A novel and simple model of the uptake of organic chemicals by vegetation from air and soil

Using the method of dynamic factors for assessing the transfer of chemical elements from soil to plants from various perspectives

Environmental, biological, and ecosystem-specific properties may influence the transfer of chemical elements (CEs) from soils to plants, including the variation in the chemical elements' concentration, their types, and physiological parameters, such as biotransformation ability in the plants. The interface between the soil and a plant, or the concentration of a particular chemical element in a plant with respect to its concentration in the soil, is the basis for a widely used biological absorption coefficient, also known as the transfer factor, bioaccumulation factor, mobility ratio, or plant-soil coefficient, which is expressed in terms of the chemical element's concentration in the plant and soil. However, from the biogeochemical perspective, these coefficients/factors can provide a comparison of the chemical element (CE) concentration in different media (plants and soil), but only in a particular place (under typical environmental conditions) and at a particular time. However, factors that highlight the variation in the processes, rather than the variation in the chemical element quantity under the conditions of the environmental variation, are required. The second-level or dynamic factors can be used for this purpose. A quantitative method, using the dynamic factors of bioaccumulation, biophilicity, translocation, bioavailability, and phytoremediation, is offered to assess the variation in the process of the uptake of chemical elements by different plants, to evaluate the influence of soil modification on their participation in the plants' metabolism and to perform quantitative evaluation of phytoremediation efficiency over a particular period of time. The use of dynamic factors for describing the chemical elements' uptake by plants in various cases, representing aerogenic and edaphic chemical elements' transfer, is discussed.

Contribution of soil erosion to PAHs in surface water in China

In China, the total annual atmospheric emission of 16 polycyclic aromatic hydrocarbons (PAHs) reach up to approximate 100 thousand tons, part of which is preserved in soils. In this study, the contribution of soil erosion to PAHs in surface water nationwide was quantified. The results indicate that a major portion of the annual PAHs emission is lost from soils via rainfall erosivity and subsequently transported to the ocean. The national annual flux of PAHs from soil to surface water by the natural physical forces of water measures up to ~70 thousand tons, which accounts for ~62% of the annual emission of PAHs with 19% entering the sea directly. In general, both the soil erosion intensity and flux of PAHs for the regions located in the Southeast of China are over those in the Northwest of China, with the regions being divided into two different parts by the famous geographic "Hu Huanyong line", reflecting the intensive impact of human activities on environmental degradation. Comparative analysis suggested that there must be a big fraction of PAHs lost during transmission due to the river sedimentation and lake dispersion. This study closes a major gap in the national budget of PAHs and provides critical information in the context of regional environment risk assessment.

Modeling of Trace Metal Migration and Accumulation Processes in a Soil-Wheat System in Lihe Watershed, China

Samples of wheat and soil were collected in the Lihe watershed in East China, the migration and accumulation processes of four common trace metals (Cu, Pb, Cd and Ni) in each part of the wheat plant (root, stem, leaf and grain) were analyzed, and a mechanistic model was proposed to simulate these processes based on wheat growth techniques. Model results show that Cu and Cd migrate more easily with wheat grains, while most Pb and Ni accumulate in roots. Modeling results were shown to be relatively good, with an error of 25.29% in value and 26.38% in fluctuation, and had smaller dispersion degree than actual measurement results. Monte Carlo simulation results also match quite well with actual measurement results, and modeling results are slightly smaller in the simulation of Leaf-Cu, Grain-Cu and Leaf-Ni. Trace metal pollution risk in wheat is evaluated based on this model; our results show that the northwest and northeast parts in the research area are not suitable for growing wheat. In general, this model is relatively accurate, and can evaluate the wheat pollution risk before seeding wheat, providing scientific references for the early selection of wheat safety sowing areas.

Root Uptake of Pharmaceuticals and Personal Care Product Ingredients

Crops irrigated with reclaimed wastewater or grown in biosolids-amended soils may take up pharmaceuticals and personal care product ingredients (PPCPs) through their roots. The uptake pathways followed by PPCPs and the propensity for these compounds to bioaccumulate in food crops are still not well understood. In this critical review, we discuss processes expected to influence root uptake of PPCPs, evaluate current literature on uptake of PPCPs, assess models for predicting plant uptake of these compounds, and provide recommendations for future research, highlighting processes warranting study that hold promise for improving mechanistic understanding of plant uptake of PPCPs. We find that many processes that are expected to influence PPCP uptake and accumulation have received little study, particularly rhizosphere interactions, in planta transformations, and physicochemical properties beyond lipophilicity (as measured by Kow). Data gaps and discrepancies in methodology and reporting have so far hindered development of models that accurately predict plant uptake of PPCPs. Topics warranting investigation in future research include the influence of rhizosphere processes on uptake, determining mechanisms of uptake and accumulation, in planta transformations, the effects of PPCPs on plants, and the development of predictive models.

