Fire and hazmat crews were called to Havre de Grace Monday morning for a report of a "petroleum product in a stream."
The call came in around 11:15 a.m. from the 800 block of John Smith Street, according to monitored Harford County emergency radio broadcasts.
Chief Scott Hurst, of the Susquehanna Hose Company of Havre de Grace, said the amount spilled was minimal and authorities did not know the source of it.
He said booms were placed to collect the spill and the scene was cleared.
Hurst confirmed that the substance got into the stream, which flows into the Susquehanna River around the city's north side McLhinney Park.
Additional details were not immediately available.