Long-range Transport Potential Research Articles

Indicators for persistence and long-range transport potential as derived from multicompartment chemistry–transport modelling

Total environmental and compartmental residence times as a measure for persistence as well as indicators for long-range transport potential (LRTP) have been derived from global geo-referenced modelling and LRTP is characterized in two geographic directions for the first time. A dynamic multicompartment chemistry–transport model (MCTM) was used to study the fate of the insecticides DDT and α-hexachlorocyclohexane (α-HCH) during the first 2 years upon entry. The indicators for LRTP were defined such as to address the tendencies of substance distributions to migrate (‘plume displacement’, PD) and to spread into remote areas (‘spatial spreading’, SS). The indicators deliver values as function of time upon entry. With the aim to address the effect of location of entry on environmental fate, scenarios of emission from the territories of seven countries were studied. It was found that the effect of location of entry on the spatial scale of countries (400–4000 km) is significant for the compartmental distribution and the inter-compartmental mass exchange fluxes (e.g., number of atmospheric cycles, ‘hops’). Location of entry introduces uncertainties in the order of a factor of 5 for the total environmental residence time, τ overall, and a factor of 5–20 for PD and SS. For the 2nd year upon entry into the environment, τ overall=317–1527 days are predicted for DDT and 101–463 days for α-HCH. The influence of location of entry does affect the substance ranking, i.e. we cannot simply state that DDT is more persistent than α-HCH, but for one scenario studied, application in China, the opposite is predicted. Precipitation patterns proved to be significant, besides other climate parameters, for atmospheric residence time. Integration of the location of entry in chemicals risk assessments is therefore recommended. In general, persistence and some indicators for LRTP, pertinent to their definition, refer to the fate of a large fraction, e.g., 63% (=1−1/ e) or 90%, but not the total substance burden. The choice of this fraction may have the consequence of a normative step which defines the spatial and temporal extensions of a related chemicals risk assessment and may affect substance ranking.

Towards Consistent Evaluation of the Persistence of Organic, Inorganic and Metallic Substances

Several criteria, including persistence (P), bioaccumulation (B), its related factor bioavailability, toxicity (T), and potential for long range transport (LRT) are currently applied when assessing the environmental hazard and risk associated with the use of chemicals of commerce. Whereas information on B and T criteria may be obtained by experimental measurement using standard tests or by the use of mass balance models, in the case of P and LRT no such standard tests exist, except for certain degradation rates in single media. Nor can these properties be measured in the environment at large. Here we focus on the criterion of persistence and its evaluation using steady-state multimedia mass balance models. It is concluded that it is possible to assess the persistence of all chemical substances including organics, inorganics, and metals using a common methodology in which mass balance models are applied to describe the substance's behavior in a specified environment or “unit world.” This avoids inconsistent evaluation and excessive regulatory fragmentation, which is likely if assessment procedures are specific to classes of chemicals. It is essential that persistence be recognized as only one of several factors influencing hazard and risk. Regulatory actions must also thus reflect other attributes such as toxicity, bioaccumulation, quantities used, and the societal value of the substances.

Expanding the applicability of multimedia fate models to polar organic chemicals.

Reliable estimates of environmental phase partitioning are essential for accurate predictions of the environmental fate of organic chemicals. Current fate and transport models use single-parameter linear free energy relationships (SP-LFERs) to quantify equilibrium phase partitioning. The applicability of such SP-LFERs is limited because no single parameter is able to describe appropriately all the molecular interactions that contribute to environmental phase distribution processes. Environmental partitioning coefficients predicted by SP-LFERs may thus have errors of up to an order of magnitude. Ranges for several environmental partitioning equilibria are identified, where such errors can result in significantly different fate predictions for individual bulk model compartments. We propose that it is possible to reduce such errors and uncertainties by implementing polyparametric LFER (PP-LFER) approaches in multimedia fate models. A level III fugacity model was modified such that the partitioning properties of chemicals are characterized by five linear solvation energy parameters rather than vapor pressure, water solubility, and octanol-water partition coefficient. A comparison of modified and unmodified models for a set of organic chemicals shows that the approach chosen to simulate environmental phase partitioning can have a large impact on model results, including long-range transport potential, overall persistence, and concentrations in various media. It is argued that PP-LFER based environmental fate models are applicable to a much wider range of organic substances, in particular those with polar functional groups. Obstacles to the full implementation of PP-LFER in multimedia fate models are currently the lack of solute descriptors for some chemicals of environmental concern and suitable regression equations for some important environmental phase equilibria, in particular for the partitioning between gas and particle phase in the atmosphere.

