"Phytoscreening": the use of trees for discovering subsurface contamination by VOCs

Influence of delta-hexachlorocyclohexane (δ-HCH) to Phytophthora ×alni resistant Alnus glutinosa genotypes - Evaluation of physiological parameters and remediation potential

Hexachlorocyclohexanes (HCHs) are persistent organochlorine pesticides with the adverse effects on human health and the environment. The effect of delta-isomer of hexachlorocyclohexane (δ-HCH) on germination, growth parameters and physiological parameters was studied in different Alnus glutinosa (L.) Gaertn. progeny of resistant genotypes to pathogen Phytophthora ×alni. Two experiments were performed: a short-term experiment to determine the effect of δ-HCH on total germination (GT), germination energy (GE), speed of germination (SG), shoot length and biomass of seedlings, and a long-term experiment devoted to remediation aspects. In addition, changes in the hormonal system of alders were monitored in both cases. Significant differences were found between the treated and control group in most of the evaluated characteristics. Also, the content of studied phytohormones differs between groups. Furthermore, the obtained results indicate genetically determined variability in response to δ-HCH. Of the six tested, the Březové and Tuřany progeny seem to be suitable candidates for phytoremediation because of the adaptation to stress conditions or high remediation efficiency. The rest of tested progeny seems to be unsuitable due to higher mortality, lower remediation efficiency and higher levels of stress hormones resulting in significant decrease in biomass and plant height. Moreover, results indicate the role of the plant as a remediation accelerator, probably through released exudates, and a positive effect on the soil microbiome as the presence of plants increased the remediation efficiency by 20.85 - 35.89%. The obtained research findings may be helpful in better understanding the processes involved in removing these pesticides from the soil. Further research should be focused on rhizosphere microbiome, mechanism of in-plant isomerization and metabolites identification.

Detecting vinyl chloride by phytoscreening in the shallow critical zone at sites with potential human exposure

Chlorinated ethene (CE) contaminants are widespread in groundwater, and the occurrence of vinyl chloride (VC), among others, is a well-known issue due to its mobility, persistence, and carcinogenicity. Human exposure to VC may occur through inhalation after soil vapor intrusion into buildings at sites with shallow underground contamination. Soil vapor intrusion risk is traditionally assessed through indoor air and sub-slab sampling (direct evidence) or soil gas and groundwater surveys (indirect evidence). Phytoscreening (sampling and analysis of tree trunk matrices) was proven as a cost-effective alternative technique to indirectly detect shallow underground contamination by higher chlorinated ethenes and subsequent vapor intrusion risk. However, the technique has appeared barely capable to screen for the lower chlorinated VC, likely due to its fugacity and aerobic bio-degradability, with only one literature record to date showing successful detection in trees. We applied phytoscreening at two sites with severe CE contamination nearby residential buildings caused by illegal dumping of chlorinated pitches from petrochemical productions. The two sites show variable amounts of VC in the shallow groundwater (1e2 to 1e4 μg/L), posing potential sanitary risk issues. Former soil gas surveys did not detect VC in the vadose zone. At both sites, we sampled trunk micro-cores and trunk gas from poplar trees close to contaminated piezometers in different seasons. VC was detected in several instances, disproving the shared literature assumption of the inefficacy of phytoscreening towards this compound. Factors influencing the detectability of VC and other CEs in trees were analyzed through linear regressions. Two different conceptual models were proposed to explain the effective uptake of VC by trees at the two sites, i.e., direct uptake of contaminated groundwater at the first site and uptake of VC from an anoxic vadose zone at the second site. In planta reductive dechlorination of CEs is not expected based on current literature knowledge. Thus, the detection of VC in trunks would indicate its occurrence in the shallow underground, suggesting higher screening effectiveness of phytoscreening compared to soil gas; this has implications for indirect vapor intrusion risk assessment.

A quantitative review and meta-analysis on phytoscreening applied to aquifers contaminated by chlorinated ethenes

Applications and acceptance of phytoscreening, i.e., the use of trees as screening tools for underground contamination, are still limited in many countries due to the lack of awareness of application policies, the intrinsic qualitative nature of the technique, and the paucity of critical analyses on available data. To date, the conditions influencing the effectiveness of the technique have been descriptively discussed, yet rarely quantified. This review will contribute to filling this knowledge gap, shedding light on the most suitable approaches to apply phytoscreening. The focus was placed specifically on chlorinated ethene compounds since they are among the main organic contaminants in groundwater and have been the most studied in the field of phytoscreening. Chlorinated ethenes' behavior and biodegradation potential largely depend on their physicochemical properties as well as the hydrogeological features of the system in which they migrate. Besides, their fate and transport in surface ecosystems are still poorly understood. Here, phytoscreening data from sites contaminated by chlorinated ethenes were extracted from relevant literature to form a global-scale database. Data were statistically analyzed to identify the major drivers of variability in tree-cores concentration. Correlation between tree-core and groundwater concentration was quantified through Spearman's rank coefficients, whilst detectability potential was determined based on tree-cores showing non-detection of contaminants. The influence on such parameters of factors like contaminant properties, hydrogeology, tree features, and sampling/analytical protocols was assessed. Results suggest that factors controlling plant uptake and contaminant phytovolatilization regulate correlation and detectability, respectively. Conditions increasing the correlation (e.g., sites with shallow and permeable aquifers) are recommended for phytoscreening applications aimed at mapping and monitoring contaminant plumes, whereas conditions increasing detectability (e.g., sampling tree-cores near ground level) are recommended to preliminary screen underground contamination in poorly investigated areas.

