Why is storm-water runoff or drain runoff a significant contributor to non-point source pollution? ...

In most urban areas, non-point source pollution arrives at the ocean via runoff from storm drains, many of which now have labels indicating that they lead to the ocean. The U.S. National Academy of Sciences estimates that 5.8 million metric tons (13 billion pounds) of litter enters the world's oceans each year. Contrary to popular belief, water (and any other material) that goes down a storm drain does not receive any treatment before being emptied into a river or directly into the ocean. Sewage treatment plants receive enough waste to process without the additional runoff from storms, so it is important to monitor carefully what is disposed into storm drains. For instance, some people discharge used motor oil into storm drains, thinking that it will be processed by a sewage plant. A good rule of thumb is this: Don't put anything down a storm drain that you wouldn't put directly into the ocean itself.