Degree of coupling between environmental compartments in multimedia models

Multimedia fate and exposure models like EQC, USES and CalTOX take inter-media transfer directly into account and provide a degree of consistency when assessing chemicals and emissions. On the other hand, the need remains to improve transparency and facilitate model evaluation using field data, especially for applications in Life Cycle Assessment. To achieve a wider understanding and to improve transparency, the present study identifies the degree of coupling between environmental media in multimedia models and the most significant environ-mental partitioning processes. This is achieved by introducing an innovative modular approach: The mass in each environmental medium is calculated iindependently and then the interactions between modules are taken into account, using intermedia transfer fractions. These fractions reflect the amount of chemical that can be considered as an input from one medium into another. The degree of intermedia feedback (fraction leaving one medium that then returns, etc.) is determined using a Taylor series expansion of the analytical solution. The importance of higher order feedbacks is determined. Results using the multimedia modular approach are presented for 300 organic chemicals, taking into account releases to air, water and soil. When feedback is considered, the results are equivalent to those of the analytical solution. For most chemicals, the feedback is unimportant: (for 90% of tested chemicals, results within 10% of exact solution without feedback, compared to a total variation of 6 to 12 orders of magnitude amongst chemicals). In such cases the compartment modules can be coupled in a straightforward manner using the intermedia transfer fractions, with negligible error, and single medium models can also be combined on the basis of the modular approach.