"Illicit Discharges" are any discharges into the City's storm system that are not composed entirely of stormwater, with some exceptions. These exceptions can be found in Section 13.02.030 of MLMC.
Dumping of materials into the storm system is not only a bad idea: it's against the law. This includes the following:
If you see a suspicious discharge, or have had a spill and need assistance, please call the Spill Hotline (509)764-3792 or make an Illicit Discharge Citizen Report.