Multimedia Mass Balance Model Research Articles

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.

Chemical-specific representation of air--soil exchange and soil penetration in regional multimedia models.

In multimedia mass-balance models, the soil compartment is an important sink as well as a conduit for transfers to vegetation and shallow groundwater. Here a novel approach for constructing soil transport algorithms for multimedia fate models is developed and evaluated. The resulting algorithms account for diffusion in gas and liquid components; advection in gas, liquid, or solid phases; and multiple transformation processes. They also provide an explicit quantification of the characteristic soil penetration depth. We construct a compartment model using three and four soil layers to replicate with high reliability the flux and mass distribution obtained from the exact analytical solution describing the transient dispersion, advection, and transformation of chemicals in soil layers with different properties but a fixed boundary condition at the air-soil surface. The soil compartment algorithms can be dynamically linked to other compartments (air, vegetation, groundwater, surface water) in multimedia fate models. We demonstrate and evaluate the performance of the algorithms in a model with applications to benzene, benzo[a]pyrene, MTBE, TCDD, and tritium.

Complexity in multimedia mass balance models: When are simple models adequate and when are more complex models necessary?

Three environmental multimedia models of varying degrees of complexity are compared to assess when simple models are adequate and when more complex models are advantageous. The simplest model, the level II (L-II) model, assumes all environmental media are at chemical equilibrium, whereas the more complex models treat chemical disequilibrium between well-mixed media (standard level IV [L-IV] model) or the major media are subdivided into separate layers to simulate heterogeneity (high-resolution level IV [HR-IV] model). The three models are compared for their performance in predicting steady-state, regional concentrations; dynamic, local-scale concentrations; and chemical persistence in the environment. The results indicate that the L-IV model often provides adequate regional simulations when chemical emission occurs to air or water. This model also is useful for assessing chemical persistence in both steady-state and dynamic scenarios. More complex models, such as the HR-IV model, are suggested for local-scale, dynamic simulations or when the chemical emission occurs to soil because they better characterize rates of intramedia transport, which can greatly affect the model predictions. The simplest L-II model predicts environmental concentrations that can differ significantly from those of more complex models, but it is useful for establishing partitioning tendencies and for ranking chemicals for their relative persistence in steady-state situations.

Robustness of spatial ranges of environmental chemicals to changes in model dimension

A comparison is made between a circular and a more adequate spherical reaction-diffusion multi-media mass balance model. This comparison adds new aspects to ongoing debates about more effective assessments of potentially harmful substances including persistent organic pollutants (POPs). The circular model serves as a paradigm in investigations of persistence and spatial range of non-polar chemicals. An analytic solution of the spherical model is presented. It is utilized in order to establish circular spatial ranges as versatile approximations of their spherical counterparts for most cases. Deviations in the few exceptions are demonstrated as playing a minor role compared to sensitivities against parameter uncertainties which characterize these exceptional cases as well. The sensitivities are fundamentally linked to the multi-scale nature of the underlying system. Finally, the insensitivity of spatial ranges with respect to dimension – circle versus sphere – is further secured by extensive studies of the role of the mode of entry for which a set of rules is established.

Multimedia Environmental Models

In this introductory paper it is suggested that multimedia mass balance models can play a valuable role in improving our understanding of the behavior of chemicals in the environment. They can prov...

Evaluation and comparison of multimedia mass balance models of chemical fate: application of EUSES and ChemCAN to 68 chemicals in Japan

The European Union System for Evaluation of Substances (EUSES) and the ChemCAN chemical fate model are applied to describe the fate of 68 chemicals on two spatial scales in Japan. Emission information on the chemicals has been obtained from Japan's Pollutant Release and Transfer Registry and available monitoring data gathered from government reports. Environmental concentrations calculated by the two models for the four primary environmental media of air, water, soil and sediment agree within a factor of 3 for over 70% of the data, and within a factor of 10 for over 87% of the data. Reasons for certain large discrepancies are discussed. Concentrations calculated by the models are generally consistent with the lower range of concentrations that are observed in the environment. Agreement between modeled and observed concentrations is considerably improved by including an estimate of the advective input of chemicals in air from outside Japan. The agreement between the EUSES and ChemCAN models suggests that results of individual chemical assessments are not likely to be significantly affected by the choice of chemical fate model. Primary sources of discrepancy between modeled and observed concentrations are believed to be uncertainties in emission rates, degradation half-lives, and the lack of data on advective inflow of contaminants in air.

Assessment of chemical fate in the environment using evaluative, regional and local‐scale models: Illustrative application to chlorobenzene and linear alkylbenzene sulfonates

AbstractEvaluation of chemical fate in the environment has been suggested to be best accomplished using a five‐stage process in which a sequence of increasing site‐specific multimedia mass balance models is applied. This approach is illustrated for chlorobenzene and linear alkylbenzene sulfonates (LAS). The first two stages involve classifying the chemical and quantifying the emissions into each environmental compartment. In the third stage, the characteristics of the chemical are determined using the evaluative equilibrium criterion model, which is capable of treating a variety of chemicals including those that are involatile and insoluble in water. This evaluation is conducted in three steps using levels I, II, and III versions of the model, which introduce increasing complexity and more realistic representations of the environment. In the fourth stage, ChemCAN, which is a level III model for specific regions of Canada, is used to predict the chemical's fate in southern Ontario. The final stage is to apply local environmental models to predict environmental exposure concentrations. For chlorobenzene, the local model was the SoilFug model, which predicts the fate of agro‐chemicals, and for LAS the WW‐TREAT, GRiDS, and ROUT models were used to predict the fate of LAS in a sewage treatment plant and in riverine receiving waters. It is concluded that this systematic approach provides a comprehensive assessment of chemical fate, revealing the broad characteristics of chemical behavior and quantifying the likely local and regional exposure levels.