Runoff is when water on non permeable surfaces just slides down that surface which eventually will make its' way to our water ways. For instance, if you wash your car on your cement driveway then the soapy water will create runoff and go into your street sewer which eventually will make its' way to a water way. With pesticides, if it rains when you put the pesticides on the crops the water will get those toxic chemicals and create runoff.
