Whatever goes into your storm drain flows through storm pipes or open channels, then discharges into streams, lakes, rivers, and eventually Puget Sound. It doesn't stop at a treatment facility along the way, which is why it's so important to make sure pollutants stay out of your storm system.
A good rule of thumb? If you wouldn't want to swim in it or let your pets drink it, keep it out of your storm system. Remember...nothing but rain down the drain!