You can prevent water pollution by not dumping chemicals down your sewer line through your sink or toilet etc.
Those chemicals eventually wind up at the water treatment plant which has to work to filter out all those chemicals and then return it to the lake or wherever else the water originally came from.
Also avoid throwing trash, debris and other chemicals down near a storm drain since storm drains typically lead back to either a water treatment plant or sometimes storm drains go directly to lakes or other bodies of water.
Also avoid throwing medicines down the toilet to dispose of them because the medicine can pollute the water as well and try using environmentally friendly and safe cleaning liquids and cleaning solutions that don't use a lot of harsh chemicals that could pollute the water.