Review of existing terrestrial bioaccumulation models and terrestrial bioaccumulation modeling needs for organic chemicals

Protocols for terrestrial bioaccumulation assessments are far less-developed than for aquatic systems. This article reviews modeling approaches that can be used to assess the terrestrial bioaccumulation potential of commercial organic chemicals. Models exist for plant, invertebrate, mammal, and avian species and for entire terrestrial food webs, including some that consider spatial factors. Limitations and gaps in terrestrial bioaccumulation modeling include the lack of QSARs for biotransformation and dietary assimilation efficiencies for terrestrial species; the lack of models and QSARs for important terrestrial species such as insects, amphibians and reptiles; the lack of standardized testing protocols for plants with limited development of plant models; and the limited chemical domain of existing bioaccumulation models and QSARs (e.g., primarily applicable to nonionic organic chemicals). There is an urgent need for high-quality field data sets for validating models and assessing their performance. There is a need to improve coordination among laboratory, field, and modeling efforts on bioaccumulative substances in order to improve the state of the science for challenging substances.

Influence of plant root morphology and tissue composition on phenanthrene uptake: stepwise multiple linear regression analysis

Polycyclic aromatic hydrocarbons (PAHs) are contaminants that reside mainly in surface soils. Dietary intake of plant-based foods can make a major contribution to total PAH exposure. Little information is available on the relationship between root morphology and plant uptake of PAHs. An understanding of plant root morphologic and compositional factors that affect root uptake of contaminants is important and can inform both agricultural (chemical contamination of crops) and engineering (phytoremediation) applications. Five crop plant species are grown hydroponically in solutions containing the PAH phenanthrene. Measurements are taken for 1) phenanthrene uptake, 2) root morphology--specific surface area, volume, surface area, tip number and total root length and 3) root tissue composition--water, lipid, protein and carbohydrate content. These factors are compared through Pearson's correlation and multiple linear regression analysis. The major factors which promote phenanthrene uptake are specific surface area and lipid content.

Challenges in tracing the fate and effects of atmospheric polycyclic aromatic hydrocarbon deposition in vascular plants

Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous organic pollutants that raise environmental concerns because of their toxicity. Their accumulation in vascular plants conditions harmful consequences to human health because of their position in the food chain. Consequently, understanding how atmospheric PAHs are taken up in plant tissues is crucial for risk assessment. In this review we synthesize current knowledge about PAH atmospheric deposition, accumulation in both gymnosperms and angiosperms, mechanisms of transfer, and ecological and physiological effects. PAHs emitted in the atmosphere partition between gas and particulate phases and undergo atmospheric deposition on shoots and soil. Most PAH concentration data from vascular plant leaves suggest that contamination occurs by both direct (air-leaf) and indirect (air-soil-root) pathways. Experimental studies demonstrate that PAHs affect plant growth, interfering with plant carbon allocation and root symbioses. Photosynthesis remains the most studied physiological process affected by PAHs. Among scientific challenges, identifying specific physiological transfer mechanisms and improving the understanding of plant-symbiont interactions in relation to PAH pollution remain pivotal for both fundamental and applied environmental sciences.

Uptake of (14)C-atropine and/or its transformation products from soil by wheat (Triticum aestivum var Kronjet) and their translocation to shoots

Plant uptake of toxins and their translocation to edible plant parts are important processes in the transfer of contaminants into the food chain. Atropine, a highly toxic muscarine receptor antagonist produced by Solanacea species, is found in all plant tissues and can enter the soil and hence be available for uptake by crops. The absorption of atropine and/or its transformation products from soil by wheat (Triticum aestivum var Kronjet) and its distribution to shoots was investigated by growing wheat in soil spiked with unlabeled or (14)C-labeled atropine. Radioactivity attributable to (14)C-atropine and its transformation products was measurable in plants sampled at 15 d after sowing (DAS) and thereafter until the end of experiment. The highest accumulation of (14)C-atropine and/or its transformation products by plants was detected in leaves (between 73 and 90% of the total accumulated) with lower amounts in stems, roots, and seeds (approximately 14%, 9%, and 3%, respectively). (14)C-Atropine and/or its transformation products were detected in soil leachate at 30, 60, and 90 DAS and were strongly adsorbed to soil, with 60% of the applied dose adsorbed at 30 DAS, plateauing at 70% from 60 DAS. Unlabeled atropine was detected in shoots 30 DAS at a concentration of 3.9 ± 0.1 μg kg(-1) (mean ± SD). The observed bioconcentration factor was 2.3 ± 0.04. The results suggest a potential risk of atropine toxicity to consumers.

