The Environment Agency is trying to find out who is responsible for spilling oil into a beck in the city.
Officers have put booms and absorbent mats in place to contain the pollution.
It’s at Blue Beck at Rawcliffe in York.
Blue Beck is the drain that starts at Rawcliffe Lake on the other side of Shipton Road, then flows in a culvert under Shipton Road rising again in a field north of the Clifton Hospital site.
If you have any information contact the Environment Agency.