A unit shutdown resulted in high flaring and noise at Imperial's Sarnia site Friday.
The Aamjiwnaang Notification System reported the company issued a Community Awareness Emergency Response (CAER) alert just after 9 a.m.
In a Facebook post, the company said it is making equipment repairs and working hard to return the site to normal operations.
Imperial had an equipment malfunction during the start-up of a process unit Thursday.
Plant sirens sounded at about 12:45 p.m. when a malfunction caused a release of light hydrocarbon to the air that contained a small amount of hydrogen sulphide.
The all clear was issued before 2pm.
