Types and causes of urban stormwater pollution

Stormwater pollutants originate from many different sources ranging from fuel and oil from our roads, to litter dropped on our streets and sediment from building sites. There are three main types of stormwater pollution:

litter, for example cigarette butts, cans, food wrappers, plastic bags or paper
'natural' pollution, for example leaves, garden clippings or animal faeces
chemical pollution, for example fertilisers, oil or detergents.

Below is a table of the common pollutants found in urban stormwater, their likely sources and the effect of the pollutant on our waterways.

Pollutant	Effect	Urban source
Sediment	Reduces the amount of light in the water available for plant growth and thereby reducing the supply of food for other organisms. Can clog and damage sensitive tissues such as the gills of fish. Can suffocate organisms which live on or in the bed of lakes and streams by forming thick deposits when this suspended material settles out.	Land surface erosion Pavement and vehicle wear Building and construction sites Spillage, illegal discharge Organic matter (for example leaf litter, grass) Car washing Weathering of buildings/structures Atmospheric deposition.
Nutrients	An increase of nutrients in water stimulates the growth of aquatic plants. This causes excessive growth of aquatic weeds and algae that may choke lakes and streams and lead to dramatic daily fluctuations in dissolved oxygen levels.	Organic matter Fertiliser Sewer overflows, septic tank leaks Animal faeces Detergents (car washing) Atmospheric deposition Spillage, illegal discharge.
Oxygen demanding substances	Oxygen is used up more quickly than it can diffuse into the water from the atmosphere. The resulting drop in oxygen levels may then be sufficient to kill fish and other aquatic organisms. If all the oxygen in the water is used up, unpleasant odours can result.	Organic matter decay Atmospheric deposition Sewer overflows, septic tank leaks Animal faeces Spillage, illegal discharges.
pH acidity	Increased acidity damages plants and animals	Atmospheric deposition Spillage, illegal discharge Organic matter decay Erosion of roofing material.
Micro-organisms	Contain very high numbers of bacteria and viruses. Some of these organisms can cause illnesses, including hepatitis and gastroenteritis.	Animal faeces Sewer overflows, septic tank leaks Organic matter decay.
Toxic organics	Can poison living organisms or damage their life processes.	Pesticides Herbicides Spillage, illegal discharge Sewer overflows, septic tank leaks.
Heavy metals	Poison living organisms or damage their life processes in some other way. Persists in the environment for a long time.	Atmospheric deposition Vehicle wear Sewer overflows, septic tank leaks Weathering of buildings, structures Spillage, illegal discharges.
Gross pollutants (litter and debris)	Unsightly. Animals can eat and choke on this material.	Pedestrians and vehicles Waste collection systems Leaf-fall from trees Lawn clippings Spills and accidents.
Oils, detergents and shampoos (surfactants)	Highly toxic poison to fish and other aquatic life.	Asphalt pavements Spillage, illegal discharges Leaks from vehicles Car washing Organic matter.
Increased water temperature	High temperatures are lethal to fish and other aquatic organisms. Elevated water temperatures stimulate the growth of nuisance plants and algae. This and other effects can lead to decreased levels of dissolved oxygen which can threaten other aquatic life.	Runoff from impervious surfaces Removal of riparian vegetation.

(Table modified from Table 1.1 in Urban Stormwater: Best Practice Environmental Management Guidelines, 1999 CSIRO).

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