Environ. Sci. Technol. 2002, 36, 4663-4668
VOCs Fate and Partitioning in Vegetation: Use of Tree Cores in Groundwater Analysis XINGMAO MA AND JOEL G. BURKEN* Department of Civil Engineering, Butler-Carlton Hall, 1870 Miner Circle, University of MissourisRolla, Rolla, Missouri 65409
Analysis of tree cores collected from contaminated sites has shown that concentrations of VOCs in cores are related to groundwater concentrations. However, initial research was highly qualitative. To better understand the relationship of groundwater VOC concentrations to measured VOCs in tree cores, detailed understanding of contaminant behavior in vegetation is required. Work presented here investigates the interaction, with focus on the chlorinated solvents trichloroethylene, 1,1,2,2-tetrachloroethane, and carbon tetrachloride. The sorption and desorption partitioning of these compounds between air and woody biomass were investigated. Partitioning coefficients were determined for cores of trunks of large trees and smaller stem cuttings. The internal partitioning of these compounds between the transpiration stream and the woody biomass within the tree was also determined for cores. The partitioning coefficients of the compounds between air, water, and biomass of tree cores and trunks were related to the physicochemical characteristics of contaminants, mainly the Henry’s law constant and vapor pressure. These partitioning coefficients relate the contaminants’ concentration in the bulk solution and analyzed headspace of vials and therefore can be utilized to quantify the fate of contaminants in natural settings and in phytoremediation systems. Tissue analysis and determination of partitioning coefficients may provide an effective way to estimate the concentration of compounds in the transpiration stream and in the soil or groundwater in a noninvasive, extremely rapid, and cost-effective manner.
Introduction The use of vegetation for environmental cleanup (phytoremediation) has attracted considerable interest in the past decade because of its cost-effectiveness, suitability for longterm applications, low maintenance, ecological benefits, and aesthetic advantages. Among the remaining issues related to phytoremediation, contaminant fate and transport are primary concerns. Degradation in the rhizosphere is a desired contaminant fate. However, uptake and transpiration through shoots and subsequent volatilization to the atmosphere and/ or storage of parent compounds or metabolites in the vegetation biomass are often the main pathways of a compound’s removal in laboratory studies. However actual fates in full-scale systems have eluded understanding (1-4). Uptake of many VOCs was established as analysis of tree * Corresponding author e-mail:
[email protected]; phone: (573)341-6547; fax: (573)341-4728. 10.1021/es025795j CCC: $22.00 Published on Web 09/25/2002
2002 American Chemical Society
cores has shown that VOCs can be detected in tree biomass when roots are exposed to contaminated groundwater (36). There have been efforts to use samples of vegetation for the site investigation on contaminant distribution in groundwater. To date, this type of analysis has been essentially qualitative. Groundwater concentration profiles were roughly indicated, but the concentration neither in the soil nor in the transpiration water was delineated. To estimate actual concentrations, a better understanding of chemical transport, partitioning, and fate between different matrixes (soil, water, and plants) is needed. VOCs fugitive in the environment will interact with different media on the basis of their physicochemical properties. Media interactions commonly investigated are partitioning between water-air (Henry’s law constant) and between water-soil (sorption). Such partitioning mechanisms are important in the environmental fate of VOCs. Plants play a role as another media. Vegetation has an intimate interaction with soil, groundwater/surface water, and the atmosphere requiring contact with all these media for survival. Plant interactions are also substantial. Transpiration, the movement of water to the atmosphere by plants, is credited with removing more water from the continental United States than either freshwater outfalls or evaporation (7). Plants also represent the extreme majority of biological material on earth, representing more than 99% of the biomass on earth (8). A number of recent studies have taken a focus on contaminant-plant interactions. Simonich and Hites (911) investigated the uptake of contaminants from the atmosphere. Through sorption from the atmosphere, plants were estimated to remove roughly 44 ( 18% polycyclic aromatic hydrocarbons (PAHs) emitted to the atmosphere in the central-northeast United States. These estimates did not include measurements of PAHs in the bark or stems of plants, which comprise most of the mass of the