Imperial reported an overflow of water to the St. Clair River in Sunday afternoon's thunderstorm that brought heavy rain to the area.
Sarnia Refinery Manager Rohan Davis said in an email that the incident occurred at about 5:25pm but no impact to downstream water quality was expected.
Monitoring was underway and the Ontario Ministry of the Environment Spills Action Centre was notified.
Davis said the exact cause of the incident is under investigation.
He says members of the public who would like more information may call Imperial's 24-hour number at 519-339-5666 in Sarnia and toll-free in area codes 519 and 810 at 1-866-288-2202.