Urban runoff is water from rain, irrigation, and car washing that flows over the ground in cities. Impervious surfaces, such as roads, parking lots, and sidewalks, are built during development. These surfaces, made of materials like asphalt and concrete, along with rooftops, prevent water from soaking into the soil. Instead, they direct polluted stormwater into storm drains during rain or storms.

This reduces the amount of water that soaks into the ground, lowering the water table and increasing the risk of flooding because more water stays on the surface. Most city storm sewer systems send untreated stormwater directly into streams, rivers, and bays. This excess water can also enter homes through basement backups or seepage through walls and floors. Urban runoff is a significant source of flooding and water pollution in cities around the world.

Pollutants

Water that flows over surfaces that do not allow water to soak in, such as roads and parking lots in cities, often carries pollutants like gasoline, motor oil, heavy metals, trash, and other harmful substances from these areas. It also picks up fertilizers and pesticides from lawns. Roads and parking lots are major sources of polycyclic aromatic hydrocarbons (PAHs), which are produced when gasoline and other fossil fuels burn, as well as heavy metals like nickel, copper, zinc, cadmium, and lead. Water from rooftops adds high levels of synthetic organic compounds and zinc, which comes from galvanized gutters. Improper use of fertilizers on lawns, parks, and golf courses contributes nitrates and phosphorus to urban runoff.

Soil erosion or poorly managed construction sites can increase the amount of sediment in runoff. This sediment often settles at the bottom of water bodies and can harm water quality. Too much sediment can raise the risk of disease by increasing nutrients in the water. These nutrients can lower oxygen levels, encourage algae growth, and harm native plants, which can disrupt aquatic ecosystems. High levels of sediment and suspended solids can also damage infrastructure. Sediment can block underground water systems, increasing surface runoff. Too much sediment can reduce the storage space in reservoirs, leading to higher costs for public agencies and affecting the quality of water in recreational areas.

Runoff can also cause toxins to build up in ocean animals and increase in concentration as they move up the food chain. Small amounts of heavy metals from runoff can enter the ocean, where they accumulate in aquatic animals and cause metal poisoning. This poisoning can affect humans if people eat contaminated animals.

When stormwater flows into storm drains and surface waters, the natural sediment load in the water decreases, but the speed and force of the water increase. In fact, the amount of runoff from impervious surfaces in a typical city is five times greater than the runoff from a similar-sized woodland area.

Effects

Overwatering using sprinklers can cause runoff that flows into nearby water sources during times when water levels are already low. This runoff carries pollutants that build up on surfaces, leading to higher concentrations of harmful substances in streams than would occur during regular rain events.

Urban runoff is a major cause of urban flooding, which happens when heavy rain overwhelms drainage systems like storm sewers, causing water to flood land or buildings in cities. Flooding can be triggered by sudden rainstorms, rising sea levels, melting snow, or overflowing rivers. It often causes repeated and costly damage to communities, even in areas far from rivers or floodplains.

Stormwater can enter homes and buildings in several ways: through backed-up sewers, toilets, or sinks; by seeping through walls or floors; by collecting on property or public roads; or by overflowing from rivers or lakes. Homes with basements are especially at risk of basement flooding due to urban runoff.

Urban runoff harms water quality. A 2009 report by the US National Research Council noted that urban stormwater remains a major source of pollution in many areas across the United States. The report explained that water quality could worsen further if land-use changes, such as construction or agriculture, are not managed properly. These changes release hard-to-track pollutants during rain, which harm waterbodies and ecosystems.

Runoff also raises stream temperatures, which can harm fish and other aquatic life. For example, sudden rainstorms can wash hot water into streams, creating conditions that kill fish. Road salt used to melt snow can also pollute streams and underground water sources.

Urban runoff has a major impact on streams that rarely have water during dry periods, called ephemeral streams. When these areas are developed, runoff creates constant water flow that harms plants, animals, and the streambed. Unlike natural streams, urban runoff carries little sediment, which erodes the streambed and destroys features like bends and sandbars. For example, runoff from urban areas often carries trash and pollutants into Southern California beaches, creating health risks for people.

Urban runoff can lead to eutrophication, a process where excess nutrients from fertilizers and organic waste cause rapid algae growth. After heavy rain, these nutrients increase oxygen use by algae, leading to oxygen shortages in water. When algae die, their decay further reduces oxygen, harming fish and other aquatic life. Algae blooms often occur in still water, such as behind dams or in stream pools.

Too much erosion from stream banks can cause flooding and damage to property. In the past, governments have used concrete and stone to build barriers along streams to control erosion. However, these hard structures harm fish habitats and often move erosion problems to other parts of the stream.

Polluted urban runoff can harm humans by contaminating drinking water, reducing food sources, and closing beaches due to health risks. After heavy rains, runoff can pollute waterways used for recreation or fishing, leading to closures. This happens because runoff increases harmful bacteria or chemicals in water. While oil and gasoline spills are often noticed, fertilizers and insecticides also cause harm. These chemicals can wash into groundwater, killing plants, insects, and animals in new environments.

Prevention and mitigation

Effective control of urban runoff requires reducing the speed and amount of stormwater, as well as limiting the release of pollutants. Local governments use various stormwater management methods to lessen the impact of urban runoff. These methods, called best management practices for water pollution (BMPs) in some countries, may focus on controlling the amount of water, improving water quality, or doing both.

Pollution prevention methods include low impact development (LID) or green infrastructure techniques, such as Sustainable Drainage Systems (SuDS) in the UK, and Water-Sensitive Urban Design (WSUD) in Australia and the Middle East. Examples include installing green roofs and managing chemicals like motor oil, fertilizers, pesticides, and road deicers. Runoff reduction systems include infiltration basins, bioretention systems, constructed wetlands, retention basins, and similar structures.

Creating effective urban runoff solutions often requires city programs that consider the unique needs of the community. Factors like a city's average temperature, rainfall amounts, location, and levels of air pollution can influence pollution rates in runoff and create challenges for management. Human factors, such as the speed of urban growth, land use patterns, and materials used for surfaces that prevent water from soaking into the ground, can worsen these issues.

Citywide maintenance strategies, such as street sweeping programs, can help improve urban runoff quality. Street sweeping vacuums collect dust and small solid particles from public roads and parking lots that often end up in runoff.

Educational programs can also help manage urban runoff. Local businesses and individuals can play a key role in reducing pollution through their actions, but they may not know about regulations. Encouraging discussions about urban runoff and the importance of properly disposing of household items can promote environmentally friendly practices with less cost to the city and local economy.

Thermal pollution from runoff can be controlled by stormwater management systems that absorb runoff or direct it into groundwater, such as bioretention systems and infiltration basins. Bioretention basins are less effective at reducing water temperature because the water may be heated by sunlight before being sent into nearby streams.

Stormwater harvesting involves collecting runoff from creeks, gullies, temporary streams, and other ground-level paths. Stormwater harvesting projects often aim to reduce polluted runoff near sensitive areas, support groundwater recharge, and provide non-potable water for uses like toilet flushing and irrigation.