Phytoremediation of Toxic Metals: A Sustainable Green Solution for Clean Environment

Contamination of aquatic ecosystems by various sources has become a major worry all over the world. Pollutants can enter the human body through the food chain from aquatic and soil habitats. These pollutants can cause various chronic diseases in humans and mortality if they collect in the body over an extended period. Although the phytoremediation technique cannot completely remove harmful materials, it is an environmentally benign, cost-effective, and natural process that has no negative effects on the environment. The main types of phytoremediation, their mechanisms, and strategies to raise the remediation rate and the use of genetically altered plants, phytoremediation plant prospects, economics, and usable plants are reviewed in this review. Several factors influence the phytoremediation process, including types of contaminants, pollutant characteristics, and plant species selection, climate considerations, flooding and aging, the effect of salt, soil parameters, and redox potential. Phytoremediation’s environmental and economic efficiency, use, and relevance are depicted in our work. Multiple recent breakthroughs in phytoremediation technologies are also mentioned in this review.

Lability and bioavailability of Co, Fe, Pb, U and Zn in a uranium mining restoration site using DGT and phytoscreening

Mine restoration is a long and ongoing process, requiring careful management, which must be informed by site-specific, geochemical risk assessment. Paired topsoil and tree core samples from 4 sites within the uranium mining complex of INB Caldas in Minas Gerais (Brazil) were collected. Soil samples were analysed for their total content of Co, Fe, Pb, U and Zn by XRF, and subsequently, the potential environmental bioavailability of these metals were investigated by DGT and pore water analysis. In addition, results were compared with metal concentrations obtained by Tree Coring from the forest vegetation. In all sampling areas, mean total concentrations of U (C_tot. = 100.5 ± 66.5 to 129.6 ± 57.1 mg kg^-1), Pb (C_tot. = 30.8 ± 12.7 to 90.8 ± 90.8 mg kg^-1), Zn (C_tot. = 91.5 ± 24.7 to 99.6 ± 10.3 mg kg^-1) and Co (C_tot. = 73.8 ± 25.5 to 119.7 ± 26.4 mg kg^-1) in soils exceeded respective quality reference values. Study results suggest that AMD caused the increase of labile concentrations of Zn in affected soils. The high lability of the elements Pb (R = 62 ± 34 to 81 ± 29%), U (R = 57 ± 20 to 77 ± 28%) and Zn (R = 21 ± 25 to 34 ± 31%) in soils together with high bioconcentration factors found in wood samples for Pb (BCF = 0.0004 ± 0.0003 to 0.0026 ± 0.0033) and Zn (BCF = 0.012 ± 0.013 to 0.025 ± 0.021) indicated a high toxic potential of these elements to the biota in the soils of the study site. The combination of pore water and DGT analysis with Tree Coring showed to be a useful approach to specify the risk of metal polluted soils. However, the comparison of the results from DGT and Tree Coring could not predict the uptake of metals into the xylems of the sampled tree individuals.

A Multidisciplinary Approach to Evaluate the Effectiveness of Natural Attenuation at a Contaminated Site

This study evaluates the natural attenuation of chlorinated hydrocarbons as remediation action in a contaminated site downtown the city of Parma (Italy). To achieve this goal, a combination of new investigation methods (bio-molecular analysis, compound specific isotope analysis, phytoscreening) has been proposed. The approach (named circular multi step) allows to: fully understand the phenomena that occur at the study site, design new investigation activities, and manage best practices. Consequently, each step of the approach improves the conceptual and numerical models with new knowledge. The activities carried out at the study site allowed to detect a contamination of perchloroethylene in a large part of the city of Parma and, of main importance, underneath a kindergarten. The results of the study did not show significant natural attenuation of chlorinated hydrocarbons and that the detected contamination could refer to the same unknown contaminant source. Furthermore, the innovative phytoscreening technique was applied to assess the presence of chlorinated hydrocarbons at the ground level. The plume spread was estimated through numerical modeling starting from potential contaminant sources. This study enhances the knowledge of groundwater flow and contamination in Parma and allows authorities to design new investigation/reclamation activities through management actions.

Characterizing the biotransformation of hexachlorocyclohexanes in wheat using compound-specific stable isotope analysis and enantiomer fraction analysis

Hexachlorocyclohexane isomers (HCHs) are persistent organic pollutants being responsible for environmental contamination worldwide. In order to characterize transformation of HCHs in different plant compartments during uptake, a hydroponic experimental setup was designed using wheat as the test plant. The extent of transformation was determined by using compound-specific isotope analysis (CSIA) and enantiomer fraction (EF) analysis. In nutrient solutions, no change of carbon (δ¹³C) and chlorine isotope ratios (δ³⁷Cl) of α-HCH and β-HCH was detected throughout the experiment indicating no transformation there. In wheat leaves, stems and roots, however, transformation of α-HCH due to a C‒Cl bond cleavage was indicated by increasing δ¹³C and δ³⁷Cl compared to the nutrient solution. In addition, 1,3,4,5,6-pentachlorocyclohexene (PCCH) was identified as the major metabolite of α-HCH transformation. For β-HCH, in contrast, no transformation was detected. The evaluation of enantiomer fraction analysis revealed no change of the EF(-) in the nutrient solution or on root surface but a decrease in the wheat compartments, providing an evidence for the preferential biological transformation of (-)α-HCH in wheat. The current study provides the first experimental evidence for biotransformation of α-HCH in wheat using CSIA and EF and provides a concept to evaluate processes during phytoremediation.

