In 1987, an amendment to the federal Clean Water Act required implementation of a two–phase, comprehensive national program to address storm water runoff.
Phase I focused on large construction sites, 11 categories of industrial facilities, and major metropolitan municipal separate storm sewer systems (MS4s).
Phase II is the next evolutionary step in an effort to preserve, protect and improve water resources impacted by storm water runoff. The program has broadened to include smaller construction sites and small municipalities (populations of less than 100,000) that were temporarily exempted based on their industrial activity. Phase II is designed to further reduce adverse impacts to water quality, by placing control on runoff that is the most likely to cause continued environmental damage.
Storm water regulations are part of the National Pollutants Discharge Elimination System (NPDES) permit program which is mandated to states under the Clean Water Act and administered by the U.S. Environmental Protection Agency (EPA). The EPA delegated permitting authority for Minnesota's NPDES program to the Minnesota Pollution Control Agency and North Dakota's program to the North Dakota Department of Health. The three permit types—construction, industrial, and MS4— have distinct requirements.