Look down into a catch basin and observe what is inside. When too much fertilizers and pesticides are applied they get transported to the water table. You may be surprised to know that animals pollute also. Dog waste in the street can be carried by runoff water. Usually bacteria are removed as water passes through soil because it acts as a filter.
Some parking spaces have oily stains from cars.
Most of the water that falls on this parking lot ends up in a catch basin. Motor oil leaking from a car is a mixture of many different organic compounds and heavy metals. Many are toxic or can cause cancer. Storm water drain from parking lot opening into stream valley shown on right. This parking lot is slightly lower in elevation than the surrounding area.