What is an "illicit discharge" to a stormwater conveyance system?
An illicit discharge is any discharge to a stormwater conveyance system, river, stream or other waterbody that is not composed entirely of stormwater. An illicit discharge could be the result of someone dumping a pollutant (automobile fluids, paint etc) into the conveyance sytem; the result of an illicit connection into the conveyance system, such as a sewer pipe 
connected to the stormwater conveyance system; or a pipe that bypases the sanitary connection or septic drain field, producing a direct discharge into open channels or streams.
What is an "illicit connnection" to a stormwater conveyance system?
An illicit connection is an improper physical connection of illicit discharges to the stormwater conveyance system. Examples include: a sewer pipe that is connected to the stormwater conveyance system that produces a continuous discharge of raw sewage to the conveyance system, a shop floor drain that is connected to the stormwater conveyance system producing a discharge of wash water or other inappropriate flows into the storm sewer system.
What are the hazards associated with illicit discharges?
Illicit discharges can contribute high levels of pollutants to waterbodies. Pollutants commonly found in illicit discharges include raw sewage (viruses and bacteria), heavy metals, toxics, oil and grease, solvents, nutrients, . Pollutant levels from these illicit discharges have been shown in EPA studies to be high enough to significantly degrade water quality, close beaches, and threaten aquatic, wildlife, and human health.
So how can you tell a normal stormwater discharge from an illict discharge?
Dry weather flow is the best indication that the water coming from an outfall pipe is from an illicit connection. If it has not rained in at least three days and there is water coming from a stormwater outfall pipe, chances are that water is from an illegal source.
Although it is a good indication, dry weather flow is not always from an illicit source. Sometimes groundwater seeps into the stormwater conveyance system and is discharged through the outfall pipe. This is a natural source and is not illicit.