What is an MS4?

MS4 stands for "municipal separate storm sewer system." It is a drainage system owned by a municipality intended to carry only surface runoff i.e. storm water. A separate sewer is not intended to, nor should it, carry storm water combined with sanitary sewage or with any other pollutant.

What is a municipality?

Under the Phase II program, the following are considered municipalities:

  • United States, a state, city, town, borough, county, parish, district association or other public body.
  • U.S. military installations, state or federal hospitals, county prison complexes, state colleges or universities, highways and other thoroughfares.

Why is my town regulated?

A drainage system is automatically regulated if:

  • The system discharges at one or more a point sources,
  • The drainage system is a separate storm sewer system (not designed to carry combined storm water and sanitary waste water),
  • The drainage system is operated by a public body,
  • The drainage system discharges to the Waters of the United States or to another MS4, AND
  • The drainage system is located in an "Urbanized Area".

In the Phase II Rule, EPA uses the US Bureau of Census definition of Urbanized Area (UA). The mapping of Urbanized Areas is based on the most recent federal census data. This definition is uniform nationally and has a statistical basis.

My town has only a little bit of Urbanized Area. What do I do?

Towns partially in a UA need to comply with Phase II in the UA. If the population in your UA area is less than 2000 you can apply to EPA for a waiver. Otherwise, you may find it helpful or economical to combine efforts with the neighboring town which has to comply also with Phase II in a designated urbanized area.

What does the town have to do to meet the requirements of the Phase II permit?

Develop and implement a storm water management program to reduce the discharge of pollutants from your MS4 to the Maximum Extent Practicable. Read and comply with the National Pollutant Discharge Elimination System (NPDES) General Permit For Storm Water Discharges From Small Municipal Separate Storm Sewer Systems (the MS4 General Permit) (See "General Permit for Storm Water Discharges..." under the heading "Small MS4 Program.") This involves development and implementation of a Storm Water Management PROGRAM (SWMP) that reduces the discharge of pollutants from your MS4 to the Maximum Extent Practicable. "Maximum extent practicable" consists of a SWMP that covers 6 areas of work. EPA calls these "minimum control measures." For each minimum control measure the SWMP should list appropriate Best Management Practices (BMPs), who will do the work, and when it will be done.

What is a BMP?

"Best Management Practice" (BMP) is a vague term, broadly used to describe the most effective, feasible method that does the job. In the context of storm water management, it is often used to mean a structure or technology used to manage or treat the water such as a hooded catch basin, detention basin, or a filter system. The term BMP is also used for behavioral practices such as timely cleaning of catch basins, or habitual closing of the lid on a dumpster (avoiding dumpster brew when it rains). A BMP can even be restraint of a specific behavior such as minimizing the use of lawn fertilizer, or of road salt and sand. Look at the websites listed in Phase II Help for examples of BMPs.

How do we do it all?

Start with detection and elimination of illicit discharges. Map your outfalls and receiving waters. Set priorities based on where you find problems in your municipality and devise a system to locate and eliminate misconnections. Also, devise a system to prevent other illicit discharges (dumping). Go on from there, working through each of the minimum control measures. Build on (and put into the SWMP) things you are already doing, for example, Household Hazardous Waste Day can be public education, public participation, and part of the program to reduce illicit discharges. If your town already has an erosion and sedimentation control bylaw, it can become part of the construction site runoff control program required by the permit. Choose new BMPs to address the types of contamination observed at mapped outfalls.

Does Phase II have any numerical concentration limits for contaminants?


What IS allowed in a municipal separate storm sewer?

Storm water is storm water runoff, snowmelt runoff, and surface runoff and drainage and those discharges are allowed. Other allowable non-stormwater discharges are listed in Part I. F. of the MS4 General Permit.

How do we pay for all this?

The Phase II Rule acknowledges that financial considerations are part of what should be considered in determining "maximum extent practicable." You may not be able to do everything all at once but you can do something. Document your financial considerations. Budget the work and keep going.

Is there funding for Phase II?

There is no dedicated funding source at the present time. Low interest loans may be available through the Clean Water State Revolving Loan Fund (SRF) for planning or if there is a significant discharge of bacteria to be corrected. For information about SRF check the MassDEP website.

Is the Massachusetts Highway Department's drainage system an MS4?

Yes. In urbanized areas, Massachusetts Highway Department's drainage is regulated by Phase II and must comply with the MS4 general permit. This is true for all separate storm sewer systems operated by a public body, e.g. the University of Massachusetts, Massachusetts Turnpike Authority, or the Metropolitan District Commission (MDC).

How do I apply for a permit?

First, read the MS4 General Permit. The application process is described in Part I. E.