Effect of organic pollutant treatment on the growth of pea and maize seedlings

This study confirmed the considerable effect of polycyclic aromatic hydrocarbon fluoranthene (FLT; 0.01, 0.1, 1, 4 and 7 mg/l) exposure on the germination of seeds, growth and root morphology of seedlings in Zea mays and Pisum sativum. Seed germination was significantly inhibited at FLT≥0.01 mg/l in maize and at ≥1 mg/l in pea. The amount of released ethylene after 3 days of germination was significantly increased in both species at FLT≥0.1 mg/l. After 7 days of seedling cultivation a significant decrease in the dry weight of roots and shoots occurred in maize at FLT≥0.1 mg/l while in pea similar effect was observed at ≥1 mg/l. The total length of primary and lateral roots was significantly reduced by FLT≥1 mg/l in maize and by 4 and 7 mg/l in pea. The length of the non-branched part of the primary root was significantly reduced by FLT≥0.1 mg/l in maize and ≥0.01 mg/l in pea. In both species the number of lateral roots was significantly increased at FLT≤1 mg/l and inhibited at concentrations of 4 and 7 mg/l. Fluoranthene content in roots and shoots of both species positively correlated with the FLT treatment.

Plant uptake of pesticides and human health: dynamic modeling of residues in wheat and ingestion intake

Human intake of pesticide residues from consumption of processed food plays an important role for evaluating current agricultural practice. We take advantage of latest developments in crop-specific plant uptake modeling and propose an innovative dynamic model to estimate pesticide residues in the wheat-environment system, dynamiCROP. We used this model to analyze uptake and translocation of pesticides in wheat after foliar spray application and subsequent intake fractions by humans. Based on the evolution of residues in edible parts of harvested wheat we predict that between 22 mg and 2.1 g per kg applied pesticide are taken in by humans via consumption of processed wheat products. Model results were compared with experimentally derived concentrations in wheat ears and with estimated intake via inhalation and ingestion caused by indirect emissions, i.e. the amount lost to the environment during pesticide application. Modeled and measured concentrations in wheat fitted very well and deviate from less than a factor 1.5 for chlorothalonil to a maximum factor 3 for tebuconazole. Main aspects influencing pesticide fate behavior are degradation half-life in plant and time between pesticide application and crop harvest, leading to variations in harvest fraction of at least three orders of magnitude. Food processing may further reduce residues by approximately 63%. Intake fractions from residues in sprayed wheat were up to four orders of magnitude higher than intake fractions estimated from indirect emissions, thereby demonstrating the importance of exposure from consumption of food crops after direct pesticide treatment.

Dynamic plant uptake model applied for drip irrigation of an insecticide to pepper fruit plants

BACKGROUND

Drip application of insecticides is an effective way to deliver the chemical to the plant that avoids off-site movement via spray drift and minimizes applicator exposure. The aim of this paper is to present a cascade model for the uptake of pesticide into plants following drip irrigation, its application for a soil-applied insecticide and a sensitivity analysis of the model parameters.

RESULTS

The model predicted the measured increase and decline of residues following two soil applications of an insecticide to peppers, with an absolute error between model and measurement ranging from 0.002 to 0.034 mg kg fw(-1). Maximum measured concentrations in pepper fruit were approximately 0.22 mg kg fw(-1). Temperature was the most sensitive component for predicting the peak and final concentration in pepper fruit, through its influence on soil and plant degradation rates.

CONCLUSION

Repeated simulations of pulse inputs with the cascade model adequately describe soil pesticide applications to an actual cropped system and reasonably mimic it. The model has the potential to be used for the optimization of practical features, such as application rates and waiting times between applications and before harvest, through the integrated accounting of soil, plant and environmental influences.