vegetation. Other research has been aimed toward the interaction of contaminated water and plants. Both hydrophilic and hydrophobic compounds have been studied regarding their partitioning and accumulation between soil and vegetation roots. Briggs et al. (1) found that uptake and subsequent translocation of compounds correlated with the octanolwater partition coefficient. Other studies have since found similar results with a variety of compounds (3, 13, 14). Compounds with intermediate lipophilicity are most effectively translocated. Eleven different organic compounds with a wide range of chemical properties were studied to determine the mass distribution and volatilization following uptake by hybrid poplar trees (2). Volatilization was shown to correlate with vapor pressure, Vp. Compounds with a Vp > 0.01 atm were volatilized, with higher vapor pressures relating to greater volatilization. Volatilization was much lower for compounds with a Vp < 0.01. Mackay and Gschwend (15) measured the partitioning of monoaromatics (benzene, toluene, o-xylene) between water and dry wood of Ponderosa pine and Douglas fir. Measured partition coefficients ranged from 0.0066 to 0.028 (L/g). A similar study was performed by Trapp et al. (16) in which the partitioning coefficients of 10 organic chemicals (log Kow, 1.48-6.20) from water to wood (oak and willow) were experimentally determined. The correlation of partitioning coefficients to the log Kow was significant except for phenol. While the partitioning of contaminants between soilwater-vapor phases subsurface has been studied to yield standard partitioning coefficients, the plant-atmosphere VOL. 36, NO. 21, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Important Physicochemical Characteristics of Tested Compoundsa
compound
vapor pressure (kPa)
Henry’s constant (Pa m3 mol-1)
log Kow
TCE TeCA CT
7.998 0.667 15.2
970 45 2330
2.33 2.39 2.70
boiling point
water solubility (mg/L)
86.7 146.2 76.7
1100 2900 800
a
Data are cited from Mackay et al. (28), and all listed values were measured at 20 °C.
partitioning coefficient is defined in a variety of ways by different researchers. Most relationships for partitioning between biomass and air follow one of two general approaches. One approach is shown as (17)
Kv )
{Cveg}/{lipid} Ca
(1)
where Cveg is the concentration of contaminants in vegetation (ng/g dry weight), lipid is the lipid content of the vegetation (mg/g dry weight), and Ca is the atmospheric gas-phase pollutant concentration (ng/m3) (18). Another approach leads to
Kaw )
Ct (µg/g) Ca (µg/L)
(2)
where Ct is the equilibrium concentration of organics in plants, and Ca is the concentration of organics in air. The approach of eq 2 is utilized in this work because of its simplicity and similarity to other partitioning coefficients, such as Kow or Henry’s law constant. For water and wood partitioning, some researchers utilize eq 3 by assuming that the water-wood partitioning was controlled by wood lignin content (15, 19):
Klw ) fligninKlignin
(3)
where Klw is the water-wood partitioning coefficient, flignin is the mass fraction of lignin, and Klignin is the lignin-water partitioning coefficient. This definition provides a simple way to compare and calculate the partitioning coefficient for various types of wood in theory. However, the variances of different extraction techniques and the compositions of lignin itself in different woods somewhat limit the application of this definition. A definition similar to air-biomass partitioning coefficient has been utilized (16), was applied in this paper, and is expressed as
Klw )
Cdw (mg/g) Cl (mg/L)
(4)
where Cdw is the equilibrium contaminant concentration in dry wood, and Cl is the concentration in internal aqueous solution, such as the transpiration stream. The contaminants that were investigated in this study [trichloroethylene (TCE), 1,1,2,2-tetrachloroethane (TeCA), and tetrachloromethane, commonly called carbon tetrachloride (CT)] are common groundwater contaminants resulting from a variety of commercial and industrial uses and applications (20, 21). All three compounds are resistant to biodegradation and chemical degradation in the subsurface. Some important physical and chemical properties of these three compounds are listed in Table 1. As these compounds are of intermediate mobility and can cause extensive plumes, use of monitored natural attenuation and phytoremediation, which require long remedial periods, are 4664
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drawing substantial consideration as remedial choices. Along with these long-term approaches come high monitoring costs. The methods outlined herein may be used to monitor groundwater plumes through biomass sampling, thus reducing the sampling costs and increasing the number of samples that can be taken, as each tree or plant on a site may yield valuable information.
