A simple general limiting law for the overall decay of organic compounds with global pollution potential

It is rigorously shown that the effective decay rate in the environment of a chemical is between the minimum decay rate in one of its possible compartments and an upper value, which is the weighted-average decay rates in all compartments. The weights are the compartments' volumes and the equilibrium concentrations that would have occurred in the compartment due to transport alone, with no degradation. This upper value is approached, in the sense of a general limiting law, if degradation is much slower than transport. This limiting law, together with an estimate for the spatial range of a persistent chemical, could serve as a minimal base for exposure-based assessment of environmental risk. As a first illustration, the result is applied to DDT and hexachloroethane. A broader group of chemicals will be discussed elsewhere.