I'll build on part of the earlier responses from me and Mr. Carreau.
Hydrologic-type modelling can help us understand how contamination moves in the environment. Ultimately, in the case you're thinking of, that analysis would have been done by National Defence.
However, custodians work with local municipalities and provinces to identify other potential sources of contaminants, be they a garage, a repair shop or another group that uses chemicals of this nature. From there, you can start to identify those other sources and then how much they may have contributed to a given—