An assessment of correlations between chlorinated VOC concentrations in tree tissue and groundwater for phytoscreening applications

The majority of prior phytoscreening applications have employed the method as a tool to qualitatively determine the presence of contamination in the subsurface. Although qualitative data is quite useful, this study explores the potential for using phytoscreening quantitatively. The existence of site-specific and non-site-specific (master) correlations between VOC concentrations in tree tissue and groundwater is investigated using data collected from several phytoscreening studies. The aggregated data comprise 100 measurements collected from 12 sites that span a wide range of site conditions. Significant site-specific correlations are observed between tetrachloroethene (PCE) and trichloroethene (TCE) concentrations measured for tree tissue and those measured in groundwater for three sites. A moderately significant correlation (r²=0.56) exists for the entire aggregate data set. Parsing the data by groundwater depth produced a highly significant correlation (r²=0.88) for sites with shallow (<4m) groundwater. Such a significant correlation for data collected by different investigators from multiple sites with a wide range of tree species and subsurface conditions indicates that groundwater concentration is the predominant factor mediating tree-tissue concentrations for these sites. This may be a result of trees likely directly tapping groundwater for these shallow groundwater conditions. This master correlation may provide reasonable order-of-magnitude estimates of VOC concentrations in groundwater for such sites, thereby allowing the use of phytoscreening in a more quantitative mode.

Phytoforensics: Trees as bioindicators of potential indoor exposure via vapor intrusion

Human exposure to volatile organic compounds (VOCs) via vapor intrusion (VI) is an emerging public health concern with notable detrimental impacts on public health. Phytoforensics, plant sampling to semi-quantitatively delineate subsurface contamination, provides a potential non-invasive screening approach to detect VI potential, and plant sampling is effective and also time- and cost-efficient. Existing VI assessment methods are time- and resource-intensive, invasive, and require access into residential and commercial buildings to drill holes through basement slabs to install sampling ports or require substantial equipment to install groundwater or soil vapor sampling outside the home. Tree-core samples collected in 2 days at the PCE Southeast Contamination Site in York, Nebraska were analyzed for tetrachloroethene (PCE) and results demonstrated positive correlations with groundwater, soil, soil-gas, sub-slab, and indoor-air samples collected over a 2-year period. Because tree-core samples were not collocated with other samples, interpolated surfaces of PCE concentrations were estimated so that comparisons could be made between pairs of data. Results indicate moderate to high correlation with average indoor-air and sub-slab PCE concentrations over long periods of time (months to years) to an interpolated tree-core PCE concentration surface, with Spearman's correlation coefficients (ρ) ranging from 0.31 to 0.53 that are comparable to the pairwise correlation between sub-slab and indoor-air PCE concentrations (ρ = 0.55, n = 89). Strong correlations between soil-gas, sub-slab, and indoor-air PCE concentrations and an interpolated tree-core PCE concentration surface indicate that trees are valid indicators of potential VI and human exposure to subsurface environment pollutants. The rapid and non-invasive nature of tree sampling are notable advantages: even with less than 60 trees in the vicinity of the source area, roughly 12 hours of tree-core sampling with minimal equipment at the PCE Southeast Contamination Site was sufficient to delineate vapor intrusion potential in the study area and offered comparable delineation to traditional sub-slab sampling performed at 140 properties over a period of approximately 2 years.

Application of phytoscreening to three hazardous waste sites in Arizona

The great majority of prior phytoscreening applications have been conducted in humid and temperate environments wherein groundwater is relatively shallow (~1-6m deep). The objective of this research is to evaluate its use in semi-arid environments for sites with deeper groundwater (>10m). To that end, phytoscreening is applied to three chlorinated-solvent hazardous-waste sites in Arizona. Contaminant concentrations were quantifiable in tree-tissue samples collected from two of the sites (Nogales, Park-Euclid). Contaminant concentrations were detectable, but not quantifiable, for the third site. Tree-tissue concentrations of tetrachloroethene (PCE) ranged from approximately 400-5000ug/kg wet weight for burrobrush, cottonwood, palo verde, and velvet mesquite at the Nogales site. In addition to standard trunk-core samples, leaf samples were collected to test the effectiveness of a less invasive sampling method. Leaf-sample concentrations were quantifiable, but several times lower than the corresponding core-sample concentrations. Comparison of results obtained for the test sites to those reported in the literature suggest that tree species is a major factor mediating observed results. One constraint faced for the Arizona sites was the relative scarcity of mature trees available for sampling, particularly in areas adjacent to industrial zones. The results of this study illustrate that phytoscreening can be used effectively to characterize the presence of groundwater contamination for semi-arid sites with deeper groundwater.

Contaminant Gradients in Trees: Directional Tree Coring Reveals Boundaries of Soil and Soil-Gas Contamination with Potential Applications in Vapor Intrusion Assessment

Contaminated sites pose ecological and human-health risks through exposure to contaminated soil and groundwater. Whereas we can readily locate, monitor, and track contaminants in groundwater, it is harder to perform these tasks in the vadose zone. In this study, tree-core samples were collected at a Superfund site to determine if the sample-collection location around a particular tree could reveal the subsurface location, or direction, of soil and soil-gas contaminant plumes. Contaminant-centroid vectors were calculated from tree-core data to reveal contaminant distributions in directional tree samples at a higher resolution, and vectors were correlated with soil-gas characterization collected using conventional methods. Results clearly demonstrated that directional tree coring around tree trunks can indicate gradients in soil and soil-gas contaminant plumes, and the strength of the correlations were directly proportionate to the magnitude of tree-core concentration gradients (spearman's coefficient of -0.61 and -0.55 in soil and tree-core gradients, respectively). Linear regression indicates agreement between the concentration-centroid vectors is significantly affected by in planta and soil concentration gradients and when concentration centroids in soil are closer to trees. Given the existing link between soil-gas and vapor intrusion, this study also indicates that directional tree coring might be applicable in vapor intrusion assessment.