New concepts for dynamic plant uptake models

Models for the prediction of chemical uptake into plants are widely applied tools for human and wildlife exposure assessment, pesticide design and for environmental biotechnology such as phytoremediation. Steady-state considerations are often applied, because they are simple and have a small data need. However, often the emission pattern is non-steady. Examples are pesticide spraying, or the application of manure and sewage sludge on agricultural fields. In these scenarios, steady-state solutions are not valid, and dynamic simulation is required. We compared different approaches for dynamic modelling of plant uptake in order to identify relevant processes and timescales of processes in the soil-plant-air system. Based on the outcome, a new model concept for plant uptake models was developed, approximating logistic growth and coupling transpiration to growing plant mass. The underlying system of differential equations was solved analytically for the inhomogenous case, i.e. for constant input. By superposition of the results of n periods, changes in emission and input data between periods are considered. This combination allows to mimic most input functions that are relevant in practice. The model was set up, parameterized and tested for uptake into growing crops. The outcome was compared with a numerical solution, to verify the mathematical structure.

Pesticide uptake in potatoes: model and field experiments

A dynamic model for uptake of pesticides in potatoes is presented and evaluated with measurements performed within a field trial in the region of Boyacá, Colombia. The model takes into account the time between pesticide applications and harvest, the time between harvest and consumption, the amount of spray deposition on soil surface, mobility and degradation of pesticide in soil, diffusive uptake and persistence due to crop growth and metabolism in plant material, and loss due to food processing. Food processing steps included were cleaning, washing, storing, and cooking. Pesticide concentrations were measured periodically in soil and potato samples from the beginning of tuber formation until harvest. The model was able to predict the magnitude and temporal profile of the experimentally derived pesticide concentrations well, with all measurements falling within the 90% confidence interval. The fraction of chlorpyrifos applied on the field during plant cultivation that eventually is ingested by the consumer is on average 10(-4)-10(-7), depending on the time between pesticide application and ingestion and the processing step considered.

Predicting the accumulation of organic contaminants from soil by plants

Analytic expressions for maximum chemical concentration attained in plants, and time this takes for uptake from surrounding soil were derived from a simple two-compartment soil/water-plant model. To illustrate, for the antibiotic norflxacin undergoing first order loss in the soil/water phase with a rate constant of 0.544 days⁻¹, maximum concentration in soybean P(MAX) is predicted to occur after 2.79 days exposure and be independent of initial soil/water concentration SW₀ of 52.5 mg kg⁻¹ dry weight. For soybean, the relationship between P(MAX) and SW₀ is P(MAX) = 0.047SW₀, resulting in predicted maximum levels of 2.20 mg kg⁻¹ dry weight. Modelled plant concentrations agreed well with experimental data (R² = 0.91).

Soil quality assessment under emerging regulatory requirements

New and emerging policies that aim to set standards for protection and sustainable use of soil are likely to require identification of geographical risk/priority areas. Soil degradation can be seen as the change or disturbance in soil quality and it is therefore crucial that soil and soil quality are well understood to protect soils and to meet legislative requirements. To increase this understanding a review of the soil quality definition evaluated its development, with a formal scientific approach to assessment beginning in the 1970s, followed by a period of discussion and refinement. A number of reservations about soil quality assessment expressed in the literature are summarised. Taking concerns into account, a definition of soil quality incorporating soil's ability to meet multifunctional requirements, to provide ecosystem services, and the potential for soils to affect other environmental media is described. Assessment using this definition requires a large number of soil function dependent indicators that can be expensive, laborious, prone to error, and problematic in comparison. Findings demonstrate the need for a method that is not function dependent, but uses a number of cross-functional indicators instead. This method to systematically prioritise areas where detailed investigation is required, using a ranking based against a desired level of action, could be relatively quick, easy and cost effective. As such this has potential to fill in gaps and compliment existing monitoring programs and assist in development and implementation of current and future soil protection legislation.

Uptake and persistence of pesticides in plants: measurements and model estimates for imidacloprid after foliar and soil application

The uptake and persistence behaviour of the insecticide imidacloprid in tomato plants treated by (i) foliar spray application and (ii) soil irrigation was studied using two plant uptake models. In addition to a pesticide deposition model, a dynamic root uptake and translocation model was developed, and both models predict residual concentrations of pesticides in or on fruits. The model results were experimentally validated. The fraction of imidacloprid ingested by the human population is on average 10(-2) to 10(-6), depending on the time between pesticide application and ingestion, the processing step, and the application method. Model and experimentally derived intake fractions deviated by less than a factor of 2 for both application techniques. Total imidacloprid residues were up to five times higher in plants treated by foliar spray application than by soil irrigation. However, peeling tomatoes treated by spray application reduces the human intake fraction by up to three orders of magnitude. Model calculations suggest that drip-irrigation in a closed hydroponic system minimizes worker and consumer exposure to pesticides and prevents runoff of pesticide by spray drift and leaching into the environment.

