The term "illicit discharge" is defined in Environmental Protection Agency's (EPA) Phase II storm water regulations as "any discharge to a municipal separate storm sewer that is not composed entirely of storm water, except discharges pursuant to an NPDES permit and discharges resulting from fire-fighting activities."
The problem is unlike sanitary sewers that divert water to a treatment plant directly from your home, storm drains lead directly to surrounding lakes and rivers without any type of treatment. All the debris and pollutants that were picked up by storm water runoff, end up in your lakes and streams!
Many of the chemicals and substances we encounter on a given day are not easily removed or separated from water. High concentrations of even common nutrients like nitrate and phosphorus (found in soap, fertilizers, animal/pet waste) can be detrimental to health of our waters.
So what can you do? One big step is being able to recognize an illicit discharge, safely stop it if possible or report it. The next section shows you some common examples.