Stormwater originates from rainfall and other precipitation that runs off of surfaces all over the city - rooftops, streets, lawns, etc. Stormwater makes its way into natural water sources and takes everything from city surfaces with it to these areas.
Polluted stormwater runoff is commonly transported through municipal separate storm sewer systems, known as MS4s. Winchester is an MS4. Illicit discharges are non-stormwater discharges into MS4s. Common examples of illicit discharges are yard waste, swimming pool water, household wastewater, car washing wastewater, and car fluids to name a few.
When you wash your car on a compacted surface, the runoff containing soap, heavy metals, road grime, greases, and oils can enter the storm drains. Unlike sanitary drains, storm drains usually run directly into creeks, ponds, and lakes. Use a commercial car wash facility or find a grassy area at home to soak up the runoff.