The Delaware Department of Natural Resources and Environmental Control reported the release of 3,000 pounds of sulfur dioxide into the atmosphere at the Delaware City Refinery, owned by PBF Energy.
The release took place at 5:30 p.m., Wednesday.
According to a DNREC report the release took place during repairs to a boiler at at the refinery.
Sulfer dioxide, a byproduct of the refining process, is considered to be hazardous by the EPA.
