An illicit discharge is defined in the federal regulations as:
...any discharge to an Municipal Separate Storm Sewer Systems (MS4) that is not composed entirely of storm water..."
There are two modes of entry: direct and indirect.
Flows in the storm drains created by illicit/improper connections between sanitary sewer and storm drain system
Flows in the storm drains created by pollutants that enter the storm drain system through an inlet or through the joints of the pipe (infiltration)
Sewage cross-connections with storm drains
Straight pipe connections
Industrial and commercial cross-connections with storm drains
Groundwater seepage into the storm drain pipe
Spills that enter the storm drain system at an inlet
Dumping liquid into a storm drain inlet
Outdoor washing activities that create flow to a storm drain inlet
Non-target irrigation from landscaping or lawns that reaches the storm drain system