Illicit Discharge


Federal regulations define an illicit discharge as …any discharge to an MS4 that is not composed entirely of stormwater… with some exceptions. These exceptions include discharges from NPDES-permitted industrial sources and discharges from fire-fighting activities.

Illicit discharges are considered illicit because Municipal Separate Storm Sewer Systems (MS4s) are not designed to accept, process, or discharge such non-stormwater wastes.

Sources of Illicit Discharges

  • Car wash wastewaters
  • Effluent from septic tanks
  • Household toxics
  • Improper disposal of auto and
  • Improper oil disposal
  • Laundry wastewaters
  • Radiator flushing disposal
  • Sanitary wastewater
  • Spills from roadway accidents

Necessity of Illicit Discharge Detection & Elimination Efforts

Discharges from MS4s often include wastes and wastewater from non-stormwater sources. A study conducted in 1987 in Sacramento, California, found that almost one-half of the water discharged from a local MS4 was not directly attributable to precipitation runoff. A significant portion of these dry weather flows were from illicit and/or inappropriate discharges and connections to the MS4.

Illicit discharges enter the system through either direct connections (e.g, wastewater piping either mistakenly or deliberately connected to the storm drains) or indirect connections (e.g, infiltration into the MS4 from cracked sanitary systems, spills collected by drain outlets, or paint or used oil dumped directly into a drain). The result is untreated discharges that contribute high levels of pollutants to receiving water bodies, including:

  • Bacteria
  • Heavy metals
  • Nutrients
  • Oil and grease
  • Solvents
  • Toxics
  • Viruses

Pollutant levels from these illicit discharges have been shown in the Environmental Protection Agency (EPA) studies to be high enough to significantly degrade receiving water quality and threaten aquatic, wildlife, and human health.

Examples of Exempt Discharges

Not all non-stormwater discharges are considered illicit discharges. Examples of exempt discharges include:

  • Air conditioning condensation
  • Dechlorinated swimming pool discharges
  • Discharges from potable water sources
  • Diverted stream flows
  • Flows from riparian habitats and wetlands
  • Footing drains
  • Foundation drains
  • Individual residential car washing
  • Irrigation water
  • Landscape irrigation
  • Lawn watering
  • Rising ground waters
  • Springs
  • Street wash water
  • Uncontaminated ground water infiltration
  • Uncontaminated pumped groundwater
  • Water from crawl space pumps
  • Water line flushing