Materials and Methods Air-Biomass Partitioning Experiments. Tree cores from an 8-year-old freshly cut hybrid poplar tree trunk (Populus deltoides × P. nigra, clone DN 34) and tree cuttings from poplar tree branches (DN 34) were used in the experiment. The trunk obtained was 6 in. diameter. Tree cores of approximately 2 in. length and 0.2 in. diameter were taken with an 0.2-in. increment borer (Forestry Services Inc.). The tree cores include both xylem and phloem tissues. Each tree core was put in a 20-mL vial, which was closed with the Teflon rubber septa. Exact lengths of the tree cores were measured and recorded as they were placed into the vials. Tree cuttings were DN 34 tissue samples, 2 in. long and approximately 0.25-0.33 in. diameter. Cuttings were used in the same manner as tree cores. Each treatment set (n ) 3) was dosed with known concentrations of TCE, TeCA, or CT. The gas-phase compounds from prepared standards were injected into the bottom of the vials with a 1-mL gastight syringe (VICI Inc.) with the cap covering the opening but not sealed. Contaminants were carefully injected to the bottom of vials to minimize immediate mixing and potential contaminant loss. After the compounds were injected, the vials were fully closed and sealed with crimp top seals. By injecting different volumes of gas phase into the headspace of different vials, the initial concentrations were obtained. The initial concentrations ranged from 3.19 ppbv to 5.12 ppmv for TCE, from 12.3 ppbv to 2.83 ppmv for TeCA, and 3.82 ppbv to 240.1 ppbv for CT. Vials were allowed to equilibrate for 48-96 h at room temperature (23 ( 1 °C). The equilibrium time between air and tree cores/cuttings was determined in separate experiments (data not shown). Literature indicated that equilibrium between air and pine needles is more rapid, taking only a few hours (22, 23). The headspace concentration of each vial was determined via gas chromatography. The vials were then weighed, opened, and placed in a drying oven at 103 °C for 24 h. The dried cores were also weighed separately, and the moisture content and weight of the tree core were calculated. Desorption partitioning coefficients were also determined for tree cuttings. Before the cuttings were dried and weighed, the vials were de-capped, and the cuttings were immediately put into new clean vials. The vials were immediately sealed with Teflon rubber septa and crimp top seal. After 48-96 h, the headspace concentration was measured via GC as described before. The cuttings were then dried and weighed as described above. Water-Wood Partitioning Experiments. Tree cores from the same trunk as used for the air-biomass partitioning measurement were used for the water-wood partitioning measurement. Tree cores were dried in an incubator set at 103 °C for 24 h, and dry cores were weighed. Dry cores were then put into 20-mL vials, and standard solutions were transferred to the vials so that no headspace remained. Initial concentrations ranged from 11.7 to 133 ppm for TCE, from 32 to 154 ppm for TeCA, and from 17 to 223 ppm for CT. The vials were then closed with the Teflon rubber septa and sealed with crimp top seals. Three replicates were made for each concentration. The samples were analyzed via GC after equilibrating 14 days. TCE Uptake Experiments. Cuttings of about 12 in. long and 0.3-0.4 in. diameter rooted in one-quarter strength Hoagland’s solution. Each cutting was fitted with a Teflon
septa and screw cap during the rooting process as described elsewhere (3). Light was supplied by 10 40-W fluorescent bulbs (Verilux) with an approximate intensity of 1580 lux on a 16-h photoperiod. After 2 weeks the cuttings together with the Teflon septa and cap were transferred to 250-mL flasks filled with one-quarter strength Hoagland’s solution. The interface of the septum and cutting was sealed with acrylic caulk soon after the transfer. TCE concentrations were 2, 5, 20, and 50 ppm in the hydroponic solutions. Three replicates were made for each concentration. TCE concentrations in the reactors were analyzed every 12 h in the first 2 days via GC. The concentrations in the reactors remained within 5% of the initial concentrations. The solution was replaced every 24 h from the third day until the end of the experiment to maintain constant