Environment Agency officers are trying to identify chemicals that have leaked into the River Derwent following a fire at an industrial unit in Derby.
They have been checking what environmental impact the fire and ensuing explosions had on the surrounding area.
A series of explosions caused by gas cylinders were heard in Derby city centre and plumes of smoke could be seen for miles.
Derbyshire Fire Services confirmed they were called to Enterprise Way at 13:40.
People were urged to keep their windows and doors closed.
The Environment Agency have confirmed some "oily run off" reached the River Derwent.
Officers special floating 'booms' will be used to prevent the oil spreading.