What Happens When It Rains?

When it rains, some of the water soaks into the ground, some evaporates, and some flows over impervious surfaces, such as paved streets, driveways and parking lots. The water that flows over the impervious ground is called “stormwater runoff”. Parking lots, buildings or other hard surfaces tend to have more stormwater runoff than undeveloped areas because there is nowhere for the water to soak into the ground.

Stormwater runoff picks up pollutants from the ground, such as oil dripped from cars, detergents from washing cars, trash, yard clippings, etc., and carries them into the nearest storm drain or drainage ditch. Ditches and storm drains do not connect to a treatment system, so everything that flows down the stormdrain goes directly to the nearest water body, ultimately flowing into the ocean.

These storm drainage systems are separate from the sewer systems that are designed to carry wastewater to a treatment plant, where it is treated before it is released into a body of water or sometimes reused. Stormdrain systems do not include any kind of treatment; everything they carry with them is emptied into a body of water, without ever being filtered or treated. This is why it is vital for everyone to pay close attention to cleanliness of driveways, yards and sidewalks. It is also very important to be mindful of chemicals used while washing cars or rinsing items outside, especially where the runoff can’t soak into grass or dirt areas.