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 groundwater, NPDES-permitted industrial sources, fire-fighting activities, water line flushing and air conditioner condensate. Illicit discharges are considered "illicit" because MS4s are not designed to accept, process, or discharge such non-stormwater wastes.
Common sources of illicit discharges:
- Sanitary Wastewater
- Effluent from septic tanks
- Car wash wastewaters
- Improper Oil disposal
- Radiator flushing disposal
- Laundry wastewaters
- Swimming pool filter backwash and pool discharges
- Spills from roadway accidents
- Improper disposal of auto and home toxic chemicals
Illicit discharges often enter the system through storm drain inlets. The result is untreated discharges that contribute high levels of pollutants, including heavy metals, toxics, oil and grease, solvents, nutrients, viruses, and bacteria to receiving waterbodies. Pollutant levels from these illicit discharges have been shown in EPA studies to be high enough to significantly degrade receiving water quality and threaten aquatic, wildlife, and human health.