Persistence of parent compounds and transformation products in a level IV multimedia model

An approach to extend the concept of persistence to transformation products of environmental chemicals is presented. First, the residence time in a steady-state scenario and the lifetime in a pulse scenario as two commonly used persistence measures are compared with respect to their mathematical formulation, and concentration functions as time-dependent solutions of a dynamic multimedia model are calculated. Second, it is shown how the model system needs to be modified in order to calculate concentration functions for any number of transformation products. On the basis of these concentration functions, two new persistence measures describing the additional exposure of the environment that is caused by the presence of transformation products are introduced. These two measures, called Joint Persistence and Secondary Persistence, are explicitly calculated for two pairs of parent compounds and transformation products, namely, methyl tert-butyl ether (MTBE) and atrazine and their transformation products tert-butyl alcohol (TBA) and desisopropyl atrazine (DIA), respectively. The suitability of both persistence definitions as measures for providing information on transformation products for regulatory purposes is discussed.