Addressing temporal variability when modeling bioaccumulation in plants

Steady state models are commonly used to predict bioaccumulation of organic contaminants in biota. However, the steady state assumption may introduce errors when complex dynamic processes such as growth, temperature fluctuations, and variable environmental concentrations significantly affect the major chemical uptake and elimination processes. In this study, a strategy for addressing temporal variability in bioaccumulation modeling is proposed. Chemical partitioning space plots are used to show the time necessary for organic contaminants to approach steady state in plant leaves and roots as well as the dominant uptake/elimination fluxes of chemicals as a function of the contaminants' physical chemical properties. The plots were produced with a novel nonsteady state model of bioaccumulation in plants, which is presented, parameterized, and evaluated. The first prerequisite identified for using a steady state model is that the duration of chemical exposure exceeds the time to approach steady state. Next, the dominant chemical transport processes for the chemical in question should be identified and the variability of parameters affecting these processes compared to the time to approach steady state. A major systematic variation in one of these parameters on a time scale similar to the time to approach steady state may cause an unacceptable deviation between the predicted and true chemical concentrations in vegetation. In such cases a nonsteady state model such as the one presented here should be used. The chemical partitioning plots presented provide guidance for understanding the dominant uptake/elimination processes and the time to approach steady state in relation to the partitioning properties of organic compounds.

A semi-quantitative approach to deriving a model structure for the uptake of organic chemicals by vegetation

An expert elicitation exercise was undertaken to determine those components and processes that are most important for modeling plant uptake of organic chemicals. The state of our knowledge of these processes was also assessed. This semi-quantitative analysis allowed the construction of an idealized model with seven compartments; soil bulk, soil water, roots, stem, leaves, fruit, and air. Three main areas were identified further research: (1) the uptake of organic chemicals by fruit; (2) the internal transfer of organic chemicals between plant structures (e.g., stem and leaves); and (3) the transfer via the soil-air-plant pathway. Until new data becomes available to quantify these processes, it is proposed that an equilibrium partitioning approach is used between plant components other than fruit or that models consist of both an edible and inedible compartment.

Estimating half-lives of pesticides in/on vegetation for use in multimedia fate and exposure models

Degradation half-lives in/on vegetation are needed in environmental risk assessment of pesticides, but these data are often not available for most active ingredients. To address this, we first correlated experimental soil degradation half-life data of 41 pesticides obtained from the reviewed literature with the corresponding experimental half-lives on plant surface. Degradation half-lives in soil were found to be four times slower compared with half-lives on plant surfaces. In a second step, we explored measured plant surface half-lives directly with those in vegetation. The results were validated by comparing computed values with results obtained from an experimental set-up. The uptake and dissipation of alpha-cypermethrin (insecticide) and bromopropylate (acaricide) was studied by detecting pesticide residues in whole and peeled tomato fruits using gas chromatography. Half-lives within vegetation were found to be four times faster compared with plant surface half-lives. Using this experimental based approach, it is concluded that the estimation of degradation half-lives of pesticides in/on vegetation to be used as input data in environmental mass balance models can be directly correlated from the more abundant ready experimental degradation half-life data for soil.

Improved approaches for modeling the sorption of phenanthrene by a range of plant species

Equilibrium sorption of phenanthrene and its relationship with plant lipid contents were investigated using roots and shoots of alfalfa, ryegrass, tomato, potato, carrot, cucumber, zucchini, and pumpkin. Lipid extractions using chloroform and hexane were compared, and the influence of dechlorophyllization on lipid determinations was evaluated. The sorption isotherms were close to linear (R2 > 0.923, P < 0.05) and the plant-water partition coefficients (K(pl)) of phenanthrene obtained from the isotherms exhibited significant and positive correlations with plantlipid contents (R2 > 0.664, P < 0.05). The correlations were more significant (R2 > 0.906, P < 0.001) when dechlorophyllization was included in the lipid extraction. The measured sorption was higher than that estimated using the octanol-water partition coefficient (K(ow)) but was very close to the estimate using the triolein-water partition coefficient (K(tw)). This study leads us to conclude that dechlorophyllization is necessary for plant lipid determination and that K(tw) is more accurate as a substitute for the lipid-water partition coefficient (K(lip)) than K(ow). These novel approaches may provide substantial improvements in the application of partition-limited models for the estimation of plant uptake of organic contaminants.

