Phase 1 of the U.S. Environmental Protection Agency's (EPA) stormwater program was promulgated in 1990 under the Clean Water Act (CWA). Phase 1 relies on National Pollutant Discharge Elimination System (NPDES) permit coverage to address stormwater runoff from incorporated places with a population of 100,000 or greater on the 1990 U.S. census and construction sites larger than five acres.
The Stormwater Phase 2 Final Rule is the next step in EPA's effort to preserve, protect, and improve the nation's water resources from polluted stormwater runoff. The Phase 2 program expands the Phase 1 program by requiring additional operators of MS4s in urbanized areas and operators of small construction sites, through the use of NPDES permits, to implement programs and practices to control polluted stormwater runoff. Phase 2 is intended to further reduce adverse impacts to water quality and aquatic habitat by instituting the use of controls on the unregulated sources of stormwater discharges that have the greatest likelihood of causing continued environmental degradation.