It is hard to treat water pollution, but at least the polluting substances are all dissolved in the water and so they can be treated as the water passes through a treatment works. Oil is insoluble, it does not dissolve in water. Instead, it clings to, and coats, everything it touches.
If we use water to move it, the oil simply forms globules on the water and the problem gets worse. The only way to treat oil is with a detergent, and this is often dangerous for wildlife.
