When it rains or snows, the water flows on the ground, runs off solid (impervious) surfaces collecting pollutants (such as oil, pesticides, sediments, bacteria, and other pollutants) that are on those surfaces, and carries them into our waterways. This “runoff” can make our waterways an unhealthy place to live, work, and play. Untreated stormwater runoff entering our streams can result in the contamination of our drinking water supplies; prohibitions on swimming, fishing or boating uses; injury or death to aquatic plants and animals; dangers to public health; and increased flooding.
In many places, this stormwater runoff is conveyed through distinct sewer pipes that bypass the wastewater treatment plants and discharge the polluted runoff directly into our streams and lakes. This system is known as a “municipal separate storm sewer system” or MS4.