Role of vegetation on the overall persistence and long-range transport potential

Role of vegetation on the overall persistence and long-range transport potential

Vegetation-air exchange facilitates the long-range transport of some SVOCs

Question 1 Wania and McLachlan (2001) suggested that the presence of a forest canopy compartment in a dynamic multimedia model significantly alters the overall mass balance through the FFE by transferring chemical from the atmosphere to forest soils, where it is stored, thus inhibiting the potential for long-range atmospheric transport (LRAT). We agree that the FFE occurs for SVOCs with the properties suggested by Wania and McLachlan (2001) and thus that the presence of vegetation will lower the transport potential of a large proportion of the chemical. However, in this letter we further hypothesize that vegetation has the potential to enhance the air-surface exchange of SVOCs, thus increasing the LRAT potential of a significant mass fraction of chemical.

Assessing the long‐range transport potential of polybrominated diphenyl ethers: A comparison of four multimedia models

Data from a comprehensive literature search of environmentally relevant physical-chemical properties for nine polybrominated diphenyl ethers (PBDE), ranging from a monobrominated congener to the fully brominated decabromodiphenyl ether, were evaluated and adjusted to achieve both internal and interhomologue consistency. These data were then used in four model-based long-range transport potential (LRTP) assessment methods. The models TaPL3-2.10, ELPOS-1.1.1, Chemrange-2, and Globo-POP-1.1 were found to yield comparable predictions. A comparison of the LRTP estimates for the PBDEs with those of benchmark chemicals (polychlorinated biphenyls [PCBs]) suggest that the lower-brominated congeners have a LRTP comparable to that of PCBs known to be subject to significant LRT, whereas the highly brominated congeners have a very low potential to reach remote areas. This is in agreement with field measurements in remote regions that indicate that the lighter components of commercially produced PBDE mixtures predominate. Deviations between Chemrange and the models based on the concept of a characteristic travel distance were due to differences in the assumed height of the air compartment, which influences the relative importance of atmospheric degradation and deposition processes. The three models assuming a uniform temperature of 25 degrees C may underestimate the LRTP of the smaller congeners. Only atmospheric parameters had a notable influence on the LRTP estimates by TaPL3, ELPOS, and Chemrange. whereas the relative enrichment of chemicals in the Arctic calculated by Globo-POP is additionally sensitive to the parameters related to the interaction of temperature with air-surface exchange and degradation in surface compartments.

Modelling the environmental fate of the polybrominated diphenyl ethers

In response to growing alarm over the occurrence of polybrominated diphenyl ethers (PBDEs) in remote regions, this study considers their physical chemistry, environmental partitioning and considerations regarding potential for long-range atmospheric transport (LRAT). Internally consistent physical–chemical property data are presented for five representative congeners (PBDE-15, -28, -47, -99, -153) and used in a multimedia modelling approach. Results of the Level II model indicate that PBDEs will largely partition to organic carbon in soil and sediment and that their persistence will be strongly influenced by degradation rates in these media that are not well known. TaPL3 model estimates of their characteristic travel distance (CTD) suggest limited LRAT potential. The LRAT is also evaluated qualitatively, in terms of surface–air exchange behaviour. PBDEs are shown to be sensitive to seasonally and diurnally fluctuating temperatures. When vegetation is included in the model, 50% of the total mass of PBDE-47 deposited to vegetation returns to the atmosphere, suggesting that it may migrate through a series of deposition/volatilisation hops. Key data that needs to be identified in this evaluation include a better understanding of air–surface exchange, particularly to foliage, and measurements of degradation rates in soil, sediment and vegetation.

Temperature dependence of the characteristic travel distance.

