Mass exposure and mass transport can be viewed as competing processes with regard to controlling the rate of release of a species from a Source. Typically, one of these processes dominates for a given species in a Source.
For example, for a Source consisting of a highly insoluble species spilled directly onto the ground surface, the concept of containment or matrix degradation is not applicable, since the contaminant is exposed immediately and available for mass transport. In this case, the release rate would be equal to the mass transport rate, which in turn is controlled by the solubility limit.
If, however, water containing a soluble species (e.g., chloride) was stored in a number of drums which gradually failed over time, then the exposure rate (i.e., the rate of drum failure) would likely control release since the species would be transported away as quickly as it was exposed.
In the first case, you would know beforehand that mass transport always controls release, and you could therefore simulate the system without even using a Source. Note, however, that within a Source, different species could be controlled by completely different mechanisms, and you may not be able to specify which mechanism is dominant for a given species beforehand. Moreover, the dominant mechanism may change for any given species from realization to realization.
For example, if the contaminated water within the drums contained a relatively insoluble species (e.g., lead) in addition to the chloride, then, depending on the solubility and the container failure rate, the release rate for this species could be controlled by mass transport considerations, even though the species was being exposed relatively gradually due to drum failure. This is because the low solubility could limit the rate at which mass was transported away. In such a case, exposed lead would simply “accumulate” at the Source as more drums failed, with the release rate from the Source remaining relatively steady due to the solubility limitation on mass transport.