Protecting Public Health and the Environment.

Stormwater NPDES Permits

Stormwater is water from rain or melting snow that does not immediately soak into the ground. Stormwater runs off of land and hard surfaces such as streets, parking lots, and rooftops, and picks up pollutants, such as fertilizers, dirt, pesticides, and oil and grease. Eventually, stormwater soaks into the ground or discharges to surface water (usually through storm drains), bringing the pollutants with it. Most stormwater discharges are considered point sources and require coverage by an NPDES permit.

Regulated businesses and industry must obtain coverage under an NPDES stormwater permit and implement stormwater pollution prevention plans or stormwater management programs that effectively reduce or prevent the discharge of pollutants into receiving waters.

An NPDES stormwater permit may be required for the following:

  • Construction activities that disturb one acre or more of land, including clearing, grading, and excavation activities
  • Industrial activities specifically listed by EPA
  • Municipal separate storm sewer systems (MS4), which are a city's or town's storm drains


Stormwater runoff from construction activities can have a significant impact on water quality, as it carries sediment and other pollutants exposed at construction sites to surface waters. The NPDES Stormwater Program requires operators of construction sites that disturb 1 acre or more to obtain authorization to discharge stormwater under an NPDES construction stormwater permit. 

Construction activities in Idaho are covered by a general permit for stormwater discharges from construction sites. This permit outlines a set of provisions construction operators must follow to comply with the requirements of the NPDES stormwater regulations. This permit covers any site 1 acre and above, including smaller sites that are part of a larger common plan of development or sale.

In order to be covered under the construction general permit, a site-specific stormwater pollution prevention plan must be developed. The construction manager must document the erosion, sediment, and pollution controls she intends to use, inspect the controls periodically, and maintain the controls throughout the life of the project.

When a stream is on Idaho's list of impaired waters (the "§303(d) list"), DEQ must incorporate a gross wasteload allocation for anticipated construction stormwater activities into the stream's water quality improvement plan (TMDL). If a TMDL has been established for the water where a project will discharge, and the TMDL indicates that it applies to construction or stormwater discharges, then the stormwater pollution prevention plan must be consistent with the requirements of that TMDL.


Activities that take place at industrial facilities, such as material handling and storage, are often exposed to stormwater. The runoff from these activities discharges industrial pollutants into nearby storm sewer systems and water bodies. This may adversely impact water quality.

Operators of industrial facilities included in one of 11 categories of industrial stormwater dischargers that discharge stormwater to a MS4 or directly to waters of the United States require authorization under a NPDES industrial stormwater permit.  EPA's Multi-Sector General Permit is the general permit currently available to facility operators in Idaho. If this general permit is not applicable to a specific facility, the facility operator must obtain coverage under an individual NPDES permit.

Municipal Separate Storm Sewer Systems

An MS4 is a city sewer system designed to collect only stormwater. By definition, an MS4 must be owned by public entity, discharge to surface waters of the United States, and not be part of a combined sewer or a publicly owned treatment works.

MS4s are classified in the federal regulations based on the population they serve (large, population >250,000; medium, 100,000 - 250,000; small <100,000). Under the NPDES Stormwater Program, stormwater discharges from large, medium, and "regulated" small MS4s must be authorized under an NPDES permit. A "regulated small MS4" is any small MS4 located in an "urbanized area" as defined by the Bureau of the Census and some small MS4s located outside of urbanized areas that have been designated as such.

Permitted MS4s must reduce and control stormwater runoff and detect and eliminate non-stormwater discharges to the system. See specific requirements for medium and large MS4s and for small MS4s. 

Industries and businesses that are located in areas with permitted MS4s may be subject to the MS4's stormwater requirements. Many communities also have local stormwater ordinances.