and dichloroacetic acid (DCAA) were identified in plant tissues of the 10 mg/L treatment.

ABSTRACT A trichloroethylene (TCE)-contaminated groundwater plume has migrated beneath communities surrounding Hill Air Force Base in Northern Utah.  Due to the combination of a relatively high water table and an abundance of orchards and vegetable gardens in this area, there exists the potential for translocation of TCE into edible fruits and vegetables.  Results from a preliminary field sampling survey conducted in the fall of 2001 indicated the presence of trace levels of TCE in the fruit of several trees growing above the TCE plume.  This discovery prompted a follow-up field study to determine if the presence of TCE in fruit was real or an anomaly.  Sites visited in 2001 will be revisited from August to October of 2002.  Fruit samples will be taken from apple and peach trees as well as from a variety of garden vegetables.  Sampling, QA/QC, and analytical methods have been refined to maximize replicability and detection.  Results obtained from the field study from 2001 and 2002 will be presented as will QA/QC and analytical methodology.

• Greenhouse Study to Determine the
  Uptake of Trichloroethylene by Fruit Trees
  Brandon Chard, W. Doucette, J. Chard, B. Bugbee, and K. Gorder
  Symposium on In Situ and On-Site Bioremediation
  Jun. 2-5, 2003; Orlando, FL

ABSTRACT Results from a preliminary field sampling survey conducted in the fall of 2001 indicated the presence of trace levels of trichloroethylene (TCE) in the fruit of several trees growing above a contaminated groundwater plume.  This discovery prompted a follow-up greenhouse study, currently in progress, to examine the extent, kinetics and physiological pathway of TCE transport into fruit. 

Nine each of mature dwarf apple and peach trees growing in individual soil-filled containers were continuously watered via subsurface irrigation during a six-month period from just after winter dormancy to the production and maturation of fruit.  Six each of the two tree species were exposed to ¹⁴C[TCE] while three each served as non-dosed controls.  The two exposure levels used (5 and 500 mg/L), bracketed the observed field groundwater concentrations.  ¹⁴C[TCE] was delivered to the trees as an aqueous solution with the irrigation water.  Samples of irrigation water, soil water, fruit, leaves and stems were collected and analyzed for TCE at regular intervals throughout the exposure period using liquid scintillation counting (LSC) and headspace gas chromatography-mass spectrometry (GC/MS) methods. After the fruit ripened, samples of leaves, stems, tree core, fruit peel and fruit flesh were collected and analyzed. 

Fruit samples are currently being analyzed for the presence of ¹⁴C.  Samples from stems and leaves, combusted with a biological oxidizer, were found to have ¹⁴C as measured by LSC.  However, TCE was not detected in fruit, stem or leaf samples by headspace GC/MS, with a detection limit of 0.1 ppb.  Trace levels of TCE were quantified in tree cores via GC/MS, however the results are significantly lower than those found via LSC.  The results indicate that ¹⁴C [TCE] was taken up by the trees and metabolized.  Methods are currently under development to determine TCE metabolites present in the plant tissue.  The results of this study and the implications associated with risk assessment will be presented.

• Phytoremediation: Physiological Procedures
  for Scaling from Laboratory to Field
  Bruce Bugbee and W. Doucette

ABSTRACT Plants can increase the removal of organic compounds from soil by three basic mechanisms: rhizosphere degradation; uptake, translocation, and volatilization of unmetabolized compounds; and uptake, metabolism or storage.  The importance of each of these mechanisms is typically estimated from measurements made on plants in containers in controlled environments or from field studies of single plants and it is necessary to scale this data to the community level.  Over the past century physiologists have developed and refined procedures for scaling from measurements made in small chambers to determine mass transport in ecosystems.  Here we review procedures and apply the principles to scaling from measurements of volatile and non-volatile contaminants in small chambers to plant communities.  Numerical examples from measured literature values are given.

