Combined Transport and Degradation Rates Screening

Using combined parallel rates of intermedia removal and degradation in persistence screening with existing single media criteria, D.W. Pennington, ORISE Research Fellow, US EPA, Cincinnati. In a number of national and international efforts, screening criteria are based on the "degradation" rates in specific media to help identify chemicals of potential concern in terms of their persistence. The association of these single-medium degradation criteria with the multimedia persistence or even the persistence in a given medium (a function of both intermedia transport and degradation) is usually undefined and not considered. Single media degradation criteria do not account for the potential ability of a chemical to transfer and then degrade more readily, or more slowly, in other media. There is therefore an obvious potential to misclassify chemicals on this basis. One proposed possibility is a two tier approach to screen chemicals in terms of persistence; using the single medium degradation criterion followed by an evaluation using a multimedia approach. However, the role of multimedia models remains unclear in practice, partially due to reluctance to specify an associated multimedia persistence pass/fail criterion. We will therefore discuss an alternative that can be used in conjunction with established single media criteria: The consideration of combined parallel rates of intermedia removal and degradation in conjunction with existing single media "persistence" criteria.