The third element of the CWA addresses the National Pollutant Discharge Elimination System (NPDES). Under this act, any person responsible for the discharge of a pollutant or pollutants into any waters of the United States from any point source discharged from an industrial area and municipalities above 100,000 must apply for and obtain a permit. In 1987, this act was expanded to include non-point sources with amendments .
As amended, the act now requires that five categories of municipal or industrial storm water discharges be regulated as NPDES discharges:
Discharges which have NPDES permits issued as of February 1987
Discharges associated with industrial activity
Discharges from a municipal separate storm water sewer system (MS4) serving a population of 250,000 or more
Discharges from a municipal separate storm water sewer system (MS4) serving a population of 100,000 or more but less than 250,000
Other discharges designated by the EPA administrator or the State if such discharge "contributes to a violation of a water quality standard or is a significant contributor of pollutants to waters of the United States"