The effect of temperature variation on the environmental fate of organic chemicals can be evaluated in steady-state multimedia box models by expressing chemical partitioning data and reaction rate coefficients as functions of temperature. Using such a modelthetemperature dependence of the characteristic travel distance in air L(A), which is a measure for the atmospheric long-range transport potential of organic chemicals, is calculated. Simulations are reported for a set of 40 chemicals of environmental interest. Increasing temperature is shown to have two opposing effects on L(A). Rates of chemical transformations in the atmosphere (k(air)) and surface media are increased, which reduces L(A). Rates of atmospheric deposition (k(dep)) are reduced leading to increased mobility and L(A). Accordingly, L(A) can monotonically increase or decrease with increasing temperature, or it can have a maximum in the modeled temperature range, but it cannot have a minimum. For chemicals with a strong temperature dependence of k(air) relative to k(dep), L(A) will increase with increasing temperature. Results for selected polychlorinated biphenyls are compared to monitoring data yielding qualitative agreement when chemical properties are adjusted to mean temperatures for the measurement period. The results demonstrate that the temperature dependence of the characteristic travel distance is highly dependent on chemical characteristics and can be counterintuitive. The use of mass balance models is thus essential. The difference between the L(A) values at 5 degrees C and 30 degrees C can be up to a factor of 6. Accordingly, chemical ranking with respect to L(A) can change significantly if performed at different temperatures. Implications of the different temperature dependencies on long-range transport to polar regions are discussed.

A perspective on environmental models and QSARs

A general review is presented of the roles of QSARs and mass balance models as tools for assessing the environmental fate and effects of chemicals of commerce. It is argued that all such chemicals must be assessed using a consistent and transparent methodology that uses chemical property data derived from QSARs, or experimental determinations when possible and applies evaluative or region-specific environmental models. These data and models enable an assessment to be made of the key chemical features of persistence, bioaccumulation, potential for long-range transport and toxicity. The other key feature is quantity used or discharged to the environment. A taxonomy of environmental models is presented in which it is suggested that rather than develop a single comprehensive model, the aim should be to establish a set of coordinated and consistent models treating evaluative and real environmental systems at a variety of scales from local to global and including food web models, organism-specific models and human exposure and pharmacokinetic models. The concentrations derived from these models can then be compared with levels judged to be of toxic significance. A brief account is given of perceived QSAR needs in terms of partitioning, reactivity, transport and toxicity data to support these models.

A Multimedia Assessment of the Environmental Fate of Bisphenol A

A comprehensive multimedia assessment of the environmental fate of bisphenol A (BPA) is presented. Components of the assessment include an evaluation of relevant partitioning and reactive properties, estimation of discharge quantities in the U.S. and the European Union (E.U.) resulting in conservative and realistic emission scenarios, and a review of monitoring data. Evaluative assessments of chemical fate using the Equilibrium Criterion (EQC) model are described from which it is concluded that the low volatility of BPA will result in negligible presence in the atmosphere. It is relatively rapidly degraded in the environment with half-lives in water and soil of about 4.5 days and less than 1 day in air, and with an overall half-life of 4.5 to 4.7 days, depending on the medium of release. The degradation rate in water is such that it may be transported some hundreds of kilometres in rivers, but long-range transport potential in air is negligible. Its low bioconcentration factor is consistent with rapid metabolism in fish (half-life less than 1 day). The estimated concentrations were generally consistent with the monitoring data, with the exception of sediment-water concentration ratios. Several hypotheses for the apparent nonequilibrium sediment-water partitioning are presented.

On the validity of classifying chemicals for persistence, bioaccumulation, toxicity, and potential for long‐range transport

It is argued that chemical substances can be meaningfully ranked or classified according to their persistence (P), bioaccumulation (B), toxicity (T), and potential for long-range transport (LRT) only if these attributes can be shown to be intensive, as distinct from extensive, properties of the substance, i.e., they are independent of quantity of substance. It is shown that P, B, and LRT can be considered intensive or quasi-intensive properties, but toxicity is more problematic. To obtain an intensive metric of toxicity requires selection of one of several possible extensive quantities that define exposure or dose. Ranking of a group of chemicals by toxicity is shown to be very dependent on which quantity is selected. It is suggested that toxicity metrics, such as lethal concentration to 50% of the population (LC50), lethal dose to 50% of the population (LD50), and threshold limit value (TLV) suffer the severe disadvantage of being dependent on the efficiency of delivery of the substance to the site(s) of toxic action in the organism. The use of measured or calculated internal dose is a preferable measure of toxicity since it reduces ambiguities inherent in the other metrics. Also, the primary concern is not the quasi-intensive property of toxicity; rather, it is the risk of toxic effects, an extensive quantity. To adequately assess the risk of toxic effects, both the toxic hazard and the degree of exposure must be characterized. Since exposure cannot be estimated without knowledge of the emission rate of chemicals to the environment, a compelling case can be made that screening to identify priority P, B, T, and LRT substances should be expanded to include quantity released to the environment as an additional factor.