• Water and Nutrient Stresses Increase Root Exudation
  Amelia Henry, J. Chard, M. Petersen,
  M. Hamilton, C. Palmer, J. Hess and B. Bugbee
  American Society of Agronomy
  Nov. 10-14, 2002; Indianapolis, IN

OBJECTIVES
DEVELOP PROCEDURES TO:  Grow healthy plants under sterile conditions; Manipulate root exudation with stress; Quantify total organic carbon in exudates; Determine composition of exudates using GC-MS

IMPLICATIONS FOR PHYTOREMEDIATION:  Our focus for the qualitative analysis: organic acids.  The chelating properties of these compounds can be useful for phytoremediation, and they are a class of compound most likely to be found in root exudate.

• Characterization of Root Exudates From
  Crested Wheatgrass (Agropyron cristatum)
  Amelia Henry, J. Chard, B. Doucette, J. Norton, and B. Bugbee - 2001
  American Society of Agronomy
  Oct 20-25, 2001; Charlotte, NC

• Sterile Culture Techniques for Characterization of Root Exudates
  Julie Chard, A. Henry, B. Doucette, S. Jones,
  J. Norton, C. Palmer, R. Hess, and B. Bugbee
  American Society of Agronomy
  Oct. 20-25, 2001; Charlotte, NC

ABSTRACT  Creating and maintaining sterile conditions in the root zone while providing optimal plant growth conditions is a challenging task.  Microbial activity and stresses resulting from excessive or inadequate air, water, and nutrients can alter root exudation.  Therefore, sterile containers must provide optimal water-holding capacity and air-filled porosity while facilitating control of nutrient status.  We developed a solid-media plant growth container for use in exudates studies (see Henry, et al., Characterization of root Exudates from Crested Wheatgrass, ASA paper 2001).  All components are autoclavable.  Containers are filled from bottom to top with a five-layer sand gradient to uniformly distribute water and air.  A foam plug seals the shoot/container interface.  Sterile, dilute nutrient solution is added through a septum via a sidearm port.  A 7-cm drain tube with silanized glass wool wick provides consistent drainage and enables leachate collection for exudates analysis.  Seeds are surface-sterilized and grown on non-selective agar to confirm sterility prior to leachate.  Plants grown for one month in these containers exhibited exponential growth and uniform root distribution within each container.  Sterility was maintained in many plants for 30 days.  We continue to refine our sterile technique.

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• A Novel Laboratory System for Determining Fate of
  Volatile Organic Compounds in Planted Systems
  Brady Orchard, W. Doucette, J. Chard, and B. Bugbee - 2000
  Environmental Toxicology and Chemistry 19:888-894

ABSTRACT Contradictory observations regarding the uptake and translocation of volatile organic compounds (VOCs) by plants have been reported, most notably for trichloroethylene (TCE). Experimental artifacts resulting from the use of semistatic or low-flow laboratory systems may account for part of the discrepancy. Innovative plant growth chambers are required to rigorously quantify the movement of VOCs through higher plants while maintaining a natural plant environment. The plant must be sealed in a chamber that allows rapid exchange of air to remove the water vapor lost in transpiration, to resupply the CO₂ consumed in photosynthesis, and to resupply the O₂ consumed in root-zone respiration. Inadequate airflow through the foliar region results in high humidity, which dramatically reduces transpiration and may reduce contaminant flux. Oxygen depletion in static root zones induces root stress, which can increase root membrane permeability. The root zone must be separated from the shoots to differentiate between plant uptake and foliar deposition. Here we describe the design, construction, and testing of a dual-vacuum, continuous high-flow chamber system for accurately determining the fate of VOCs in plants. The system provides a natural plant environment, complete root/shoot separation, the ability to quantify phytovolatilization and mineralization in both root and shoot compartments, continuous root-zone aeration, and high mass recovery.

