MS4 Boundary Areas

MS4 boundaries must be established. The purpose in identifying the boundary of the MS4 is to provide IDEM the exact coverage area for the MS4 entity permit. In most situations, the boundary will be determined by the extent of the MS4 entity's jurisdictional area. However, in some instances, the MS4 area boundary may be identified by other means, such as the extent of a corresponding urbanized area.

A general permit issued under this Rule covers designated MS4 entity boundaries within Indiana and the jurisdictional area of the designated MS4 entity. The three exceptions related to coverage of MS4 entity jurisdictional areas are for:

  1. Designated counties, acceptable coverage may be limited to the boundaries of the mapped Urbanized Area (UA);
  2. MS4 entities sharing responsibilities for the rule requirements, acceptable coverage may be extended to the jurisdictional area of another MS4 entity with the written approval of that other MS4 entity; and
  3. MS4 entities sharing a jurisdictional area (such as when regulated universities or colleges overlap jurisdictions with a surrounding municipality), the jurisdictional area of the larger MS4 entity may exclude the regulated smaller MS4 entity jurisdictional area, as long as the smaller MS4 entity has a storm water permit addressing their jurisdictional area.

If a portion of a designated county is regulated, the boundaries of the regulated area must correspond to the nearest township or section containing the mapped urbanized area. Although many of the Census Bureau UA map boundaries are identified by street name, the regulated area must correspond to a township or section. The street name boundaries are simply a tool to determine the corresponding township or sectional boundaries. If a regulated municipality also has jurisdiction within a township or section shared by a regulated county, the boundary of the county can include language excluding the incorporated municipal areas within section X.

A county MS4 may choose to designate the entire county, excluding incorporated cities, towns, and municipalities. This coverage would be above and beyond the requirement to administer the program in the Census Bureau UA map boundaries. However, this option may allow the county to administer the program in an equitable manner and address storm water issues that may extend beyond the urbanized area.

The boundaries of an MS4 may also vary according to the required minimum control measures. For example, a regulated county may implement a county-wide program addressing construction site storm water runoff control or public education, but, in contrast, implement a program for illicit discharge detection and elimination in only the mapped urbanized area within their county. These boundary differences based on minimum control measures should also be provided in a narrative format and included in the SWQMP: Part C. If a county MS4 chooses to alter the boundaries beyond the urbanized area a written request should be sent to the attention of the IDEM MS4 Coordinator.

MS4 boundaries that change due to annexation should be updated with the submittal of the Annual Report or if a significant acreage change occurs the MS4 may choose to notify the IDEM MS4 Coordinator in writing.