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.

runoff
Toxic runoff can pollute surface waters, like rivers and lakes, as well as seep into underground groundwater supplies.

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.

Noun

organism that eats meat.

concentration
Noun

measure of the amount of a substance or grouping in a specific place.

consume
Verb

to use up.

Noun

agricultural produce.

Noun

community and interactions of living and nonliving things in an area.

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.

Noun

mass of ice that moves slowly over land.

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.

Noun

watering land, usually for agriculture, by artificial means.

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.)

Noun

fossil fuel formed from the remains of ancient organisms. Also called crude oil.

Plural Noun

(singular: plankton) microscopic aquatic organisms.

point-source pollution
Noun

pollution from a single, identifiable source.

pollutant
Noun

chemical or other substance that harms a natural resource.

Noun

all forms in which water falls to Earth from the atmosphere.

Noun

overflow of fluid from a farm or industrial factory.

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.

stream
Noun

body of flowing fluid.

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.