Tree Sampling as a Method to Assess Vapor Intrusion Potential at a Site Characterized by VOC-Contaminated Groundwater and Soil

Vapor intrusion (VI) by volatile organic compounds (VOCs) in the built environment presents a threat to human health. Traditional VI assessments are often time-, cost-, and labor-intensive; whereas traditional subsurface methods sample a relatively small volume in the subsurface and are difficult to collect within and near structures. Trees could provide a similar subsurface sample where roots act as the "sampler' and are already onsite. Regression models were developed to assess the relation between PCE concentrations in over 500 tree-core samples with PCE concentrations in over 50 groundwater and 1000 soil samples collected from a tetrachloroethylene- (PCE-) contaminated Superfund site and analyzed using gas chromatography. Results indicate that in planta concentrations are significantly and positively related to PCE concentrations in groundwater samples collected at depths less than 20 m (adjusted R² values greater than 0.80) and in soil samples (adjusted R² values greater than 0.90). Results indicate that a 30 cm diameter tree characterizes soil concentrations at depths less than 6 m over an area of 700-1600 m², the volume of a typical basement. These findings indicate that tree sampling may be an appropriate method to detect contamination at shallow depths at sites with VI.

Phytoscreening as an efficient tool to delineate chlorinated solvent sources at a chlor-alkali facility

Chlorinated ethenes (CE) are among the most common volatile organic compounds (VOC) that contaminate groundwater, currently representing a major source of pollution worldwide. Phytoscreening has been developed and employed through different applications at numerous sites, where it was generally useful for detection of subsurface chlorinated solvents. We aimed at delineating subsurface CE contamination at a chlor-alkali facility using tree core data that we compared with soil data. For this investigation a total of 170 trees from experimental zones was sampled and analyzed for perchloroethene (PCE) and trichloroethene (TCE) concentrations, measured by solid phase microextraction gas chromatography coupled to mass spectrometry. Within the panel of tree genera sampled, Quercus and Ulmus appeared to be efficient biomonitors of subjacent TCE and PCE contamination, in addition to the well known and widely used Populus and Salix genera. Among the 28 trees located above the dense non-aqueous phase liquid (DNAPL) phase zone, 19 tree cores contained detectable amounts of CE, with concentrations ranging from 3 to 3000 μg L^-1. Our tree core dataset was found to be well related to soil gas sampling results, although the tree coring data were more informative. Our data further emphasized the need for choosing the relevant tree species and sampling periods, as well as taking into consideration the nature of the soil and its heterogeneity. Overall, this low-invasive screening method appeared useful to delineate contaminants at a small-scale site impacted by multiple sources of chlorinated solvents.

Technological change in revitalization - Phytoremediation and the role of nonknowledge

In the 1990s, remediation methods that use plants to investigate and extract contaminations were developed. According to their proponents, these technologies have considerable potential for greening remediation and to develop a more sustainable trajectory for revitalization. Although a great deal of research has been conducted on these technologies they have, so far, rarely been applied in European countries. This article will contribute a perspective from social science to this debate. By taking an approach inspired by social science practice theories, attention will be drawn to a specific socio-cultural aspect: dealing with nonknowledge. The analysis of remediation practices reveals how the phenomenon of nonknowledge becomes relevant in the process of inventing and applying new technologies. This allows for a better understanding of technological innovation processes in remediation. Based on this understanding strategies are suggested which might increase the use of plant-based technologies in remediation and site management.

Trichloroethylene (TCE) in tree cores to complement a subsurface investigation on residential property near a former electroplating facility

Tree cores were collected and analyzed for trichloroethylene (TCE) on a private property between a former electroplating facility in Asheville, North Carolina (USA), and a contaminated wetland/spring complex. TCE was detected in 16 of 31 trees, the locations of which were largely consistent with a "plume core" delineated by a more detailed subsurface investigation nearly 2 years later. Concentrations in tree cores and nearby soil borings were not correlated, perhaps due to heterogeneities in both geologic and tree root structure, spatial and temporal variability in transpiration rates, or interferences caused by other contaminants at the site. Several tree cores without TCE provided evidence for significantly lower TCE concentrations in shallow groundwater along the margins of the contaminated spring complex in an area with limited accessibility. This study demonstrates that tree core analyses can complement a more extensive subsurface investigation, particularly in residential or ecologically sensitive areas.