The spatial scale of organic chemicals in multimedia fate modeling. Recent developments and significance for chemical assessment.

In the last years, the spatial range (SR) or characteristic travel distance (CTD) of organic chemicals has found increasing scientific interest as an indicator of the long-range transport (LRT) potential and, in combination with persistence, as a kind of 'hazard' indicator on the exposure level. This development coincides with European debates about more effective and more preventive approaches to the chemicals assessment, and about an international, legally-binding instrument for the phase out of persistent organic pollutants (POPs). Persistence and LRT potential are important issues in these debates. Here, the development of the concept of assessing the spatial scale from early ideas in the 1970s and 1980s to recent studies in the field of multimedia fate and transport modeling is summarized. Different approaches to the modeling of environmental transport (advective and dispersive) and different methods for quantifying the SR or CTD are compared. Relationships between SR or CTD and different persistence measures are analyzed. Comparison of these relationships shows that conclusions for chemical assessment should be based on an evaluation of different persistence and spatial scale measures. The use of SR or CTD and persistence as hazard indicators in the chemicals assessment is illustrated.

BETR North America: a regionally segmented multimedia contaminant fate model for North America.

We present the Berkeley-Trent North American contaminant fate model (BETR North America), a regionally segmented multimedia contaminant fate model based on the fugacity concept. The model is built on a framework that links contaminant fate models of individual regions, and is generally applicable to large, spatially heterogeneous areas. The North American environment is modeled as 24 ecological regions, within each region contaminant fate is described using a 7 compartment multimedia fugacity model including a vertically segmented atmosphere, freshwater, freshwater sediment, soil, coastal water and vegetation compartments. Inter-regional transport of contaminants in the atmosphere, freshwater and coastal water is described using a database of hydrological and meteorological data compiled with Geographical Information Systems (GIS) techniques. Steady-state and dynamic solutions to the 168 mass balance equations that make up the linked model for North America are discussed, and an illustrative case study of toxaphene transport from the southern United States to the Great Lakes Basin is presented. Regionally segmented models such as BETR North America can provide a critical link between evaluative models of long-range transport potential and contaminant concentrations observed in remote regions. The continent-scale mass balance calculated by the model provides a sound basis for evaluating long-range transport potential of organic pollutants, and formulation of continent-scale management and regulatory strategies for chemicals.

Long-range transport potential of semivolatile organic chemicals in coupled air-water systems.

Ongoing deliberations on the regulation of semivolatile organic chemicals require the assessment of chemical transport in atmospheric and marine systems. The characteristic travel distance was proposed as a measure for the transport potential in air and water. However, the existing definition treats the transport processes separately. It is shown that combined transport in coupled air-ocean systems can accelerate the overall transport into remote regions. Concentration ratios in air and water change with distance from sources depending on the initial concentration ratio and on the difference between the transport velocities. A measure is suggested facilitating the chemical screening with respect to transport potentials in such air-ocean systems. A case study for alpha and gamma-hexachlorocyclohexane shows that the suggested measure qualitatively reveals the transport potentials of these chemicals and exemplifies possible concentration patterns.

QSAR approach to POPs screening for atmospheric persistence

The environmental behaviour of a persistent organic pollutant (POP) is mainly controlled by its persistence, its tendency to undergo long-range transport (LRT) and its physicochemical properties. Atmospheric half-life is one of the criteria commonly used to study air persistence and LRT potential. For the 12 UNEP POPs and another 48 possible POPs, the mean and maximum half-life estimations for degradation in air are modelled using different molecular structure descriptors (atom and fragment counts, topological and WHIM descriptors), selected by Genetic Algorithm, in QSAR regression models. Both values are modelled to obtain an average estimate and a precautionary value for ranking and screening purposes. The models, validated for their predictivity, could be applied to predict unavailable data. Principal component analysis (PCA) was then used to explore the half-life data in addition to the physicochemical properties that are most relevant to atmospheric mobility; the aim has been to screen and rank POPs with regard to their tendency towards atmospheric persistence and mobility, and to obtain a persistence index in air and an LRT index. These indexes were also modelled by molecular descriptors, thus allowing a preliminary screening of new compounds.

