Can all water pollution be seen?

Not all can be seen. Some water contaminants that can be seen are floatable materials--anything that floats making disrupting the intent use of the water. Oil and grease--coulbe from boats, ships, oil spills, etc. Of course, water is supoose to be colorless. When this property is not present, the water is polluted. I think most of the pollutants cannot be seen; these are dissolved organics and metals.