Analyzing tree cores to detect petroleum hydrocarbon-contaminated groundwater at a former landfill site in the community of Happy Valley-Goose Bay, eastern Canadian subarctic

This research examines the feasibility of analyzing tree cores to detect benzene, toluene, ethylbenzene, and m, p, o-xylene (BTEX) compounds and methyl tertiary-butyl ether (MTBE) in groundwater in eastern Canada subarctic environments, using a former landfill site in the remote community of Happy Valley-Goose Bay, Labrador. Petroleum hydrocarbon contamination at the landfill site is the result of environmentally unsound pre-1990s disposal of households and industrial solid wastes. Tree cores were taken from trembling aspen, black spruce, and white birch and analyzed by headspace-gas chromatography-mass spectrometry. BTEX compounds were detected in tree cores, corroborating known groundwater contamination. A zone of anomalously high concentrations of total BTEX constituents was identified and recommended for monitoring by groundwater wells. Tree cores collected outside the landfill site at a local control area suggest the migration of contaminants off-site. Tree species exhibit different concentrations of BTEX constituents, indicating selective uptake and accumulation. Toluene in wood exhibited the highest concentrations, which may also be due to endogenous production. Meanwhile, MTBE was not found in the tree cores and is considered to be absent in the groundwater. The results demonstrate that tree-core analysis can be useful for detecting anomalous concentrations of petroleum hydrocarbons, such as BTEX compounds, in subarctic sites with shallow unconfined aquifers and permeable soils. This method can therefore aid in the proper management of contamination during landfill operations and after site closures.

Using pre-screening methods for an effective and reliable site characterization at megasites

This paper illustrates the usefulness of pre-screening methods for an effective characterization of polluted sites. We applied a sequence of site characterization methods to a former Soviet military airbase with likely fuel and benzene, toluene, ethylbenzene, and xylene (BTEX) contamination in shallow groundwater and subsoil. The methods were (i) phytoscreening with tree cores; (ii) soil gas measurements for CH4, O2, and photoionization detector (PID); (iii) direct-push with membrane interface probe (MIP) and laser-induced fluorescence (LIF) sensors; (iv) direct-push sampling; and (v) sampling from soil and from groundwater monitoring wells. Phytoscreening and soil gas measurements are rapid and inexpensive pre-screening methods. Both indicated subsurface pollution and hot spots successfully. The direct-push sensors yielded 3D information about the extension and the volume of the subsurface plume. This study also expanded the applicability of tree coring to BTEX compounds and tested the use of high-resolution direct-push sensors for light hydrocarbons. Comparison of screening results to results from conventional soil and groundwater sampling yielded in most cases high rank correlation and confirmed the findings. The large-scale application of non- or low-invasive pre-screening can be of help in directing and focusing the subsequent, more expensive investigation methods. The rapid pre-screening methods also yielded useful information about potential remediation methods. Overall, we see several benefits of a stepwise screening and site characterization scheme, which we propose in conclusion.

Plant uptake, translocation, and return of polycyclic aromatic hydrocarbons via fine root branch orders in a subtropical forest ecosystem

Fine roots of woody plants are a heterogeneous system differing markedly in structure and function. Nevertheless, knowledge about the plant uptake of organic pollutants via fine roots is scarce to date. In the present study, plant uptake, translocation, and return of polycyclic aromatic hydrocarbons (PAHs) via fine roots in a subtropical forest ecosystem were investigated. Levels of Σ15PAHs in different fine root branch orders of Michelia macclurei, Cryptocarya concinna, Cryptocarya chinensis, and Canthium dicoccums varied from 5072±1419 ng g(-1) to 6080±1656 ng g(-1), 4037±410 ng g(-1) to 6101±972 ng g(-1), 3308±1191 ng g(-1) to 4283±237 ng g(-1), and 3737±800 ng g(-1) to 4895±1216 ng g(-1), respectively. Overall, concentrations of low-molecular-weight PAHs with 2-3 aromatic rings were higher than high-molecular-weight PAHs with 4-6 aromatic rings in all fine root branch orders. There were obvious translocations of PAHs between adjacent branch orders and a net accumulation of PAHs may occur in the fourth- and fifth-order roots. The storage of PAHs in the fine root system showed an obvious increasing trend along the branch orders ascending for all tree species. The return flux of PAHs via fine roots mortality showed an obvious decreasing trend with the branch orders ascending across the four tree species. Lower order roots contributed greatly to the total PAHs return flux. Our results indicated that fine roots turnover is an effective pathway for perennial tree species to remove environmental toxicants absorbed into them.

Phytoscreening with SPME: Variability Analysis

Phytoscreening has been demonstrated at a variety of sites over the past 15 years as a low-impact, sustainable tool in delineation of shallow groundwater contaminated with chlorinated solvents. Collection of tree cores is rapid and straightforward, but low concentrations in tree tissues requires sensitive analytics. Solid-phase microextraction (SPME) is amenable to the complex matrix while allowing for solvent-less extraction. Accurate quantification requires the absence of competitive sorption, examined here both in laboratory experiments and through comprehensive examination of field data. Analysis of approximately 2,000 trees at numerous field sites also allowed testing of the tree genus and diameter effects on measured tree contaminant concentrations. Collectively, while these variables were found to significantly affect site-adjusted perchloroethylene (PCE) concentrations, the explanatory power of these effects was small (adjusted R(2) = 0.031). 90th quantile chemical concentrations in trees were significantly reduced by increasing Henry's constant and increasing hydrophobicity. Analysis of replicate tree core data showed no correlation between replicate relative standard deviation (RSD) and wood type or tree diameter, with an overall median RSD of 30%. Collectively, these findings indicate SPME is an appropriate technique for sampling and analyzing chlorinated solvents in wood and that phytoscreening is robust against changes in tree type and diameter.

