Applicability of compartmental models for simulating the transport of radionuclides in soil

The applicability of multi-compartmental models to describe the transport of radioactive trace substances in soil is reconsidered. It is shown that the widely used serial compartmental model implicitly assumes convection-dominated flow and has to be replaced by a model with backflow for diffusion-controlled transport. A compartmental approach provides a valid model of the transport processes only if the number of compartments is set to a specific value which is determined by the physical flow conditions in the soil studied. It is demonstrated that describing diffusional transport by the commonly used serial compartmental model results in apparently time-dependent residence times. Modifications of compartment models made in order to account for complex sorption behavior of the radionuclides are discussed. Consequences for the design and interpretation of field studies are outlined.