Despite their limitations in practice, mass balance models (MBMs) appear to offer the most convenient means for estimating a pollutant’s persistence in a single medium or in a multimedia environment. Insofar as overall environmental persistence is of greater concern, multimedia MBMs are more popular than those gearing towards persistence in a single medium. MBMs can provide an understanding of the chemical’s behavior and fate in one or more environmental media. These models typically link the emission rates to prevailing environmental concentrations and identify such critical issues as persistence for excessive times, tendency to bioaccumulate, and the potential to undertake intermedia transport.

Simple and multimedia MBMs are actually mathematical constructs designed to gain a qualitative and quantitative understanding of the environmental behavior of chemicals, which are likely to be present in more than one medium. Such models subdivide the environment into a number of compartments which each are assumed to have homogeneous characteristics and chemical concentrations. The models then calculate the chemical’s distribution pattern within that simplified system. These models thus integrate information on multiple and interacting processes of partitioning, transport, and transformation into a comprehensive and yet comprehensible picture of a chemical’s fate in a specific medium or the environment.

In all cases, the use of simple or multimedia MBMs is often limited by the availability of data on degradation half-lives. For students interested in this type of modeling, they should start with the textbook by Mackay (2001), and with the work by Wania and Mackay (2000) on comparing the overall persistence values calculated by various multimedia models.