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WHAT IS AN MS4?
An MS4 is a conveyance or system of conveyances that is:
owned by a state, city, town, village or other public entity that discharges to waters of the U.S.; designed or used to collect or convey stormwater (including storm drains, pipes, ditches, etc.); not a combined sewer; and not part of a publicly owned treatment works (sewage treatment plant).
Polluted stormwater runoff is commonly transported through municipal separate storm sewer systems (MS4s), from which it is often discharged untreated into local waterbodies. To prevent harmful pollutants from being washed or dumped into an MS4, operators must obtain a Kentucky Pollutant Discharge Elimination System (KPDES) permit and develop a stormwater management program.
Phase I, issued in 1990, requires medium and large cities or certain counties with populations of 100,000 or more to obtain National Pollutant Discharge Elimination System (NPDES) permit coverage for their stormwater discharges. There are two Phase I MS4 programs in Kentucky, Lexington and Louisville. They are both large MS4s.
Phase II, issued in 1999, requires regulated small MS4s in urbanized areas, as well as small MS4s outside the urbanized areas that are designated by the permitting authority, to obtain NPDES permit coverage for their stormwater discharges. There are nearly 100 Phase II MS4 programs.
Generally, Phase I MS4s are covered by individual permits and Phase II MS4s are covered by a general permit. Each regulated MS4 is required to develop and implement a stormwater management program to reduce the contamination of stormwater runoff and prohibit illicit discharges.
The MS4 program consists of several requirements, referred to as the six minimum control measures and are as follows: public education and outreach; public participation and involvement; illicit discharge detection and elimination; construction site runoff control; post-construction runoff control; and pollution prevention and good housekeeping for municipal operations. The Phase I communities have additional requirements of monitoring outfalls and inspection of industrial facilities located in the MS4 boundary.
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