Plant sirens sounded Thursday morning at Sarnia's Suncor refinery.
A message on the Community Awareness and Emergency Response Line says there was an on-site leak of boiler feedwater just after 9:30am.
The company says no hazardous materials were involved and the affected unit is being shut down to address the issue.
The Ministry of Environment has been notified.
The site is undergoing preparation work in advance of a spring turnaround next month.