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• Uptake of Trichloroethylene by Hybrid Poplar Trees
  Grown Hydroponically in Flow-Through Plant Growth Chambers
  Brady Orchard, W. Doucette, J. Chard, and B. Bugbee - 2000
  Environmental Toxicology and Chemistry 19:895-903

ABSTRACT Phytoremediation is being promoted as a cost-effective treatment option for shallow groundwater and soils contaminated with trichloroethylene (TCE). However, its effectiveness is difficult to assess due to contradictory reports regarding the magnitude of plant uptake and phytovolatilization. Experimental artifacts and plant stress, resulting from the use of static or low-flow plant growth laboratory systems, may account for part of the discrepancy. High exposure concentrations and short durations may also cause artifacts in laboratory studies. A dual-chamber plant growth system designed to minimize experimental artifacts was used to determine the uptake of [ 14 C] TCE by hydroponically grown hybrid poplar as a function of plant stress (aerobic and oxygen-reduced root zone), exposure concentration (1–70 mg/L), and exposure duration (12 or 26 d). The [ 14 C]TCE recoveries ranged from 92 to 101% in 11 dosed chambers. Trichloroethylene mass equivalent concentrations in the shoot tissue (2–168 mg/kg) were dependent on the amount of water transpired and the exposure concentration. Root-zone oxygen status did not significantly impact TCE uptake.   Transpiration stream concentration factors (TSCFs) determined in these studies (0.02–0.22) were independent of exposure duration and are much lower than those previously reported and predicted. The role of TSCF and other factors in estimating the significance of plant uptake in the phytoremediation of TCE-contaminated groundwater is discussed.

• Phytovolitalization of Trichloroethylene from Trees
  Bruce Bugbee, W. Doucette, J. Chard, and B. Orchard   
  American Society of Agronomy
  Nov. 2000; Minneapolis, MN

ABSTRACT  The seasonal magnitude of the gaseous efflux of volatile compounds from the leaves of plants has been one of the most difficult parameters to quantify in studies of the potential use of vegetation to help remediate contaminated soils. Measurements are typically made of the efflux of organics per unit leaf area of a small sample of leaves in a small chamber. Unfortunately, scaling from a small leaf sample to the entire community of leaves is difficult. Scaling from a short interval to the entire growing season is equally difficult. To facilitate scaling in space and time, we measured the efflux of trichloroethylene (TCE) per unit water vapor in transpiration. This approach eliminates many of the assumptions that must be made to extrapolate based on a sub-sample of a tree's leaf area. Our recent findings suggest that volatilization of TCE from groundwater can be a significant fate mechanism in climates with limited summer rainfall such as Hill Air Force Base, Utah. However, in a climate with rainfall that replenishes surface water such as Cape Canaveral FL, phytovolatilization of TCE was not a significant fate mechanism. The design and operation of chambers to quantify the ratio of TCE to water vapor will be discussed.

• Phytovolitalization of Trichloroethylene from Trees
  Bruce Bugbee, W. Doucette, J. Chard, and B. Orchard
  American Society of Agronomy
  Oct. 1999; Salt Lake City, UT

ABSTRACT Phytoremediation, the use of plants to stabilize, extract, and/or metabolize contaminants, may be a cleanup alternative for trichloroethylene (TCE)-contaminated sites. A laboratory system that provides a realistic plant growth environment while obtaining high mass recoveries was used to accurately determine plant-contaminant interactions and to minimize experimental artifacts. Experimental variables examined include exposure duration (10 to 43 days) and TCE concentration (1, 10, 70 mg/L). Mass recovery of (^14)C was 92 to 100%. Transpiration stream concentration factors (TSCFs) varied from 0.02 to 0.19 and were greatest at an intermediate concentration (10 mg/L). TCE metabolites were found in plant tissues from only the higher exposure treatments (10 and 70 mg/L), with evidence of increased accumulation over time. In contrast with several recent reports, our data indicates little, if any, phytovolatilization. 

• Hycrest Crested Wheatgrass Accelerates the
  Degradation of Pentachlorophenol in Soil
  Ari Ferro, R. Sims, and B. Bugbee - 1994
  Journal of Environmental Quality 23:272-279

ABSTRACT We investigated the effects of vegetation on the fate of pentachlorophenol (PCP) in soil using a novel high-flow sealed test system. Pentachlorophenol has been widely used as a wood preservative, and this highly toxic biocide contaminates soil and ground water at many sites. Although plants are known to accelerates the rates of degradation of certain soil contaminants, this approach has not been thoroughly investigated for PCP. The fate of [14C]PCP, added to soil at a concentration of 100 mg/kg, was compared in three unplanted and three planted systems. The plant used was Hycrest, a perennial, drought-tolerant cultivar of crested wheatgrass [Agropyron desertorum (Fisher ex Link) Schultes].

