Urban runoff is surface runoff of rainwater, landscape irrigation, and car washing created by urbanization. Impervious surfaces (roads, parking lots and sidewalks) are constructed during land development. During rain, storms, and other precipitation events, these surfaces (built from materials such as asphalt and concrete), along with rooftops, carry polluted stormwater to storm drains, instead of allowing the water to percolate through soil. This causes lowering of the water table (because groundwater recharge is lessened) and flooding since the amount of water that remains on the surface is greater. Most municipal storm sewer systems discharge untreated stormwater to streams, rivers, and bays. This excess water can also make its way into people's properties through basement backups and seepage through building walls and floors. Urban runoff can be a major source of urban flooding and water pollution in urban communities worldwide.
Pollutants Water running off impervious surfaces in urban areas tends to pick up gasoline, motor oil, heavy metals, trash, and other pollutants from roadways and parking lots, as well as fertilizers and pesticides from lawns. Roads and parking lots are major sources of polycyclic aromatic hydrocarbons (PAHs), which are created as the byproducts of the combustion of gasoline and other fossil fuels, as well as of the heavy metals nickel, copper, zinc, cadmium, and lead. Roof runoff contributes high levels of synthetic organic compounds and zinc (from galvanized gutters). Fertilizer use on residential lawns, parks and golf courses is a measurable source of nitrates and phosphorus in urban runoff when fertilizer is improperly applied or when turf is over-fertilized. Eroding soils or poorly maintained construction sites can often lead to increased sedimentation in runoff. Sedimentation often settles to the bottom of water bodies and can directly affect water quality. Excessive levels of sediment in water bodies can increase the risk of infection and disease through high levels of nutrients present in the soil. These high levels of nutrients can reduce oxygen and boost algae growth while limiting native vegetation growth, which can disrupt aquatic ecosystems. Excessive levels of sediment and suspended solids have the potential to damage existing infrastructure as well. Sedimentation can increase surface runoff by plugging underground injection systems. Increased sedimentation levels can also reduce storage behind reservoir. This reduction of reservoir capacities can lead to increased expenses for public land agencies while also impacting the quality of water recreational areas. Runoff can also induce bioaccumulation and biomagnification of toxins in ocean life. Small amounts of heavy metals are carried by runoff into the oceans, which can accumulate within aquatic animals to cause metal poisoning. This heavy metal poisoning can also affect humans, since ingesting a poisoned animal increases the risk of heavy metal poisoning. As stormwater is channeled into storm drains and surface waters, the natural sediment load discharged to receiving waters decreases, but the water flow and velocity increases. In fact, the impervious cover in a typical city creates five times the runoff of a typical woodland of the same size.
Effects
Dry weather discharges Overwatering through irrigation by sprinkler may produce runoff reaching receiving waters during low flow conditions. Runoff carries accumulated pollutants to streams with unusually low dilution ratios causing higher pollutant concentrations than would be found during regional precipitation events.
Urban flooding
Urban runoff is a major cause of urban flooding, the inundation of land or property in a built-up environment caused by rainfall overwhelming the capacity of drainage systems, such as storm sewers. Triggered by events such as flash flooding, storm surges, overbank flooding, or snow melts, urban flooding is characterized by its repetitive, costly, and systemic impacts on communities, even when not within floodplains or near any body of water. There are several ways in which stormwater enters properties: backup through sewer pipes, toilets and sinks into buildings; seepage through building walls and floors; the accumulation of water on the property and in public rights-of-way; and the overflow of water from water bodies such as rivers and lakes. Where properties are built with basements, urban flooding is the primary cause of basement flooding.
Water pollution Urban runoff contributes to water quality problems. In 2009 the US National Research Council published a comprehensive report on the effects of urban stormwater and stated that it continues to be a major contamination source in many watersheds throughout the United States. The report explained that "...further declines in water quality remain likely if the land-use changes that typify more diffuse sources of pollution are not addressed... These include land-disturbing agricultural, silvicultural, urban, industrial, and construction activities from which hard-to-monitor pollutants emerge during wet-weather events. Pollution from these landscapes has been almost universally acknowledged as the most pressing challenge to the restoration of waterbodies and aquatic ecosystems nationwide."
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