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Runoff occurs when there is more water than land can absorb. The excess liquid flows across the surface of the land and into nearby creeks, streams, or ponds. Runoff can come from both natural processes and human activity.
The most familiar type of natural runoff is snowmelt. Mountains that cannot absorb water from heavy snowfalls produce runoff that turns into streams, rivers, and lakes. Glaciers, snow, and rain all contribute to this natural runoff.
Runoff also occurs naturally as soil is eroded and carried to various bodies of water. Even toxic chemicals enter waterways through natural processes, such as volcanic eruptions. Toxic gases released by volcanoes eventually return to the water or soil as precipitation.
Runoff from human activity comes from two places: point sources and nonpoint sources. Point source pollution is any source that empties directly into a waterway. This might include a pipe from specific sewage treatment plant, factory, or even a home. Regulations determine what type of runoff, and how much, industries are allowed to release. These regulations vary by region, state, and nation.
Nonpoint source pollution is any source where runoff does not go directly into a waterway. Nonpoint sources of runoff can be large urban, suburban, or rural areas. In these areas, rainwater and irrigation wash chemicals into local streams. Runoff from nonpoint sources includes lawn fertilizer, car exhaust, and even spilled gasoline from a car. Farms are a huge nonpoint source of runoff, as rainwater and irrigation drain fertilizers and pesticides into bodies of water.
Impervious surfaces, or surfaces that can't absorb water, increase runoff. Roads, sidewalks, and parking lots are impervious surfaces. Materials as diverse as car-washing soaps, litter, and spilled gas from a gas station all become runoff.
Reducing Runoff
Runoff is a major source of water pollution. As the water runs along a surface, it picks up litter, petroleum, chemicals, fertilizers, and other toxic substances. From California to New Jersey, beaches in the U.S. are regularly closed after heavy rainfall because of runoff that includes sewage and medical waste.
These chemical pollutants can harm not just a beach, but an entire ecosystem. Tiny microbes, such as plankton or algae, absorb pollutants in the runoff. Fish or shellfish consume the microbes or absorb the pollutants directly. Animals such as birds consume the fish, increasing the level of pollutants in their own bodies. This process in which the concentration of a substance increases as it passes up the food chain is called biomagnification.
Biomagnification means organisms high on the food chain, including people, have a higher concentration of pollutants in their bodies than organisms such as seagrass or algae. As people eat foods such as oysters, they may be ingesting runoff from farms, sewage treatment plants, and city streets.
Runoff is an economic threat, as well as an environmental one. Agribusiness loses millions of dollars to runoff every year. In the process of erosion, runoff can carry away the fertile layer of topsoil. Farmers rely on topsoil to grow crops. Tons of topsoil are lost to runoff every year.
People can limit runoff pollution in many ways. Farmers and gardeners can reduce the amount of fertilizer they use.
Urban areas can reduce the number of impervious surfaces. Soil acts as a natural sponge, filtering and absorbing many harmful chemicals.
Communities can plant native vegetation. Shrubs and other plants prevent erosion and runoff from going into waterways.
Toxic runoff can pollute surface waters, like rivers and lakes, as well as seep into underground groundwater supplies.
Photograph by James P. Blair
Stormwater Runoff
Stormwater runoff is the runoff drained into creeks, bays, and other water sources after a storm. Stormwater runoff includes all debris, chemicals, and other pollutants picked up by the rain or snow.
absorb
Verb
to soak up.
agribusiness
Noun
the strategy of applying profit-making practices to the operation of farms and ranches.
algae
Plural Noun
(singular: alga) diverse group of aquatic organisms, the largest of which are seaweeds.
biomagnification
Noun
process in which the concentration of a substance increases as it passes up the food chain.
concentration
Noun
measure of the amount of a substance or grouping in a specific place.
consume
Verb
to use up.
erode
Verb
to wear away.
excess
Noun
extra or surplus.
exhaust
Noun
gases and particles expelled from an engine.
farm
Noun
land cultivated for crops, livestock, or both.
fertile
Adjective
able to produce crops or sustain agriculture.
fertilizer
Noun
nutrient-rich chemical substance (natural or manmade) applied to soil to encourage plant growth.
filter
Verb
to remove particles from a substance by passing the substance through a screen or other material that catches larger particles and lets the rest of the substance pass through.
Noun
group of organisms linked in order of the food they eat, from producers to consumers, and from prey, predators, scavengers, and decomposers.
gasoline
Noun
liquid mixture made from oil and used to run many motor vehicles.
impervious surface
Noun
boundary that does not allow water to penetrate it.
ingest
Verb
to take material, such as food or medicine, into a body.
lawn
Noun
area of grass mowed, watered, and maintained by people.
litter
Noun
trash or other scattered objects left in an open area or natural habitat.
medical waste
Noun
material thrown away from healthcare facilities such as hospitals, including blood, tissue, and medical instruments.
microbe
Noun
tiny organism, usually a bacterium.
mountain
Noun
landmass that forms as tectonic plates interact with each other.
native
Adjective
indigenous, or from a specific geographic region.
nonpoint-source pollution
Noun
toxic chemicals that enter a body of water from many sources.
oyster
Noun
type of marine animal (mollusk).
pesticide
Noun
natural or manufactured substance used to kill organisms that threaten agriculture or are undesirable. Pesticides can be fungicides (which kill harmful fungi), insecticides (which kill harmful insects), herbicides (which kill harmful plants), or rodenticides (which kill harmful rodents.)
point-source pollution
Noun
pollution from a single, identifiable source.
pollutant
Noun
chemical or other substance that harms a natural resource.
Noun
regions with low population density and large amounts of undeveloped land. Also called "the country."
seagrass
Noun
type of plant that grows in the ocean.
sewage
Noun
liquid and solid waste material from homes and businesses.
shellfish
Noun
any aquatic organism that has a shell or exoskeleton.
shrub
Noun
type of plant, smaller than a tree but having woody branches.
snow
Noun
precipitation made of ice crystals.
snowmelt
Noun
water supplied by snow.
soil
Noun
top layer of the Earth's surface where plants can grow.
specific
Adjective
exact or precise.
topsoil
Noun
the most valuable, upper layer of soil, where most nutrients are found.
toxic
Adjective
poisonous.
Noun
an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.
water pollution
Noun
introduction of harmful materials into a body of water.
waterway
Noun
body of water that serves as a route for transportation.
