An oil spillage near the river Calder is being investigated as suspected fly-tipping.
A clean-up was carried out after the incident near Denby Dale Road which saw black waste oil seep into ponds and lagoons near the river.
Environmental officers visited the site to protect wildlife after reports that an oil container had been dumped there.
Wakefield Council and the Environment Agency said they were both investigating the incident.
A spokesman for the Agency said: “The Environment Agency is investigating a pollution incident in a watercourse near Denby Dale Road, Wakefield, following reports of oil in the water.
“Our officers have visited the site to assess the situation and to minimise any potential impact on the environment.
“It appears that the oil has entered the watercourse from a fly-tipped oil container.”
Officers worked to contain the spillage and prevent the waste oil seeping further into watercourses, the spokesman said.
He added: “Our contractors have carried out work along the watercourse to clean up the oil to minimise the impact downstream.
“If anyone sees any pollution, they are urged to report the matter to our incident hotline on 0800 807060 so we can investigate.”
Fly-tipping can be reported to Wakefield Council’s enforcement team by calling 0345 8 506 506. Dumping household, industrial and commercial waste illegally carries a fine of up to £50,000.