The quantity of runoff generated depends on the frequency, intensity, and duration of precipitation in an area. The quantity of pollutants originating from highways and motor vehicles, however, is not well understood runoff is hard to measure and varies by location. Indirectly, vehicles also contribute by carrying solids from parking lots, urban roadways, construction sites, farms, and dirt roads. Secondary runoff pollutant sources associated with vehicular traffic include gas stations and other auto-related facilities, oil production and transportation operations, petroleum refineries, and improper disposal of used motor oil. Nitrogen and phosphorus-based nutrients generally originate from atmospheric and roadside fertilizer applications.
Atmospheric deposition is the main source of PCBs.
These pollutants can harm the environment in various ways. Toxic substances can affect human health or various plant or animal species.