Phytomonitoring of chlorinated ethenes in trees: a four-year study of seasonal chemodynamics in planta

Long-term monitoring (LTM) of groundwater remedial projects is costly and time-consuming, particularly when using phytoremediation, a long-term remedial approach. The use of trees as sensors of groundwater contamination (i.e., phytoscreening) has been widely described, although the use of trees to provide long-term monitoring of such plumes (phytomonitoring) has been more limited due to unexplained variability of contaminant concentrations in trees. To assess this variability, we developed an in planta sampling method to obtain high-frequency measurements of chlorinated ethenes in oak (Quercus rubra) and baldcypress (Taxodium distichum) trees growing above a contaminated plume during a 4-year trial. The data set revealed that contaminant concentrations increased rapidly with transpiration in the spring and decreased in the fall, resulting in perchloroethene (PCE) and trichloroethene (TCE) sapwood concentrations an order of magnitude higher in late summer as compared to winter. Heartwood PCE and TCE concentrations were more buffered against seasonal effects. Rainfall events caused negligible dilution of contaminant concentrations in trees after precipitation events. Modeling evapotranspiration potential from meteorological data and comparing the modeled uptake and transport with the 4 years of high frequency data provides a foundation to advance the implementation of phytomonitoring and improved understanding of plant contaminant interactions.

In planta passive sampling devices for assessing subsurface chlorinated solvents

Contaminant concentrations in trees have been used to delineate groundwater contaminant plumes (i.e., phytoscreening); however, variability in tree composition hinders accurate measurement of contaminant concentrations in planta, particularly for long-term monitoring. This study investigated in planta passive sampling devices (PSDs), termed solid phase samplers (SPSs) to be used as a surrogate tree core. Characteristics studied for five materials included material-air partitioning coefficients (Kma) for chlorinated solvents, sampler equilibration time and field suitability. The materials investigated were polydimethylsiloxane (PDMS), low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), polyoxymethylene (POM) and plasticized polyvinyl chloride (PVC). Both PDMS and LLDPE samplers demonstrated high partitioning coefficients and diffusivities and were further tested in greenhouse experiments and field trials. While most of the materials could be used for passive sampling, the PDMS SPSs performed best as an in planta sampler. Such a sampler was able to accurately measure trichloroethylene (TCE) and tetrachloroethylene (PCE) concentrations while simultaneously incorporating simple operation and minimal impact to the surrounding property and environment.

Phytoscreening and phytoextraction of heavy metals at Danish polluted sites using willow and poplar trees

The main purpose of this study was to determine typical concentrations of heavy metals (HM) in wood from willows and poplars, in order to test the feasibility of phytoscreening and phytoextraction of HM. Samples were taken from one strongly, one moderately, and one slightly polluted site and from three reference sites. Wood from both tree species had similar background concentrations at 0.5 mg kg(-1) for cadmium (Cd), 1.6 mg kg(-1) for copper (Cu), 0.3 mg kg(-1) for nickel (Ni), and 25 mg kg(-1) for zinc (Zn). Concentrations of chromium (Cr) and lead (Pb) were below or close to detection limit. Concentrations in wood from the highly polluted site were significantly elevated, compared to references, in particular for willow. The conclusion from these results is that tree coring could be used successfully to identify strongly heavy metal-polluted soil for Cd, Cu, Ni, Zn, and that willow trees were superior to poplars, except when screening for Ni. Phytoextraction of HMs was quantified from measured concentration in wood at the most polluted site. Extraction efficiencies were best for willows and Cd, but below 0.5% over 10 years, and below 1‰ in 10 years for all other HMs.

A solid-phase microextraction method for the in vivo sampling of MTBE in common reed (Phragmites australis)

Phytoscreening of phytoremediation-based plantings is discussed as a promising monitoring tool in literature. We developed and applied an analytical procedure for the in vivo sampling of methyl tert-butyl ether (MTBE) in the common reed (Phragmites australis) from a phytoremediation site highly polluted with MTBE. The approach uses solid-phase microextraction (SPME) with the SPME fibre directly introduced into the aerenchyma of the plant stem. For optimising the analytical procedure and estimating the capability of the proposed method, laboratory tests on the microcosm scale and field studies over one vegetation period were carried out. Furthermore, the results of in vivo SPME sampling were compared with those obtained with the traditional approach for analysing plants using dynamic headspace analysis. The MTBE signals detected within the plants were also correlated with the concentration in the water phase. The discussion of results showed the feasibility of the proposed method for a qualitative phytoscreening of volatile organic compounds present in wetland plants.

Directional phytoscreening: contaminant gradients in trees for plume delineation

Tree sampling methods have been used in phytoscreening applications to delineate contaminated soil and groundwater, augmenting traditional investigative methods that are time-consuming, resource-intensive, invasive, and costly. In the past decade, contaminant concentrations in tree tissues have been shown to reflect the extent and intensity of subsurface contamination. This paper investigates a new phytoscreening tool: directional tree coring, a concept originating from field data that indicated azimuthal concentrations in tree trunks reflected the concentration gradients in the groundwater around the tree. To experimentally test this hypothesis, large diameter trees were subjected to subsurface contaminant concentration gradients in a greenhouse study. These trees were then analyzed for azimuthal concentration gradients in aboveground tree tissues, revealing contaminant centroids located on the side of the tree nearest the most contaminated groundwater. Tree coring at three field sites revealed sufficiently steep contaminant gradients in trees reflected nearby groundwater contaminant gradients. In practice, trees possessing steep contaminant gradients are indicators of steep subsurface contaminant gradients, providing compass-like information about the contaminant gradient, pointing investigators toward higher concentration regions of the plume.

