The water in this world is being contaminated every day by hundreds of different reasons. One of these reasons is people throwing rubbish in our rivers and streams at the top of mountains polluting water downstream.
As the toxic chemicals (PAHs) in rubbish break down in the water and it pollutes everything around it, which contaminates the water.
As the water starts to reach the coast and the ocean it gets salty and does not filter out any of the harmful chemicals.
As it reaches the ocean all the harmful toxins enter the ocean and it pollutes everything. This just shows how 1 little piece of rubbish can harm the environment.