As stated in Slide 4, a pollutant’s persistence can be separated into overall environmental persistence and persistence within a single (environmental) medium. For either case, both the measurement and the quantity are often based on some mass balance theories. Insofar as overall environmental persistence is viewed as a set of interconnected media, mass balance models used for its assessment are simply referred to as of the multimedia type.

Mass balance is a simple term in theory but often difficult to appreciate in practice. It is based on the first law of thermodynamics, and is often called the law of conversion of mass. That is, it is built upon the concept that physical or chemical changes do not destroy or create matter, they merely change the material’s appearance or form. Thus, in one form or another, the material is still there in a confined geographic space. Of course the difficult task here is to track down the material in all its possible forms in all the environmental media considered.

For measuring a substance’s persistence, the concept of mass balance is on knowing, for a given time period, how much of the material is left in the (original) form of interest and how much of it (eventually) in this same form is transported to another place. One problem here is that in practice, we seldom know enough about the amount of material that starts out or the exact rate of degradation in a particular place, less the rates of transports or losses to other places. Nor do we know for certain the number of possible places for such transports or losses, since the term environment is a highly abstract concept which means different things to different people. There is also the problem of accounting for the number of places as sources for additional amounts of the material released into the environment(s) of concern.