|Urban areas create problems with both water quality and water quantity. When impervious surfaces in an urbanizing area cover more than ten percent of the land surface in a watershed, without BMP's, the streams begin to degrade. Above twenty-five percent impervious surface, streams become severely impacted. Impervious surfaces prevent rainwater from infiltrating into the ground, thus creating large volumes of surface runoff. Many urban areas have a storm drain network to carry runoff away rapidly to the nearest stream. Thus tremendous volumes of stormwater may flow rapidly into streams and rivers causing flooding.
- Urban runoff impacts water quality because the runoff can carry concentrations of heavy metals from transportation areas
- Excess nutrients from over application of fertilizers around homes, golf courses, and parks
- Petroleum products from spills and leaks
- Bacteria and pathogens from pet waste and leaking sewer systems
- Trash thrown into streams or washed off streets into the storm drains
- Sediment from construction