Pesticide bioconcentration modelling for fruit trees

The model presented allows simulating the pesticide concentration evolution in fruit trees and estimating the pesticide bioconcentration factor in fruits. Pesticides are non-ionic organic compounds that are degraded in soils cropped with woody species, fruit trees and other perennials. The model allows estimating the pesticide uptake by plants through the water transpiration stream and also the time in which maximum pesticide concentration occur in the fruits. The equation proposed presents the relationships between bioconcentration factor (BCF) and the following variables: plant water transpiration volume (Q), pesticide transpiration stream concentration factor (TSCF), pesticide stem-water partition coefficient (K(Wood,W)), stem dry biomass (M) and pesticide dissipation rate in the soil-plant system (k(EGS)). The modeling started and was developed from a previous model "Fruit Tree Model" (FTM), reported by Trapp and collaborators in 2003, to which was added the hypothesis that the pesticide degradation in the soil follows a first order kinetic equation. The FTM model for pesticides (FTM-p) was applied to a hypothetic mango plant cropping (Mangifera indica) treated with paclobutrazol (growth regulator) added to the soil. The model fitness was evaluated through the sensitivity analysis of the pesticide BCF values in fruits with respect to the model entry data variability.

Plant uptake of non ionic organic chemicals

Plant uptake of organic chemicals is an important process when considering the risks associated with land contamination, the role of vegetation in the global cycling of persistent organic pollutants, and the potential for industrial discharges to contaminate the food chain. There have been some significant advances in our understanding of the processes of plant uptake of organic chemicals in recent years; most notably there is now a better understanding of the air to plant transfer pathway, which may be significant for a number of industrial chemicals. This review identifies the key processes involved in the plant uptake of organic chemicals including those for which there is currently little information, e.g., plant lipid content and plant metabolism. One of the principal findings is that although a number of predictive models exist using established relationships, these require further validation if they are to be considered sufficiently robust for the purposes of contaminated land risk assessment or for prediction of the global cycling of persistent organic pollutants. Finally, a number of processes are identified which should be the focus of future research.

Beneficial reuse and sustainability: the fate of organic compounds in land-applied waste

Land application systems, also referred to as beneficial reuse systems, are engineered systems that have defined and permitted application areas based on site and waste characteristics to determine the land area size requirement. These terrestrial systems have orders of magnitude greater microbial capability and residence time to achieve decomposition and assimilation compared with aquatic systems. In this paper we focus on current information and information needs related to terrestrial fate pathways in land treatment systems. Attention is given to conventional organic chemicals as well as new estrogenic and pharmaceutical chemicals of commerce. Specific terrestrial fate pathways addressed include: decomposition, bound residue formation, leaching, runoff, and crop uptake. Molecular decomposition and formation of bound residues provide the basis for the design and regulation of land treatment systems. These mechanisms allow for assimilation of wastes and nondegradation of the environment and accomplish the goal of sustainable land use. Bound residues that are biologically produced are relatively immobile, degrade at rates similar to natural soil materials, and should present a significantly reduced risk to the environment as opposed to parent contaminants. With regard to leaching and runoff pathways, no comprehensive summary or mathematical model of organic chemical migration from land treatment systems has been developed. For the crop uptake pathway, a critical need exists to develop information for nonagricultural chemicals and to address full-scale performance and monitoring at more land application sites. The limited technology choices for treatment of biosolids, liquids, and other wastes implies that acceptance of some risks and occurrence of some benefits will continue to characterize land application practices that contribute directly to the goal of beneficial reuse and sustainability.

Plant uptake and transport models for neutral and ionic chemicals

BACKGROUND

Models for predicting uptake and transport of chemicals in plants are applied in pesticide design, risk assessment, and environmental biotechnology.

OBJECTIVE

This review considers the theoretical basics of the most popular models, and discusses what they have in common. The line is drawn between models for neutral compounds, and models for weak and strong electrolytes.

MAIN FEATURES

Neutral Compounds. Neutral compounds undergo only very few processes inside plants (lipophilic interactions, metabolism), in contrast to weak electrolytes. The models developed for neutral compounds are widely applied in the risk assessment of environmental contaminants, but are not of much use for weak electrolytes, such as pesticides. Weak electrolytes. A very important process for weak electrolytes is the 'ion trap', which traps chemicals that dissociate inside plant cells. This is considered in the popular models of Kleier, Satchivi and Briggs. Other relevant processes for electrolytes are electrophilic interactions, speciation and complex formation. None of the currently used models considers these processes.

