• Roads are major sources of urban runoff pollution.Rolfo Rolf Brenner/Photodisc/Getty Images
  
  Urban runoff is the water which enters rivers and other water courses from urban areas. There's more runoff in urban areas than in rural environments because the pavements, roads and buildings in urban areas do not let water soak into the ground. Instead the water is channeled straight into the water system. The U.S. Environmental Protection Agency (EPA) found that urban runoff is the greatest source of pollution in estuaries and the third greatest in lakes.
  
  

Roads

• Motor vehicles pollute urban runoff in two main ways, according to the EPA. Vehicles both emit and deposit from their exhausts, while at the same time discharging both solid particles and fluids as they move and brake. These pollutants tend to be concentrated in places of high vehicle use -- one 1996 study cited by the EPA found that convenience stores -- one of the places of highest vehicle use -- had the highest concentration of hydrocarbons in runoff water. Hydrocarbons are highly toxic and several U.S.-based studies have shown that fish in areas where the water has been polluted with hydrocarbons from roads are much more likely to suffer from tumors. Other potentially hazardous materials found in highway runoff include lead, pesticides and cyanide.
  
  

Construction Sites

• Construction sites can also be a source of pollution for urban runoff, although the nature of pollution varies with the nature of construction activity, the physical characteristics of the site and how close the water course is to the construction activity, says the EPA. Runoff from construction sites is characterized by high levels of sediment -- erosion from construction sites ranges from 7.2 to 500 tons per acre per year, compared to 1 ton per acre per year in undisturbed forest. Pesticides, herbicides and other chemicals used to kill vermin also contribute to pollution in construction site runoff, as do the fertilizers used to encourage new plant growth in the construction area.
  
  

Thermal Pollution

• Urban areas are also responsible for thermal pollution. Unlike other forms of water pollution, this does not involve the presence of other materials in the water itself. Instead it occurs when water is heated above the ambient temperature. Urban areas tend to be warmer than rural areas because materials like concrete, tarmac and brick have a higher specific heat capacity than soil. When water falls on these surfaces it is heated before it reaches the water course, raising its temperature.