Comparing representations of the environmental spatial scale of organic chemicals

Long-range transport potential is one of the criteria proposed to identify chemicals as persistent organic pollutants. Here, different approaches to determine the spatial scale of an environmental area or region that can be impacted by an emitted chemical are investigated. A ranking of chemicals according to the spatial range R on a limited scale, as suggested by Scheringer, is equivalent to the characteristic travel distance L according to Bennett et al. on a linear, open scale. However, the methods are different with respect to the weighting of differences between the chemicals and according to their comparability with persistence. The characteristic travel distance L and the overall persistence are both plotted on open log-linear scales, whereas the spatial range R is represented on a limited scale. Various approaches of how to account for the mode of entry of chemicals are compared, and it is argued that relative measures are more appropriate to describe the long-range transport potential of chemicals than absolute numbers.

Predicting long-range transport: a systematic evaluation of two multimedia transport models.

The United Nations Environment Program has recently developed criteria to identify and restrict chemicals with a potential for persistence and long-range transport (persistent organic pollutants or POPs). There are many stakeholders involved, and the issues are not only scientific but also include social, economic, and political factors. This work focuses on one aspect of the POPs debate, the criteria for determining the potential for long-range transport (LRT). Our goal is to determine if current models are reliable enough to support decisions that classify a chemical based on the LRT potential. We examine the robustness of two multimedia fate models for determining the relative ranking and absolute spatial range of various chemicals in the environment. We also consider the effect of parameter uncertainties and the model uncertainty associated with the selection of an algorithm for gas-particle partitioning on the model results. Given the same chemical properties, both models give virtually the same ranking. However, when chemical parameter uncertainties and model uncertainties such as particle partitioning are considered, the spatial range distributions obtained for the individual chemicals overlap, preventing a distinct rank order. The absolute values obtained for the predicted spatial range or travel distance differ significantly between the two models for the uncertainties evaluated. We find that to evaluate a chemical when large and unresolved uncertainties exist, it is more informative to use two or more models and include multiple types of uncertainty. Model differences and uncertainties must be explicitly confronted to determine how the limitations of scientific knowledge impact predictions in the decision-making process.

Pesticides in the atmosphere of the Mississippi River Valley, part II — air

Weekly composite air samples were collected from early April through to mid-September 1995 at three paired urban and agricultural sites along the Mississippi River region of the Midwestern United States. The paired sampling sites were located in Mississippi, Iowa, and Minnesota. A background site, removed from dense urban and agricultural areas, was located on the shore of Lake Superior in Michigan. Each sample was analyzed for 49 compounds; of these, 21 of 26 herbicides, 13 of 19 insecticides, and 4 of 4 related transformation products were detected during the study, with most pesticides detected in more than one sample. The maximum number of pesticides detected in an air sample was 18. Herbicides were the predominant type of pesticide detected at every site. Detection frequencies of most herbicides were similar at the urban and agricultural sites in Iowa and Minnesota. In Mississippi, herbicides generally were detected more frequently at the agricultural site. The insecticides chlorpyrifos, diazinon, and carbaryl, which are used in agricultural and non-agricultural settings, were detected more frequently in urban sites than agricultural sites in Mississippi and Iowa. Methyl parathion was detected in 70% of the samples from the Mississippi agricultural site and at the highest concentration (62 ng/m 3 air) of any insecticide measured in the study. At the background site, dacthal (100%), atrazine (35%), cyanazine (22%), and the (primarily atrazine) triazine transformation products CIAT (35%) and CEAT (17%) were detected most frequently, suggesting their potential for long-range atmospheric transport.

Controlling persistent organic pollutants–what next?