concentrations in the reactors. After 10 days, the experiment was terminated. A 2-in. section of each stem immediately above the septa was cut, weighed, measured, placed into 20-mL vials, and sealed. Following an equilibration period, the vials were analyzed via GC. The vials were then opened, and the mass of the dry biomass and water in each cutting was determined as discussed above. Using the TCE concentration in headspace, the TCE concentration in total biomass and the transpiration stream were determined according to eqs 2, 4, 6, and 7. Computation and Data Analysis. On the basis of the laws of conservation of mass and the assumption that no degradation took place, partitioning coefficients were obtained. For the air-biomass partitioning, the mass balance equation is expressed as
Mp ) CaVa + CtMt
(5)
where Mp is the total compound mass injected (mg), Va is the headspace volume (L), and Mt is the tree core or cutting mass (g). The headspace volume is equal to the volume of vial (0.02 L) less the tree core/cutting volume (Vt). Substituting into eq 2 for Ct, the following equation can be derived:
Kaw )
[Mp - (0.02 - Vt)Ca]/Mt Ca
(6)
(7)
(Vvial - Vcore + Kaw) C (Vtrans + KlwMd) a
(8)
where Ctrans is defined as the contaminant concentration in the transpiration stream through the stem. By definition, TSCF is equal to the concentration of the analyte in the transpiration stream within the plant divided by the concentration in the bulk solution from which uptake is taking
(9)
If eq 8 is substituted for Ctrans, the following equation can be obtained:
Cbulk solution )
where Mp is the total mass of the compounds (mg), Vt is the volume of dry tree cores (mL). Mw is the mass of the dry cores (g), and Cl is the aqueous concentration at equilibrium. In situ, the aqueous concentration (Cl) is the concentration of compounds in the internal solution of the tree, including the transpiration stream. A combination of eqs 2 and 4 and the determined partitioning coefficients allows the measured concentrations in headspace to be used to calculate the concentrations in the transpiration stream as shown in
Ctrans )
place and is expressed as
TSCF ) Ctrans/Cbulk solution
The tree core and cutting volume were calculated after measuring the lengths and diameters and approximating as cylindrical. Similarly, the partitioning coefficient between woody tissue and water was calculated according to
[Mp - (0.02 - Vt)Cl]/Mw Klw ) Cl
FIGURE 1. Isotherms for partitioning from air to tree cores or cuttings after >48 h of equilibrium time. Compounds tested were (A) trichloroethylene, (B) tetrachloroethane, and (C) carbon tetrachloride.
(Vvial - Vcore + Kaw) C /TSCF (Vtrans + KlwMd) a
(10)
This equation directly relates the concentration in bulk solution to the measured concentration in headspace, providing a simple way to estimate the concentration in bulk solution (groundwater concentration) from the headspace concentration of tree core samples.
Results and Discussion Air-Biomass Partition. Isotherms were obtained for the biomass-air partitioning for three compounds (Figure 1). Partitioning coefficients are taken as the slopes of the isotherms (Table 2). The partitioning coefficients for tree cores and air are higher than those of tree cuttings and air. The observed difference is likely due to the structural diversity between tree cores and cuttings. Cuttings are composed of wood and bark and have different xylem-to-phloem ratios than tree cores. Bark is totally lacking the longitudinal tracheid, a dominant makeup of the volume of wood xylem. The composition of wood and bark is also different. The lignin content of hardwood bark is about 40-50% as compared with the 18-25% of wood. The lignin of bark is VOL. 36, NO. 21, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Air-Biomass Partitioning Coefficients for the Three Compounds Testeda tree core and air (sorption) tree cutting and air (sorption) tree cutting and air (desorption) pine needle and air (sorption)b spruce needle and air (sorption)b a
The ( represents 95% confidence interval.
b
TCE (L/g)
TeCA (L/g)
CT (L/g)
0.135 ( 0.056 0.064 ( 0.010 0.072 ( 0.010 0.029 0.019
0.319 ( 0.060 0.242 ( 0.041 0.343 ( 0.183
0.055 ( 0.008 0.042 ( 0.007 0.053 ( 0.008 0.010 0.019
Values calculated from data gathered by Brown et al. (23).
