When it rains, or when water is used outdoors, some of the water soaks into the ground. However, with increasing impervious surface (roofs of houses, pavement, etc), more water runs off the land, because impervious surfaces do not allow water to soak through. This is stormwater runoff, and it picks up pollutants are from the ground and carries them into the nearest storm drain or drainage ditch alongside the street. Ditches and storm drains do not connect to a treatment system, so everything that flows down the drain goes directly to the nearest water body, ultimately flowing into the ocean.
The storm drainage system is separate from the sewer system. The sewer system carries water through a water treatment plant where it is treated before it is released into a body of water. The storm drainage system does not include a water treatment plant. Stormwater and everything it carries with it is emptied into a body of water, without ever being treated. This is why it is vital for everyone to pay close attention and work to prevent stormwater pollution!