Storm water is one of the leading causes of water pollution nationally, and urban runoff is the top category of nonpoint source pollution in the New Hampshire Nonpoint Source Management Plan. Storm water is regulated by the U.S. Environmental Protection Agency under the Clean Water Act. Since March 2003, municipalities and developers have been subject to new requirements dealing with storm water management.

The new requirements are called Phase II Storm Water Regulations since they are the second round of storm water rules implemented by EPA. Phase I, passed in 1992, dealt with larger municipalities (none in New Hampshire), privately-owned industries, and construction sites of 5 acres or larger. (Fact Sheet on Phase I and Phase II in New Hampshire)

The National Pollutant Discharge Elimination System (NPDES) program is administered by EPA in New Hampshire. This site is designed to assist regulated entities in New Hampshire in complying with Phase II.