FIGURE 2. Partitioning coefficients plotted versus the vapor pressure and the Henry’s law constant. Partitioning between woody tissues and water, Klw, showed linear increase with increasing H and Vp. Partitioning between the biomass and water, Kaw, decreased exponentially with increasing H and Vp, exponential regression provided R2 values of 0.97-0.99. also somewhat different from that of wood: the methoxyl content of bark lignin being only about one-half that of xylem lignin (24). The larger exposed surface area of tree cores as compared with cuttings constitutes another potential reason for the difference of partitioning coefficients. The severed xylem tissues are assumed to have a larger specific surface area. However, the gas-phase chlorinated solvents can easily diffuse through the organic matrix of the core or cutting, making surface adsorption and absorption indistinguishable and negating the higher external surface area. Measured partitioning coefficients for both trees and cuttings were compared with a limited number of partitioning coefficients measured by other researchers (Table 2). While partitioning values in Table 2 are similar in magnitude, the determined values presented here for poplar biomass are higher than those cited for other compounds and tissues. Structural variability between needles and stems undoubtedly contributes to the difference. Needles have higher surfaceto-volume ratio than stems or trunks. Length of experiment may also contribute as cited values were from experiments lasting only a few hours (22, 23). Complete equilibrium may not have been reached, potentially leading to a low estimation of the partitioning coefficient. When comparing the partitioning coefficients of these three compounds, the relative order was TeCA > TCE > CT for both cuttings and trunk cores. The relative partitioning is as expected from their vapor pressures and Henry’s law constants (Table 1). When the partitioning coefficients were 4666
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FIGURE 3. Isotherms for tree cores in water for (A) trichloroethylene, (B) tetrachloroethane, and (C) carbon tetrachloride. The error bars represent 95% confidence interval, n ) 3. plotted versus the vapor pressure and Henry’s law constants, apparent relationships existed (Figure 2). Both tree cores and cuttings showed exponential correlations between partitioning coefficients and vapor pressures and Henry’s law constants. Water-Wood Partitioning Coefficient. Poplar tree cores from aqueous solution sorbed all three compounds. The chemicals’ aqueous concentrations versus their sorbed concentrations in liquid after equilibrium are shown in Figure 3. Isotherms were linear over the range of concentrations used, but caution should be taken when the measured coefficients are extended for concentrations beyond the range. A relative comparison of the measured values and similar coefficients from literature sources are listed in Table 3. The cited numbers were reported in a log-log figure; therefore, great variability exists in determining the 95% confidence intervals, and only a range of probable values are listed in the table. Research by Mackay and Gschwend
TABLE 3. Wood-Water Partitioning Coefficientsa poplarb oakc willowc poplare
TCE (L/g)
TeCA (L/g)
CT (L/g)
0.0385 ( 0.0068 0.0182d (0.0072-0.0459) 0.0217 (0.0049-0.0968) 0.0141 (0.0048-0.0414)
0.0207 ( 0.0030 0.0174 (0.0069-0.0438) 0.0207 (0.0047-0.0918) 0.0134 (0.0046-0.0395)
0.0593 ( 0.0066 0.0285 (0.0108-0.0754) 0.0350 (0.0073-0.1678) 0.0239 (0.0076-0.0748)
a The ( represents 95% confidence interval. b Measured values. Calculated values from Trapp et al. (16). d The number stands for the average of the estimated water-oak wood partitioning coefficient for TCE. The numbers in parentheses show the range of the partitioning coefficient. e Values calculated from equation by Mackay and Gschwend (15), assuming a lignin content of 20%. c
FIGURE 4. Aqueous TCE concentration in poplar cuttings vs TCE concentration in the hydroponic bulk solution. The error bars represent 95% confidence interval, n ) 3. established relationships of water-lignin partitioning (15). The fraction of lignin in poplar trees was assumed to be 20%. The lignin content of hardwood is about 18-25%, and the content of softwood is a little higher and is 25-30% (24). Lignin is the chief noncarbonhydrate constituent of wood binding to cellulose fibers and strengthening the cell walls. Lignin is extremely hydrophobic and shows strong affinity to hydrophobic organics. The measured values reported here are in agreement with the range of literature values, reinforcing the findings. Wood-water partitioning depends dominantly on water- lignin partitioning. Previous research has shown that wood-water partitioning coefficients for many