As stormwater runs off of rooftops and lawns and down driveways and streets, it picks up pollutants and debris. In developed areas where much of the land surface is covered by buildings and pavement, water cannot soak into the ground.
As a result, the stormwater flows over these impervious surfaces and picks up many toxic chemicals from motor oil, lead from auto exhaust, and zinc from roof drains and tires. Stormwater may also contain fertilizer, pesticides, leaves and grass clippings, sediment, bacteria, viruses, and other disease-causing microorganisms can abound in storm water that carries pet waste and litter. These chemicals may kill or impair the health of aquatic life. They make waterways unsafe for swimming and other types of water recreation.