The flow-through test system allowed us to maintain a budget for 14C-label as well as monitor mineralization (breakdown to 14CO2) and volatilization of the test compound in a 155-d trial. In the unplanted systems, an average of 88% of the total radiolabel remained in the soil and leachate and only 6% was mineralized. In the planted systems, 33% of the radiolabel remained in the soil plus leachate, 22% was mineralized, and 36% was associated with plant tissue (21% with the root fraction and 15% with shoots). Mineralization rates were 23.1 mg PCP mineralized kg-1 soil in 20 wk in the planted system, and for the unplanted system 6.6 mg PCPkg-1 soil for the same time period.

Similar amounts of volatile organic material were generated in the two systems (1,5%). Results indicated that establishing crested wheatgrass on PCP-contaminated surface that establishing crested wheatgrass on PCP-contaminated surface soils may accelerate the removal of the contaminant.

• Phytoremediation of Trichloroethylene Using Hybrid Poplar
  Julie Chard, B. Orchard, W. Doucette, and B. Bugbee
  American Society of Agronomy, 1999

ABSTRACT Phytoremediation, the use of plants to stabilize, extract, and/or metabolize contaminants, may be a cleanup alternative for trichloroethylene (TCE)-contaminated sites. A laboratory system that provides a realistic plant growth environment while obtaining high mass recoveries was used to accurately determine plant-contaminant interactions and to minimize experimental artifacts. Experimental variables examined include exposure duration (10 to 43 days) and TCE concentration (1, 10, 70 mg/L). Mass recovery of (^14)C was 92 to 100%. Transpiration stream concentration factors (TSCFs) varied from 0.02 to 0.19 and were greatest at an intermediate concentration (10 mg/L). TCE metabolites were found in plant tissues from only the higher exposure treatments (10 and 70 mg/L), with evidence of increased accumulation over time. In contrast with several recent reports, our data indicates little, if any, phytovolatilization.

• Phytoremediation: Effect of Root-Zone Stress
  on the Uptake of Organic Contaminants
  Bruce Bugbee, J. Chard, B. Orchard, and W. Doucette
  American Society of Agronomy, 1998

ABSTRACT  There is a widespread belief that plants have enormous potential to absorb contaminants from the root-zone and degrade or release them into the atmosphere. Literature reports indicate either significant or non-significant uptake, with no clear explanation for the discrepancies. We designed a system to rigorously examine movement of trichloroethylene (TCE) in the soil-plant-atmosphere continuum. Mass balance recovery from 4 chambers in two, 10-day studies was 93 to 101%. Recovery of radiolabel transpired from leaves was <0.03%. Recovery of the ¹⁴C label in leaves and stems was negligible (<0.08%). We hypothesized that anaerobic root-zone conditions would increase membrane permeability and increase TCE uptake, but we did not find that these conditions increased uptake. Our studies were conducted at 1mg L^-1 to simulate typical groundwater concentrations. Published studies showing significant uptake have been conducted with root-zone TCE concentrations of 10 to 100 mg L^-1 . We are currently examining the effect of these high concentrations of TCE on uptake.

• Phytoremediation of Dissolved Phase
  Trichloroethylene Using Mature Vegetation
  S. Hayhurst, W. Doucette, B. Orchard,
  C. Pajak, B. Bugbee, and G. Koerner

ABSTRACT Plants have profound effects on physical, chemical and biological processes and can significantly impact the environmental fate of many contaminants. Phytoremediation is being evaluated as a potential remediation option for many organic contaminants including chlorinated solvents, such as trichloroethylene (TCE) in shallow contaminated soils and groundwater. Reports of both insignificant and significant plant uptake, transpiration and/or degradation of TCE have appeared in the literature. Most of these results have come from laboratory studies.