The NPDES Stormwater Phase II Final Rule covers a small subset of small MS4s called regulated small MS4s. A small MS4 is designated as a regulated small MS4 in one of three ways:
Automatic Nationwide Designation - An MS4 is automatically designated if it is located within the boundaries of an urbanized area (UA) as determined by he Bureau of the Census based on data from the current census.
Potential Designation by the NPDES as a result of Meeting Specific Criteria - The Phase II Rule requires the NPDES permitting authority to establish and apply a set of criteria to MS4s outside of a UA, located in an area with a population of at least 10,000 and a population density of at least 1,000 people per square mile.
Physically Interconnected - Any small MS4 located outside of a UA that contributes a large amount of pollutants to a physically interconnected MS4, which is already permitted by the NPDES stormwater program, must be designated.
Potential Designation After Evaluation by the NPDES - Additional MS4s may be designated as regulated small MS4s if the NPDES permitting authority determines that stormwater discharges into a local water body cause, or have the potential to cause water quality problems.