test compounds had significant linear correlation with log Kow (15, 16). A linear relationship was observed between the partitioning coefficients and vapor pressure and Henry’s law constants (Figure 2). Uptake and Transpiration of TCE. Poplar cuttings showed no signs of toxicity or inhibition in these short-term experiments at a concentration up to 50 ppm. No leaf wilting, chlorosis, or water usage reduction was observed. A linear correlation of calculated transpiration stream concentration and known bulk solution is shown in Figure 4. The linear relationship provides evidence that the aqueous contaminant is taken up by trees and that the concentration in the transpiration stream is directly proportional to the aqueous concentration at the roots. The slope represents the TSCF value if an assumption is made. The assumption is that the concentration over the 2-in. section of stem analyzed is constant. Making this assumption yields a TSCF of 0.26 (Figure 4). The assumption made is likely to produce a conservative value for two reasons. One is that this method measures total mass of TCE in the sample and averages this mass for all water in cuttings, including xylem and phloem flows and intercellular water that may reside in nonconductive tissues such as vacuoles. The other reason considers that diffusion of VOCs from plant tissues is documented here through partition coefficients and in other
TABLE 4. Estimated Values of Transpiration Stream Concentration Factors (TSCF) for TCE chemical TCEa
1 mM 1 mM TCEa 1 mM TCE & TCAa 1 mM TCE & methanola 0.6-70 mg/L TCEb about 50 mg/L TCEc mixture of chemicalsd
plant
results
1 salt cedar 2 salt cedar 1 poplar 1 poplar hybrid poplars hybrid poplars barley
0.1 0.26 0.58 (TCE) 0.1 (TCE) 0.02-0.22 0.75 0.69 (TCE)e
a Cited from Davis et al. (26). b Values cited from Orchard et al. (6). 0.02 was observed at concentration of 70 mg/L. TSCFs for 10 mg/L tended to be higher than those measured in 1 mg/L but were not different statistically. c Eleven different chemicals from hydrophilic to hydrophobic (log Kow ) 0.87-5.04) were used in the experiment (3). d The mixtures included 15 different compounds with log Kow from -0.57 to 4.6 (1). e TSCF value calculated from correlation curve developed by Briggs et al. (1).
works (5, 25, 26). These research efforts have suggested that TCE and other chlorinated VOCs can diffuse from xylem tissues. Reported TSCF values were compared with the TSCF estimate here (Table 4). The calculated results from previous TSCF relationships (1, 3) are higher than the conservative estimation presented here, while the TSCF calculated by Orchard et al. (6) and Davis et al. (27) are lower. The difference between reported TSCF was discussed briefly (6) and might arise from many factors, including the concentration differences used, the experimental arrangement introducing the contaminants, and the methods used for contaminant detection. Other factors may include the use of different plant species (27). There is still much to be learned regarding plant interactions with VOCs in the soil profile and plant-contaminant interactions once chemicals have entered plant tissues. However, this work does provide the first direct evidence that parent compound VOCs are taken up by plants and that contaminant concentrations in tissues are directly related to the contaminant concentrations in the growth media. Using this information and the relationships outlined herein, plant tissue analysis and specifically tree core analysis can become a useful tool in site characterization and monitoring. Indepth understanding and accurate measurements of TSCF for various vegetation and compounds will greatly broaden the application for the characterization process, potentially leading to quantitative measurements. However, the application of the parameter Cbulk solution to an in-situ setting is somewhat ambiguous at this time as it has been developed in the laboratory for a hydroponic arrangement, void of any vapor phase or soil interactions. In terms of monitoring, this method can provide a simple and cost-effective method to examine the effectiveness of phytoremediation processes, where long-term monitoring costs have been a concern. The potential use of this method in conjunction with monitored natural attenuation could also be of great use and benefit to remediation efforts and in reducing long-term monitoring costs.
Acknowledgments This research was supported by the National Science Foundation BES 9984064. The authors thank Harry Compton, Steve Hirsh, Lee Newman, Dave Rieske, and Louis Licht for their assistance and contributions to this work.
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Received for review May 16, 2002. Revised manuscript received July 13, 2002. Accepted August 8, 2002. ES025795J
