A 'slick' of pollution appeared on Thornton Beck near Thornton-le-Dale this week and Environment Agency staff were sent to inspect it.
The EA said: "We're investigating reports of pollution on Thornton Beck near Thornton le Dale, which we believe is caused by cooking oil. This will degrade naturally but an iridescent slick may be seen for a time. If you're disposing of cooking oil please do so correctly - not down the drain!"
Yorkshire Water have in the past run campaigns targeted at restaurants and takeaways on the correct disposal of oils and fats. By-products can cause blockages and 'fatbergs' to form in sewage networks, eventually polluting water courses.
Matthew Crawshaw, network protection team leader at Yorkshire Water, said: “The incorrect disposal of fats, oils and grease down drains and into the sewers can have a significant impact on our wastewater network. Once in the sewer fats can restrict the normal flow of wastewater and ultimately could cause a complete blockage of the network, resulting in sewage escapes and in the worst-case pollution to the local environment and watercourses."