What is an MS4? 

The initialism “MS4” (pronounced M-S-4) is short for municipal separate storm sewer system.  It is used to refer to an entity who is regulated by an MS4 Clean Water Act permit or the separate storm sewer conveyance system.


Basic 101

Stormwater discharges from MS4 conveyances (e.g., pipes and ditches) do not flow to a local wastewater treatment plant.  Instead, they are going directly into our local creeks, streams, and rivers without treatment.  This is concerning because of the potential pollutants stormwater runoff can carry to our local waterbodies from various land uses.


Indiana MS4 Program Background

The Indiana Department of Environmental Management (IDEM) has updated its Clean Water Act, National Pollutant Discharge Elimination System (NPDES) stormwater Phase II MS4 general permit (GP).  An MS4 is defined as “a conveyance or system of conveyances (including roads with drainage systems, municipal streets, catch basins, curbs, gutters, ditches, man-made channels, or storm drains)” that is (1) designed or used for collecting or conveying stormwater and (2) is owned or operated by a city, county, or other governmental entity (including federal and state entities).  The term “MS4” specifically excludes (1) combined sewers and (2) systems that are part of a publicly owned treatment works. 

The first MS4 GP became effective in 2003.  Approximately 180 Indiana Counties, Cities, Towns, Conservancy Districts, Universities (including Purdue), Colleges, etc. were designated by IDEM and US EPA as being subject to permit coverage and are referred to as “MS4s”.  On April 13, 2022, IDEM issued notification to approximately 45 additional local entities that IDEM has designated them as new MS4s. 

Annotated MS4 General Permit 


Components or MCMs of MS4 Program

The primary components of the MS4 program are permit requirements called “Minimum Control Measures” or MCMs and are the following:

  • MCM 1 - Public Education and Outreach – includes educating the general public and selected stakeholders
  • MCM 2 - Public Participation/Involvement – includes complying with official public notice requirements and engaging the general public
  • MCM 3 - Illicit Discharge Detection and Elimination (IDDE) – is intended to minimize non-stormwater discharges by finding and fixing issues with the separate stormwater conveyance system; includes mapping of the system and screening of stormwater outfalls as well as a local ordinance requirement
  • MCM 4 - Construction Site Runoff Control – includes a local ordinance requirement that includes at least all the requirements of the IDEM construction stormwater general permit
  • MCM 5 - Post-Construction Runoff Control – same as MCM #4 plus requires MS4s to include Low Impact Development and Green Infrastructure programs
  • MCM 6 - Pollution Prevention/Good Housekeeping (PP&GH) – includes requirements for maintenance of the separate storm system conveyance and for all municipally owned operations and facilities

LTAP has developed training videos and reference guides for MCM 5 and MCM 6. To access the videos, visit our Learning Management System. Reference guides, in the form of quick reference factsheets, are available for each training module and can be accessed by clicking the specific MCM link above.


Indiana LTAP Model Stormwater Management Ordinance and Technical Standards

MCMs 3, 4, and 5 (IDDE, construction, and post-construction) require MS4s to develop and implement a local ordinance or ordinances.  To assist with promoting statewide consistency among these local ordinances, several regulated MS4 communities requested that an entity such as LTAP develop a model stormwater management ordinance for them to use. 

LTAP accepted this challenge, and these documents are now ready!  The ordinance and technical standards meet the minimum IDEM requirements for general permitting and also contain recommended requirements for stormwater management. 

You can download the ordinance and technical standards here:


New Permit Requirements for Mandatory Training

IDEM released the final version of their newly updated MS4 GP which became effective on December 18, 2021.  This newly updated permit contains several new and more robust requirements including the following training activities with their mandated compliance schedule: