NPDES Phase II covers two classes of stormwater dischargers:
Operators of small construction activities that disturb equal to or greater than one and less than five acres of land or part of a greater common development
Operators of small municipal separate storm sewer systems (MS4s) located in "urbanized areas" Populations of 10,000 or more, as assigned by the Bureau of the Census
Phase II Rule
The EPA established the Phase II Rule making the MS4 a stormwater management program which intends to improve the nation's waterways by reducing the quantity of pollutants that stormwater collects after a rain event. These pollutants are transported to nearby waterways during a storm event. Polluted stormwater can impair waterways, leading to the closure of beaches and recreational areas, and occasionally, contaminate drinking water.