Within the context of current international initiatives on the control of persistent organic pollutants (POPs), an overview is given of the scientific knowledge relating to POP sources, emissions, transport, fate and effects. At the regional scale, improvements in mass balance models for well-characterised POPs are resulting in an ability to estimate their environmental concentrations with sufficient accuracy to be of help for some regulatory purposes. The relevance of the parameters used to define POPs within these international initiatives is considered with an emphasis on mechanisms for adding new substances to the initial lists. A tiered approach is proposed for screening the large number of untested chemical substances according to their long-range transport potential, persistence and bioaccumulative potential prior to more detailed risk assessments. The importance of testing candidate POPs for chronic toxicity (i.e. for immunotoxicity, endocrine disruption and carcinogenicity) is emphasised as is a need for the further development of relevant SAR (structure activity relationship) models and in vitro and in vivo tests for these effects. Where there is a high level of uncertainty at the risk assessment stage, decision-makers may have to rely on expert judgement and weight-of-evidence, taking into account the precautionary principle and the views of relevant stake-holders. Close co-operation between the various international initiatives on POPs will be required to ensure that assessment criteria and procedures are as compatible as possible.

Environmental fate of trifluralin.

Trifluralin, a preemergence, soil-applied and soil-incorporated herbicide, has been in agricultural use since 1963. The environmental chemistry and fate of dinitroaniline herbicides, including trifluralin, has been studied extensively in agricultural soils. Probst et al. (1975) and Helling (1976) have summarized pre-1975 data on the mobility, persistence, and degradation or metabolism of dinitroaniline herbicides as a group. Since then, numerous studies have been carried out on the fate of dinitroanilines, especially trifluralin, in the environment to understand further their degradation in soil, potential for mobility and persistence, and environmental concentration in water and air. The present review, while summarizing briefly earlier data, concentrates primarily on the post-1975 data on degradation, mobility, and persistence of trifluralin in soils and its potential concentrations in water and air. Trifluralin is readily degraded under sunlight in all media, with half-lives (t1/2) of minutes to several months, depending on the substrate. In addition, other dissipation processes, such as microbial and chemical, are also operative in soils, water, and sediments. Several degradation products of trifluralin have been identified and characterized, both under photolysis and following aerobic and anaerobic metabolism in soils and water-sediment systems. The differences between various degradative pathways of trifluralin appear to be more quantitative than qualitative in nature, leading eventually to the same end products that are subject to binding or mineralization with time. The general lack of accumulation of the breakdown products of trifluralin suggests that these are also subject to the same degradative mechanisms as the parent compound. Trifluralin has low water solubility and is strongly bound to soil components; mean Koc values range from 4,000 to 13,000. Once applied and incorporated into the soil, trifluralin remains relatively immobile with minimal or no potential for contamination of groundwaters under or near the treated zones. Trifluralin residues in soil surface layers are subject to loss via transport in runoff water or volatilization into the air. Seasonal losses in surface runoff are consistently less than 0.5% of the amounts applied, with concentrations in edge-of-the-field run-off water typically < 1.0 microgram L-1. Consequently, trifluralin is infrequently detected in surface waters and, if present, usually occurs below levels of quantification. Seasonal trifluralin losses into the atmosphere can be as high as 25% of that applied. Maximum trifluralin residues in the air above treated fields are in the 2-3 micrograms m-3 range following application, decreasing to < 100 ng m-3 in ambient air of intensive use areas, indicating its rapid dissipation in air. Trifluralin residues at < 100 pg m-3 in the atmosphere of remote nonuse regions have been reported, suggesting its potential for long-range transport. However, there is a general lack of understanding of the mechanisms controlling its potential for long-distance transport, especially considering its rapid photodegradation in vapor and solution states. The persistence of trifluralin in agricultural soils following incorporation is highly variable, depending on several factors such as depth of incorporation, soil moisture, soil temperature, soil air, and soil organic matter content. Estimated half-lives under a variety of agronomic conditions range from 25 to > 201 d, thus categorizing its persistence from 'moderate' to 'persistent'. The estimated half-life data for trifluralin under agronomic conditions, however, cannot be extrapolated to other potential scenarios, such as its dissipation in nontarget areas where trifluralin residues, if any, are essentially deposited on surfaces. Surface deposits on nontarget areas, unlike soil-incorporated residues, would be subject to volatilization and photolysis and thus more short lived. (ABSTRACT TRUNCATED)