CONCLUSIONS

The accuracy of models for neutral compounds is satisfactory, but the prediction of electrolyte behavior inside plants is still quite difficult due to gaps in knowledge.

Model intercomparison for the uptake of organic chemicals by plants

Currently, a variety of models are available for predicting the uptake, translocation, and elimination of organic contaminants by plants. These models range from simple deterministic risk assessment screening tools to more complex models that consider physical, chemical, and biological processes in a mechanistic manner. This study evaluates the performance of a range of such models and model types against experimental data sets. Three dynamic, three regression-based, and three steady-state and equilibrium models have been selected for evaluation. These models differ in terms of their scope, methodological approach, and complexity. Data from nine published experiments were used to create scenarios to test model performance. These experiments consider plant contamination via both soil and aerial exposure pathways. A total of 19 different organic chemicals were used in the experiments along with 7 different plant species. Model predictions of chemical concentrations in the relevant plant compartments were compared with the experimentally recorded values. The results indicate that dynamic models offer performance advantages for acute exposure durations and for rapidly changing environmental media. Equilibrium/steady-state and regression-based models perform better for chronic exposure durations, where stable conditions are more likely to exist. The selection of an appropriate plant uptake model will therefore be dependent on the requirements of the assessment, the nature of the environmental media, and the duration of the source term. The results generated by the regression-based models suggest that in their current form these models are unsuitable for evaluating the uptake of organic chemicals from the air into plants.

Exposure chamber measurements of mass transfer and partitioning at the plant/air interface

Dynamic measures of air and vegetation concentrations in an exposure chamber and a two-box mass balance model are used to quantify factors that control the rate and extent of chemical partitioning between vegetation and the atmosphere. A continuous stirred flow-through exposure chamber was used to investigate the gas-phase transfer of pollutants between air and plants. A probabilistic two-compartment mass balance model of plant/air exchange within the exposure chamber was developed and used with measured concentrations from the chamber to simultaneously evaluate partitioning (Kpa), overall mass transfer across the plant/air interface (Upa), and loss rates in the atmosphere (Ra) and aboveground vegetation (Rp). The approach is demonstrated using mature Capsicum annuum (bell pepper) plants exposed to phenanthrene (PH), anthracene (AN), fluoranthene (FL) and pyrene (PY). Measured values of log Kpa (V[air]/V[fresh plant]) were 5.7, 5.7, 6.0, and 6.2 for PH, AN, FL, and PY, respectively. Values of Upa (m d(-1)) under the conditions of this study ranged from 42 for PH to 119 for FL. After correcting for wall effects, the estimated reaction half-lives in air were 3, 9, and 25 h for AN, FL and PY. Reaction half-lives in the plant compartment were 17, 6, 17, and 5 d for PH, AN, FL, and PY, respectively. The combined use of exposure chamber measurements and models provides a robust tool for simultaneously measuring several different transfer factors that are important for modeling the uptake of pollutants into vegetation.

A design of two simple models to predict PCDD/F concentrations in vegetation and soils

The vegetation and soil levels of the 17 polychlorinated dibenzo-p-dioxin and dibenzofuran (PCDD/F) toxic congeners were calculated by means of a vegetation and a soil model, respectively. Both models predicted the levels of the 17 PCDD/F congeners in quite good agreement with the observed results although the soil model was more accurate than the vegetation model. Four different pathways of contribution to the vegetation concentrations were taken into account: vapour-phase absorption, dry particle deposition, wet particle deposition and uptake by root. The most important pathway was the vapour-phase absorption and the less was the uptake by root. In the soils model four pathways were considered: background soil concentration, dry particle deposition, wet particle deposition and uptake by root. After the background concentration, the most important pathway was the wet deposition.

Improved uptake models of nonionized pesticides to foliage and seed of crops

The residual amount of nonionized pesticides incorporated to foliage and stem (foliage) and seed (fruit) of crops via root hairs from the water phase was estimated using the uptake models newly including metabolic parameters by which the amount of intact pesticide remaining in crops was considered with its proportion in a transpiration stream. A new parameter was also introduced for the seed model that accounts for the pesticide loss by adsorption to the inner surface of xylem tissue. Validation of the model was conducted using six pesticides with soybean and spinach plants. The ratio of the predicted concentration of pesticide to the measured one was 0.44-1.49 and 0.57-2.93 with foliage and seed models, respectively, showing that these improved models would be effective as a prediction tool.

