The Environmental Protection Agency defines illicit discharges in general as any discharge into a storm drain system that is not composed entirely of storm water. The exceptions include water from fire fighting activities, discharges from dechlorinated waterlines, fire hydrant flushing and discharges from facilities already under an NPDES (National Pollutant Discharge Elimination System) Permit.
Illicit discharges are a problem because unlike wastewater, which flows to a wastewater treatment plant, storm water generally flows to waterways without any additional treatment. Illicit discharges often include pathogens, nutrients, surfactants, and various toxic pollutants.
Examples of Illicit Discharges
Motor vehicle fluid spills or illegal disposal into a storm drain
Improper disposal of household or other hazardous materials, including paints and stains
Vehicle wash water from loading areas in the vicinity of storm drain inlets
Improper storage of oil, pesticides, dirt, or fertilizers
Improper pet waste disposal
Erosion of dirt or landscaping materials being swept into a storm drain or water body (including from construction sites)
Damaged sanitary sewer lines or sewer cross connections to storm water systems
Septic system failure
Examples of What You Might See That Result From Illicit Discharges