Estimating overall persistence and long-range transport potential of persistent organic pollutants: a comparison of seven multimedia mass balance models and atmospheric transport models.

Two different approaches to modeling the environmental fate of organic chemicals have been developed in recent years. The first approach is applied in multimedia box models, calculating average concentrations in homogeneous boxes which represent the different environmental media, based on intermedia partitioning, transport, and degradation processes. In the second approach, used in atmospheric transport models, the spatially and temporally variable atmospheric dynamics form the basis for calculating the environmental distribution of chemicals, from which also exchange processes to other environmental media are modeled. The main goal of the present study was to investigate if the multimedia mass balance models CliMoChem, SimpleBox, EVn-BETR, G-CIEMS, OECD Tool and the atmospheric transport models MSCE-POP and ADEPT predict the same rankings of the overall persistence (P(ov)) and long-range transport potential (LRTP) of POPs, and to explain differences and similarities between the rankings by the mass distributions and inter-compartment mass flows. The study was performed for a group of 14 reference chemicals. For P(ov), the models yield consistent results, owing to the large influence of phase partitioning parameters and degradation rate constants, which are used similarly by all models. Concerning LRTP, there are larger differences between the models than for P(ov), due to different LRTP calculation methods and spatial model resolutions. Between atmospheric transport models and multimedia fate models, no large differences in mass distributions and inter-compartment flows can be recognized. Deviations in mass flows are mainly caused by the geometrical design of the models.

Influence of vegetation on the environmental partitioning of DDT in two global multimedia models.

Two multimedia models are used to investigate the effect of a vegetation compartment on the environmental partitioning of dichlorodiphenyltrichloroethane (DDT): a steady-state unit world model using global averages of vegetation cover and land-to-sea ratio and a dynamic model with latitudinal zones and zone-specific vegetation types and annual temperature courses. The vegetation compartment represents canopies of deciduous and coniferous forests and blades of grasses; the organic carbon content of the vegetation-covered soil is higher than in the bare soil. In the steady-state model, transfer from the air to the vegetation and the underlying soil as well as revolatilization from the foliage and reduced deposition to the soil is observed, depending on the chemical's degradation rate constant in vegetation and the deposition velocities of the gaseous and particle-bound fractions. In both models, a significant effect of the organic carbon content of the vegetation-covered soil increasing the effect of the vegetation compartment is observed. In the steady-state model, the changes in the DDT concentrations in air do not exceed 7% difference between the cases with and without vegetation; the soil concentrations differ by maximally a factor of 2.7. In the spatially and temporally resolved model, however, air concentration differences up to 90% are observed, depending on the type and amount of vegetation in the latitudinal zones. Long-range transport is less pronounced in the model with vegetation.

Long-range transport and global fractionation of POPs: insights from multimedia modeling studies.

The long-range transport of persistent organic pollutants (POPs) is investigated with two multimedia box models of the global system. ChemRange is a purely evaluative, one-dimensional steady-state (level III) model; CliMoChem is a two-dimensional model with different temperatures, land/water ratios and vegetation types in different latitudinal zones. Model results are presented for three case studies: (i) the effect of atmospheric aerosol particles on the long-range transport of POPs, (ii) the effect of oceanic deposition on the long-range transport of different PCB congeners, (iii) the global fractionation of different PCB congeners. The model results for these case studies show: (i) the low atmospheric half-lives estimated for several organochlorine pesticides are likely to be inconsistent with the observed long-range transport of these compounds; (ii) export to the deep sea reduces the potential for long-range transport of highly hydrophobic compounds (but does not remove these chemicals from the biosphere); (iii) there are different meanings of the term global fractionation that refer to different aspects of the fractionation process and need to be distinguished. The case-study results further indicate that the influences of varying environmental conditions on the physicochemical properties and the degradation rate constants of POPs need to be determined.

Scenario-based risk assessment of multi-use chemicals: application to solvents.

The risk through chemical exposure is commonly characterized by ratios of exposure concentrations and effect levels (risk quotients). For chemicals with many different applications such as solvents, however, in addition to the risk quotients of different exposure situations it is useful to determine the corresponding numbers of exposed individuals, that is, not only the magnitude but also the extent of the risk. To this end, the Scenario-Based Risk Assessment (SceBRA) method has been developed that makes use of a large set of scenarios, each of which describes a typical situation regarding handling a solvent or solvent-containing product. The scenarios cover the life-cycle steps of production, distribution, and use of solvents. For each scenario, SceBRA provides the risk quotient, r, and the number of exposed individuals, N. This study investigated seven solvents that are used in large amounts in Switzerland. For each solvent, characteristic distributions of r and N values were calculated, making it possible to compare different solvents with respect to their risk profile. Graphical representations of the r, N data provide an informative way for analyzing and communicating the results of SceBRA.

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.

Comparison of different life-cycle impact assessment methods for aquatic ecotoxicity.

Human and ecotoxicity impact categories are problematic to quantify within life-cycle impact assessment (LCIA) because their local scope makes them difficult to aggregate with the traditional global-impact categories used in life-cycle assessment (LCA). For being able to assess local impacts such as toxicity, LCIA developers increasingly include fate modeling into LCA. This article follows this development by comparing different LCIA methods for aquatic ecotoxicology and by investigating the importance of fate within LCIA, the necessity of considering freshwater and seawater compartments separately, and the key degradation and intermedia transfer processes involved. The methods are compared by assessing an example study of domestic clothes washing in former West Germany. Four LCIA methods are selected and applied to four substances emitted during the washing process. The conclusion is that the consideration of environmental fate does matter and that aquatic ecotoxic impacts can differ significantly for the same substance in freshwater and in marine ecosystems. The way (bio)degradation, photolysis, volatilization, and transfer from agricultural soils are considered plays an important role as do the system boundaries chosen. This means that the LCIA methodology should remain flexible so that appropriate methods can be chosen for different applications. Fate models being developed in the environmental risk assessment of chemicals can contribute to the further improvement of LCIA methods.

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.