Strategies for including vegetation compartments in multimedia models

The incentives for including vegetation compartments in multimedia Level I, II and III fugacity calculations are discussed and equations and parameters for undertaking the calculations suggested. Model outputs with and without vegetation compartments are compared for 12 non-ionic organic chemicals with a wide variety of physical-chemical properties. Inclusion of vegetation compartments is shown to have a significant effect on two classes of chemicals: (1) those that are taken up by atmospheric deposition and (2) those that are taken up by transpiration through the plant roots. It is suggested that uptake from the atmosphere is important for chemicals with logK(OA) greater than 6 and a logK(AW) of greater than -6. Plant uptake by transpiration is important for chemicals with logK(OW) less than 2.5 and a logK(AW) of less than -1. At logK(OA) > 9 atmospheric uptake is dominated by particle-bound deposition and the importance of partitioning to vegetation is largely dependent on the relative magnitude of the particle deposition velocities to soil and vegetation. These property ranges can be used to determine if a chemical will significantly partition to vegetation. If the chemical falls outside the property ranges of the two classes it will probably be unnecessary to include vegetation in models for assessing environmental fate. The amount of chemical predicted to partition to vegetation compartments in the model is shown to be highly sensitive to certain model assumptions. Further experimental research is recommended to obtain more reliable equations describing equilibrium partitioning and uptake/depuration kinetics.

Estimating the influence of forests on the overall fate of semivolatile organic compounds using a multimedia fate model

On the basis of recently reported measurements of semivolatile organic compound (SOC) uptake in forest canopies, simple expressions are derived that allow the inclusion of a canopy compartment into existing non-steady-state multimedia fate models based on the fugacity approach. One such model is used to assess how the inclusion of the canopy compartment in the model affects the calculated overall behavior of SOCs with specific physical--chemical properties. The primary effect of the forest is an increase in the net atmospheric deposition to the terrestrial environment, reducing atmospheric concentrations and accordingly the extent of deposition to the agricultural and aquatic environments. This effect was most pronounced for chemicals with log KOA around 9-10 and log KAW -2 to -3; their average air concentrations during the growing season decreased by a factor of 5 when the canopy compartment was included. Concentration levels in virtually all compartments are decreased at the expense of increased concentrations in the forest soil. The effect of the forest lies not in a large capacity for these chemicals but in the efficiency of pumping the chemicals from the atmosphere to the forest soil, a storage reservoir with high capacity from which the chemicals can return to the atmosphere only with difficulty. Because of seasonal variability of canopy size and atmospheric stability, uptake into forests is higher during spring and summer than in winter. The model suggests that this may dampen temperature-driven seasonal fluctuations of air concentrations and in regions with large deciduous forests may lead to a temporary, yet notable dip in air concentrations during leaf development in spring. A sensitivity analysis revealed a strong effect of forest cover, forest composition, and degradation half-lives. A high degradation loss on the plant surface has the effect of preventing the saturation of the small plant reservoir and can cause very significant reductions in atmospheric concentrations of those SOCs for which uptake in the canopy is limited by the size of the reservoir.

The evolution of mass balance models of persistent organic pollutant fate in the environment

Current approaches to modelling the fate of persistent organic pollutants (POPs) in the environment have evolved in response to four dominant characteristics of these substances; namely: (1) the presence of POPs in virtually all environmental phases and the ease with which they move from one to the other requires multi-compartmental modelling. Describing transport across phase boundaries becomes as, or even more, important as quantifying transport within the phases; (2) POPs may persist in the environment for many decades. For chemicals that 'have time', concepts such as equilibrium partitioning and steady-state become more important than for short-lived substances whose fate is more controlled by the rates of transformation; (3) measuring POPs is difficult and expensive and observed concentrations of POPs are not available in high spatial or temporal resolution. Consequently, high resolution tends not to be a high priority in POP models; and (4) detrimental effects of POPs often manifest themselves in top predators, which has led to a focus on modelling biotic uptake and transfer within food chains. The task of building a POPs model is viewed as combining the four 'building blocks' of partitioning, transport, transformation and source data with the help of the law of the conservation of mass. Process models, evaluative models, models of real local, regional and global fate, as well as biological uptake models are presented and references to numerous examples are provided. An attempt is made to forecast future directions in the field of POPs modelling. It is expected that modelling techniques that do not rely on quantitative emission estimates as well as approaches that take into account spatial, temporal and climatic variability as well as parameter uncertainty will increase in importance. Finally, the relationship between modelling POPs and models of other pollutant issues is addressed, as are potential interactions between POPs and pollutant issues such as eutrophication, acidification and global climate change.