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Municipal Separate Storm Sewer Systems (MS4)

On December 18, 2021, IDEM issued a Municipal Separate Storm Sewer General Permit (MS4GP).  The MS4GP replaces 327 IAC 15-13 (Rule 13) that had previously been the Administrative Code that established the permitting requirements for all designated MS4s in Indiana. stormwater

MS4s are defined as a conveyance or system of conveyances owned by a state, city, town, or other public entity that discharges to waters of the United States and is designed or used for collecting or conveying stormwater. Regulated conveyance systems include roads with drains, municipal streets, catch basins, curbs, gutters, storm drains, piping, channels, ditches, tunnels, and conduits. It does not include combined sewer overflows and publicly owned treatment works.

MS4 conveyances within urbanized areas have one of the greatest potentials for polluted stormwater run-off. The Federal Register Final Rule explains the reason as: “urbanization alters the natural infiltration capacity of the land and generates...pollutants...causing an increase in stormwater run-off volumes and pollutant loadings.” Based on increased population and proportionally higher pollutant sources, urbanization results “in a greater concentration of pollutants that can be mobilized by, or disposed into, stormwater discharges.”

The federal Clean Water Act requires stormwater discharges from certain types of urbanized areas to be permitted under the National Pollutant Discharge Elimination System (NPDES) program. In 1990, Phase I of these requirements became effective, and municipalities with a population served by a municipal separate storm sewer system (MS4) of 100,000, or more, were regulated. In 1999, Phase II became effective, and any entity responsible for an MS4 conveyance, regardless of population size, could potentially be regulated.

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The MS4GP provides permit coverage for Phase II MS4 entities. A general permit is a single permit that is written to cover multiple permittees. A specific permit is not written for each entity. Instead, the requirements and conditions of the MS4GP (effective December 18, 2021) applies to all Phase II MS4s upon submittal of a Notice of Intent (NOI).

As required under the MS4GP, the NOI is submitted to the agency, IDEM may conclude that conditions associated with the MS4 entity requesting permit coverage under the MS4GP are inappropriate for a general permit. In these situations, a specific permit or Individual MS4 Permit may be requested by IDEM. The rationale used by IDEM to make this determination would be provided to the MS4 entity asked to submit an individual MS4 permit application.

During development of the MS4GP, a workgroup was established to provide comments and input into the state's permit.

About Municipal Separate Storm Sewer Systems

Where can I find the Permit for MS4 entities in Indiana?

What criteria is used to designate MS4s in Indiana?

How can I find out which communities and entities are designated as MS4s by IDEM?

Quick References to MS4 Topics

For more information regarding MS4s and the MS4GP, please contact IDEM Stormwater Program Staff.