Rainwater runoff which fl ows along impervious surfaces (roads, parking lots and sidewalks) is a major source of polluti on of receiving waters.
Such pollution sources derive from the formation of rainfall, as harmful substances are released in the atmosphere. Raindrops absorb substances while falling through the atmosphere and, when this is polluted, rainwater is added with such pollutants as: suspended solids, organic substances, nitrogen, phosphorus,
chlorides, etc. Once it reaches the ground, rainwater runoff is collected in stormwater drainage systems and discharged into receiving waters. Rainwater picks up further contaminants along the way, due to: pneumati c and brake abrasion, oil and fuel spills, anti freeze and road salt, urban waste, vegetati on and impervious surfaces.
Nowadays, specific laws have been introduced in many countries to regulate the collection and the treatment of the fi rst fl ush of runoff, which can seriously pollute receiving waters.