Stormwater starts off clean and flows DIRECTLY into our rivers, lakes and streams. Along the way, it picks up everything it comes in contact with as it flows over the land surface, roadways, sidewalks, parking lots, construction sites, business parks, etc. These materials become part of the stormwater runoff and are funneled through gutters, storm drains, canals, and drainage ways into local surface waters - UNTREATED! It is estimated that more than one-half of the pollution in our nation’s waterways comes from stormwater runoff.
In the past, it was thought that water pollution was caused mainly by industrial and municipal wasterwater treatment plant discharges. A lot of effort was put into cleaning up these "point sources" of waste water. Now, the effort is being expanded to clean up "non-point source" pollution, water pollution that is generated all over and carried to rivers and streams in pipes and ditches.
The problem with non-point source pollution is that it is very expensive to treat and discharge. Treatment facilities would have to be very large to treat storm peak flows and would sit unused more that 95% of the time.
During runoff events, pollutants carried by stormwater degrade the quality of the lakes, rivers, wetlands and other waterways they eventually enter. Nutrients such as phosphorus and nitrogen can promote the overgrowth of algae, deplete oxygen in the waterway and be harmful to other aquatic life. Toxic chemicals from automobiles, sediment from construction activities, and careless application of pesticides, herbicides and fertilizers threaten the health of the receiving waterway and can kill fish and other aquatic life. Bacteria from animal waste and illicit connections to sewerage systems can make lakes unsafe for wading, swimming and fishing.
In addition, uncontained stormwater runoff from construction sites may have devastating effects on local water bodies, particularly smaller streams, lakes and wetlands. During storms, construction sites may be the source of sediment-laden runoff, which can overwhelm a small stream’s channel, resulting in streambed scour, streambank erosion and destruction of near stream vegetative cover. Uncontrolled sediment-laden runoff is a principal contributor to the loss of in-stream habitats for fish and other aquatic species and flood storage capacity. Construction activities may also yield pollutants such as pesticides, petroleum products, construction chemicals, solvents, asphalts and acids that can contaminate storm water runoff.
Stormwater runoff cannot be treated using the same end-of-pipe controls appropriate for single point sources such as wastewater treatment plants. The best way to improve stormwater quality is to treat the source - don't let runoff get polluted in the first place. These methods are called Best Management Practices (BMPs).