One-year measurements of chloroethenes in tree cores and groundwater at the SAP Mimoň Site, Northern Bohemia

Chlorinated ethenes (CE) are among the most frequent contaminants of soil and groundwater in the Czech Republic. Because conventional methods of subsurface contamination investigation are costly and technically complicated, attention is directed on alternative and innovative field sampling methods. One promising method is sampling of tree cores (plugs of woody tissue extracted from a host tree). Volatile organic compounds can enter into the trunks and other tissues of trees through their root systems. An analysis of the tree core can thus serve as an indicator of the subsurface contamination. Four areas of interest were chosen at the experimental site with CE groundwater contamination and observed fluctuations in groundwater concentrations. CE concentrations in groundwater and tree cores were observed for a 1-year period. The aim was to determine how the CE concentrations in obtained tree core samples correlate with the level of contamination of groundwater. Other factors which can affect the transfer of contaminants from groundwater to wood were also monitored and evaluated (e.g., tree species and age, level of groundwater table, river flow in the nearby Ploučnice River, seasonal effects, and the effect of the remediation technology operation). Factors that may affect the concentration of CE in wood were identified. The groundwater table level, tree species, and the intensity of transpiration appeared to be the main factors within the framework of the experiment. Obtained values documented that the results of tree core analyses can be used to indicate the presence of CE in the subsurface. The results may also be helpful to identify the best sampling period for tree coring and to learn about the time it takes until tree core concentrations react to changes in groundwater conditions. Interval sampling of tree cores revealed possible preservation of the contaminant in the wood of trees.

Plants as bio-indicators of subsurface conditions: impact of groundwater level on BTEX concentrations in trees

Numerous studies have demonstrated trees' ability to extract and translocate moderately hydrophobic contaminants, and sampling trees for compounds such as BTEX can help delineate plumes in the field. However, when BTEX is detected in the groundwater, detection in nearby trees is not as reliable an indicator of subsurface contamination as other compounds such as chlorinated solvents. Aerobic rhizospheric and bulk soil degradation is a potential explanation for the observed variability of BTEX in trees as compared to groundwater concentrations. The goal of this study was to determine the effect of groundwater level on BTEX concentrations in tree tissue. The central hypothesis was increased vadose zone thickness promotes biodegradation of BTEX leading to lower BTEX concentrations in overlying trees. Storage methods for tree core samples were also investigated as a possible reason for tree cores revealing lower than expected BTEX levels in some sampling efforts. The water level hypothesis was supported in a greenhouse study, where water table level was found to significantly affect tree BTEX concentrations, indicating that the influx of oxygen coupled with the presence of the tree facilitates aerobic biodegradation of BTEX in the vadose zone.

Plants as bio-indicators of subsurface conditions: impact of groundwater level on BTEX concentrations in trees

Numerous studies have demonstrated trees' ability to extract and translocate moderately hydrophobic contaminants, and sampling trees for compounds such as BTEX can help delineate plumes in the field. However, when BTEX is detected in the groundwater, detection in nearby trees is not as reliable an indicator of subsurface contamination as other compounds such as chlorinated solvents. Aerobic rhizospheric and bulk soil degradation is a potential explanation for the observed variability of BTEX in trees as compared to groundwater concentrations. The goal of this study was to determine the effect of groundwater level on BTEX concentrations in tree tissue. The central hypothesis was increased vadose zone thickness promotes biodegradation of BTEX leading to lower BTEX concentrations in overlying trees. Storage methods for tree core samples were also investigated as a possible reason for tree cores revealing lower than expected BTEX levels in some sampling efforts. The water level hypothesis was supported in a greenhouse study, where water table level was found to significantly affect tree BTEX concentrations, indicating that the influx of oxygen coupled with the presence of the tree facilitates aerobic biodegradation of BTEX in the vadose zone.

Test of tree core sampling for screening of toxic elements in soils from a Norwegian site

Tree core samples have been used to delineate organic subsurface plumes. In 2009 and 2010, samples were taken at trees growing on a former dump site in Norway and analyzed for arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni), and zinc (Zn). Concentrations in wood were in averages (dw) 30 mg/kg for Zn, 2 mg/kg for Cu, and < 1 mg/kg for Cd, Cr, As and Ni. The concentrations in wood samples from the polluted test site were compared to those derived from a reference site. For all except one case, mean concentrations from the test site were higher than those from the reference site, but the difference was small and not always significant. Differences between tree species were usually higher than differences between reference and test site. Furthermore, all these elements occur naturally, and Cu, Ni, and Zn are essential minerals. Thus, all trees will have a natural background of these elements, and the occurrence alone does not indicate soil pollution. For the interpretation of the results, a comparison to wood samples from an unpolluted reference site with same species and similar soil conditions is required. This makes the tree core screening method less reliable for heavy metals than, e.g., for chlorinated solvents.

Time-weighted average SPME analysis for in planta determination of cVOCs

The potential of phytoscreening for plume delineation at contaminated sites has promoted interest in innovative, sensitive contaminant sampling techniques. Solid-phase microextraction (SPME) methods have been developed, offering quick, undemanding, noninvasive sampling without the use of solvents. In this study, time-weighted average SPME (TWA-SPME) sampling was evaluated for in planta quantification of chlorinated solvents. TWA-SPME was found to have increased sensitivity over headspace and equilibrium SPME sampling. Using a variety of chlorinated solvents and a polydimethylsiloxane/carboxen (PDMS/CAR) SPME fiber, most compounds exhibited near linear or linear uptake over the sampling period. Smaller, less hydrophobic compounds exhibited more nonlinearity than larger, more hydrophobic molecules. Using a specifically designed in planta sampler, field sampling was conducted at a site contaminated with chlorinated solvents. Sampling with TWA-SPME produced instrument responses ranging from 5 to over 200 times higher than headspace tree core sampling. This work demonstrates that TWA-SPME can be used for in planta detection of a broad range of chlorinated solvents and methods can likely be applied to other volatile and semivolatile organic compounds.

