The Storm Water Phase II Rule is the next step in U.S. EPA's effort to preserve, protect, and improve the nation's water resources from polluted storm water runoff. Phase II is intended to further reduce adverse water quality and aquatic habitat conditions by instituting the use of controls on the unregulated sources of storm water discharges that have the greatest likelihood of causing environmental degradation.

The Phase II rule automatically covers on a nationwide basis all small municipal separate storm sewer systems (MS4s) located in "urbanized areas" as delineated by the Census Bureau, and those potentially designated MS4s located outside the urbanized areas. This regulation also affects runoff from prisons, hospitals, military bases, etc. which are located within the urban areas. The 2000 Census will determine the final list of communities. The Ohio EPA sent a letter [PDF 32K] to notify those automatically designated MS4s located within the urbanized area.