A fire at a waste disposal site could lead to pollution of the nearby River Holme, the Environment Agency has said.
The blaze started at Hunters Waste Management in Huddersfield at about 17:00 BST on Thursday.
Firefighters are expected to be at the scene in Queens Mill Road in Lockwood until Saturday.
People living nearby have been advised to keep their windows and doors shut. West Yorkshire Police said road closures are in place around the scene.
The Environment Agency said the waste that is burning is non-hazardous, mixed household waste.
A spokesperson said: "Our officers are on site, assessing the potential impact on the environment, including that posed by the water run-off."
He said there was a potential risk to the nearby River Holme, which would be closely monitored.
"It appears that most of the run-off is entering the foul sewer, but there is a potential risk to the nearby River Holme, so we are investigating this risk further and will consider taking protective measures, if required.
"We will continue to actively monitor the situation on site."