Pollutant plume delineation from tree core sampling using standardized ranks

There are currently contradicting results in the literature about the way chloroethene (CE) concentrations from tree core sampling correlate with those from groundwater measurements. This paper addresses this issue by focusing on groundwater and tree core datasets in CE contaminated site, Czech Republic. Preliminary analyses revealed strongly and positively skewed distributions for the tree core dataset, with an intra-tree variability accounting for more than 80% of the total variability, while the spatial analyses based on variograms indicated no obvious spatial pattern for CE concentration. Using rank transformation, it is shown how the results were improved by revealing the initially hidden spatial structure for both variables when they are handled separately. However, bivariate analyses based on cross-covariance functions still failed to indicate a clear spatial correlation between groundwater and tree core measurements. Nonetheless, tree core sampling and analysis proved to be a quick and inexpensive semi-quantitative method and a useful tool.

Effect of radial directional dependences and rainwater influence on CVOC concentrations in tree core and birch sap samples taken for phytoscreening using HS-SPME-GC/MS

Phytoscreening for chlorinated volatile organic compounds (CVOC) in tree core samples is influenced by many factors. For instance, greater fluctuations are observed for CVOC concentrations in samples taken around the trunk at a fixed height compared to samples taken directly next to each other. To avoid false negatives and inaccurate interpretation of the results, we investigated this radial directional dependence as well as the influence of rainwater on measured concentrations. CVOC analysis was performed by gas chromatography/mass spectrometry (GC/MS) following Solid-Phase-Microextraction (SPME). Phytoscreening was successfully carried out at three sites using this method. In addition, sap samples were taken from white birches during their budding period as a novel phytoscreening approach. Birch sap sampling is shown to be a suitable means of characterizing contaminant distribution within the soil subsurface. Radial directional dependence of CVOC concentrations varies by almost 80% for tree core samples and 50% for birch sap samples. Variations in concentrations measured around the trunk do not, however, provide information on the inflow direction of contaminated groundwater. The weather conditions were shown to have a greater influence so that CVOC concentrations measured from samples taken during colder, rainier weather were, on average, a factor of 100 lower than those taken during a warm and dry period. Nevertheless phytoscreening is adequate for CVOC characterization in the soil subsurface if the campaign is carried out during a dry weather period, the results then can be taken as being semiquantitative.

Phytoscreening for chlorinated solvents using rapid in vitro SPME sampling: application to urban plume in Verl, Germany

Rapid detection and delineation of contaminants in urban settings is critically important in protecting human health. Cores from trees growing above a plume of contaminated groundwater in Verl, Germany, were collected in 1 day, with subsequent analysis and plume mapping completed over several days. Solid-phase microextraction (SPME) analysis was applied to detect tetrachloroethene (PCE) and trichloroethene (TCE) to below nanogram/liter levels in the transpiration stream of the trees. The tree core concentrations showed a clear areal correlation to the distribution of PCE and TCE in the groundwater. Concentrations in tree cores were lower than the underlying groundwater, as anticipated; however, the tree core water retained the PCE:TCE signature of the underlying groundwater in the urban, populated area. The PCE:TCE ratio can indicate areas of differing degradation activity. Therefore, the phytoscreening analysis was capable not only of mapping the spatial distribution of groundwater contamination but also of delineating zones of potentially differing contaminant sources and degradation. The simplicity of tree coring and the ability to collect a large number of samples in a day with minimal disruption or property damage in the urban setting demonstrates that phytoscreening can be a powerful tool for gaining reconnaissance-level information on groundwater contaminated by chlorinated solvents. The use of SPME decreases the detection level considerably and increases the sensitivity of phytoscreening as an assessment, monitoring, and phytoforensic tool. With rapid, inexpensive, and noninvasive methods of detecting and delineating contaminants underlying homes, as in this case, human health can be better protected through screening of broader areas and with far faster response times.

Phytoforensics, dendrochemistry, and phytoscreening: new green tools for delineating contaminants from past and present

As plants evolved to be extremely proficient in mass transfer with their surroundings and survive as earth's dominant biomass, they also accumulate and store some contaminants from surroundings, acting as passive samplers. Novel applications and analytical methods have been utilized to gain information about a wide range of contaminants in the biosphere soil, water, and air, with information available on both past (dendrochemistry) and present (phytoscreening). Collectively these sampling approaches provide rapid, cheap, ecologically friendly, and overall "green" tools termed "Phytoforensics".

A mass balance study of the phytoremediation of perchloroethylene-contaminated groundwater

A mass balance study was performed under controlled field conditions to investigate the phytoremediation of perchloroethylene (PCE) by hybrid poplar trees. Water containing 7-14 mg L(-1) PCE was added to the test bed. Perchloroethylene, trichloroethylene, and cis-dichloroethylene were detected in the effluent at an average of 0.12 mg L(-1), 3.9 mg L(-1), and 1.9 mg L(-1), respectively. The total mass of chlorinated ethenes in the water was reduced by 99%. Over 95% of the recovered chlorine was as free chloride in the soil, indicating near-complete dehalogenation of the PCE. Transpiration, volatilization, and accumulation in the trees were all found to be minor loss mechanisms. In contrast, 98% of PCE applied to an unplanted soil chamber was recovered as PCE in the effluent water or volatilized into the air. These results suggest that phytoremediation can be an effective method for treating PCE-contaminated groundwater in field applications.