Designation of New MS4s

If a previously unregulated MS4 entity is determined to be a documented significant contributor of pollutants (MS4 Designation Criteria) to waters of the state or a regulated MS4 area, the entity can be designated as an MS4. To designate an entity requires written documentation. This documentation should be attributable to the storm water discharge of the entity and include:

  1. Analytical pollutant concentration, biological assessment, or loading data (i.e., turbidity, pH, presence of E. coli bacteria, phosphorus, etc.);
  2. Any visible pollutant evidence (i.e., photographs of algae blooms, discoloration, petroleum sheening, dead fish, erosion, etc.);
  3. Any modeling (i.e., hydraulic, hydrological, sediment transport, pollutant loading, etc.); and
  4. Supporting statements (i.e., how is the unregulated discharge(s) impacting a water body or regulated MS4 area) as to why the MS4 entity should be designated.

An entity may also be designated if other local programs are ineffective in protecting water quality. The ineffectiveness of other programs can have many causes, and includes non-applicability of other rules to an entity, permit coverage that does not address storm water quality, unsuccessful voluntary approaches, and noncompliant actions related to storm water quality by an entity. For these designation criteria to be used, a documented water quality problem attributable to the entity’s storm water discharge must be proven.

IDEM will review all information and documentation that is submitted and determine that either:

  1. The entity is determined to be a documented significant contributor of pollutants to waters of the state or another regulated MS4 entity, and should be regulated under Rule 13, or, in some instances, require an individual NPDES storm water permit;
  2. The submitted documentation is incomplete and additional evidence is required before a determination by IDEM can be made; or
  3. The submitted documentation is inaccurate or insufficient, and the entity is determined not to be a documented significant contributor of pollutants to waters of the state or another regulated MS4 entity.

The rule also provides an option to designate an entity if discharges associated with the local conveyances are to a sensitive area. By definition in the rule, sensitive areas include habitat areas for threatened or endangered species, intake areas for public surface water supplies, areas used for full body contact recreation, and areas classified as Outstanding State Resource Waters. Designation of an entity based solely on discharge to a sensitive area is not likely, but sensitive areas will be criteria for designation consideration when used in combination with the quantity, quality, and location of the entity’s storm water discharges.

As information and data become available, the designation of MS4 entities may change. When changes occur, IDEM will notify newly designated MS4 entities in writing.