That does occur and it is certainly part of the responsibilities of those individual site owners—DND or any other—to understand whether the contamination may have travelled off-site, to inform landowners, stakeholders, the public or indigenous peoples, as required, and then to conduct the testing that's needed to confirm the risk and contaminants that are present and assess those like they would on federal lands. Those are the basics.
It's always important to work with landowners and to consider not just federal guidelines but also possibly provincial guidelines in cases where the contamination has gone off of federal lands.