Who pays to clean up contaminated soil discovered while excavating on my Ontario property?

As the current owner, you are generally the one responsible for addressing contaminated soil discovered on your own property, even if the contamination originated from a previous owner's activity or from an entirely different source such as a neighbouring property. Ontario's environmental protection framework is generally structured around the current state and current ownership of land rather than assigning cleanup duty only to whoever historically caused a problem.

That doesn't mean the cost necessarily has to stay with you in the end. If the contamination can be traced to a previous owner who knew about it and failed to disclose a known hazardous condition, or to a neighbouring property from which contamination migrated, you may have a separate claim against that party to recover cleanup costs, though pursuing that claim is a distinct legal process from the ministry's expectation that the current owner deal with the contamination itself. Practically, discovering contamination mid-excavation usually means work stops while a qualified environmental consultant assesses the extent of the problem and next steps, since continuing to disturb contaminated soil without proper handling can create additional legal and health risks.