Municipal Separate Storm Sewer Systems (MS4)

327 IAC 15-13 (Rule 13) is a storm water general permit rule. 327 IAC 15-13, regulates Municipal Separate Storm Sewer Systems (MS4s). 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 storm water. 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 storm water runoff. 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 storm water runoff 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, storm water discharges.”

The federal Clean Water Act requires storm water 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.

Under Phase I, Indianapolis was the only city to meet the population threshold criteria of 100,000. Since Indianapolis was the only city to meet the Phase I criteria, Indiana Department of Environmental Management (IDEM) chose to issue the city an individual storm water permit. Under Phase II, 327 IAC 15-13 was written to regulate most MS4 entities (cities, towns, universities, colleges, correctional facilities, hospitals, conservancy districts, homeowner's associations and military bases) located within mapped urbanized areas, as delineated by the United States Census Bureau, or, for those MS4 areas outside of urbanized areas, serving an urban population greater than 7,000 people.

The general permit rule, referred to as Rule 13, provides permit coverage for most Phase II MS4 entities. The rule was delayed by the intensive state rulemaking procedures. Although the federal deadline for state adoption was December 8, 2002, Indiana did not have its rule become effective until August 6, 2003.

IDEM foresees that the vast majority, if not all, of the Phase II MS4 entities in Indiana will be covered under general permits. A general permit is a single permit that is written to cover multiple permittees with similar characteristics. No written draft permit is issued to the permittee under a general permit. Instead, the requirements and conditions of this type of permit are found in Indiana Administrative Code, under the appropriate general permit rule. Notice of Intent (NOI) letters for this type of permit can be by either a single MS4 entity or multiple MS4 entities.

As the required information is submitted to the agency, IDEM may conclude that conditions associated with the MS4 entity requesting permit coverage under this rule are inappropriate for a general permit. In these situations, a tailored Individual MS4 Permit could 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 Rule, a workgroup was established to provide comments and input into the state's draft Rule 13 language. A guidance workgroup was also established to assist in development of a Rule 13 Guidance Document [PDF]. This guidance document was created to accompany the actual Rule 13 language. The document is written to correspond to Rule 13, and, for convenience, actually includes the majority of Rule 13 text (italicized text within quotations). In general, this document presents information that clarifies the various Sections and Sub-Sections of the Rule and provides instructions and ideas for developing a complete MS4 program. To allow for more flexibility, IDEM does not require a regulated MS4 entity to complete every item or suggestion that the guidance document specifies. Rather, the items serve as recommended or acceptable ways to meet the regulatory requirements.

An MS4 entity may develop alternative program elements to those recommended by this document, as long as they provide a similar level of protection to state waters.

About Rule 13 Municipal Separate Storm Sewer Systems

Where can I find the Rule that establishes MS4 entities in Indiana?

What criteria were used to designate MS4s in Indiana?

How can I find out which communities and entities were designated by IDEM?

Quick References to Rule 13 Topics

For more information regarding 327 IAC 15-13 (Rule 13), please contact the IDEM Storm Water Permit Coordinator.