Runoff occurs when pollutants are washed into a water body by the movement of water. When rates of precipitation are greater than rates at which water percolates into the soil, water begins moving over the land surface. As the water travels, it picks up sediments and pollutants, which are eventually deposited into water bodies. Runoff is the largest contributor to pollution in both freshwater and marine systems. It contributes to increased turbidity, causes eutrophication, adds toxins to the aquatic food web, and increases rates of erosion. Urbanization, which creates impervious surfaces, exacerbates damage caused by runoff.
Historical Background and Scientific Foundations
Runoff forms from water, usually precipitation, that hits Earth’s surface. If the rate of precipitation is greater than the rate at which water can infiltrate the ground, or if the ground is already saturated with water, water accumulates in depressions in the ground. Once small depressions are filled, the water flows across the land. As the water travels, it collects residues and wastes that have collected on the surface of the land or in the soils. If the moving water does not evaporate, it eventually runs into a water body, depositing the collected wastes there. In the continental United States, approximately 31% of the precipitation becomes runoff. Two-thirds return to the atmosphere through evaporation or transpiration of plants. Only about 2% of precipitation infiltrates the ground and returns to the aquifer.
Numerous factors affect runoff. Runoff can form from many different types of precipitation: rain, snow, snowmelt, or sleet. Each affects the quantity and movement of runoff. The intensity, quantity, and duration of the precipitation influence the quantity and speed of the runoff and the amount of material it picks up. Other meteorological factors such as the distribution of precipitation in the watershed, storm direction, air temperature, wind speed and direction, and humidity can affect how much runoff actually enters the water. In addition, the conditions of the soil, the drainage basin topography, and the locations of the water reservoirs impact runoff.
Impacts and Issues
There are several forms of runoff. Agricultural runoff refers to water that has run through agricultural lands. This runoff often contains sediment, herbicides, pesticides, and fertilizer. Urban runoff is water that has collected on city streets, parking lots, and residential areas. This runoff most often includes motor oil, gasoline, garbage, animal waste, herbicides, and pesticides from residential yards. River runoff refers to the water that enters the ocean via a river or stream. The river may carry with it pollutants that it has picked up along its entire length. These pollutants vary depending on the health of the river and the pathway along which it travels. Pollution that enters the ocean via river runoff is point-source pollution. Because the river is a known entry point, it can be monitored. This is in contrast to agricultural runoff and urban runoff, which are nonpoint-sources of pollution and much more difficult to manage. Runoff accounts for more than 40% of all marine pollution. Agricultural runoff is the leading source of environmental degradation in surveyed lakes and rivers in the United States.
The pollutants that are deposited into water bodies via runoff decrease water quality in a variety of ways. Sediment, primarily from agricultural runoff, absorbs light in the water decreasing the light energy available for photosynthesis by plants and phytoplankton. It can also clog feeding structures of filter-feeding organisms. Pollutants like oils, herbicides, pesticides, and heavy metals can kill or damage invertebrates and plants in the water, just like they kill or injure organisms on land. These effects are propagated up the food chain and concentrated in the top predators, which are often fished for human consumption. Animal waste and fertilizer add excessive quantities of nutrients to water bodies. These nutrients trigger massive blooms of microscopic plants known as phytoplankton. Cellular respiration by phytoplankton and the bacteria that consume phytoplankton waste deplete a water body of dissolved oxygen, killing fish and invertebrates that require oxygen for respiration. This condition is known as eutrophication.
Urbanization increases runoff because impervious surfaces like roads and parking lots prevent infiltration of water into the ground. This has several negative consequences. Because water from precipitation runs into streams and the ocean instead of percolating through the soil, aquifers receive less water. This slows rates of groundwater recharge and lowers the water table. When water runs over impervious surfaces, its flow rate increases. This means that it moves with more energy and has the potential to carry large amounts of sediment. The movement of sediment by water is known as erosion. Erosion removes coastal land on which delicate coastal ecosystems depend and increases turbidity to the water. Impervious surfaces also increase the quantity of
WORDS TO KNOW
AQUIFER: Rock, soil, or sand underground formation that is able to hold and/or transmit water.
EUTROPHICATION: The process whereby a body of water becomes rich in dissolved nutrients through natural or man-made processes. This often results in a deficiency of dissolved oxygen, producing an environment that favors plant over animal life.
WATERSHED: The expanse of terrain from which water flows into a wetland, water body, or stream.
runoff. Excessive runoff exacerbates the frequency and intensity of floods.
Some legislation in the United States has been aimed at monitoring runoff and decreasing the amount of pollution that reaches water bodies as a result of runoff. The Clean Water Act of 1972 created a permit system to regulate runoff from point-source polluters. In 1987, the act was amended to help monitor and regulate nonpoint-sources of runoff. However, nonpoint-source runoff is extremely hard to assess and regulate. The Coastal Zone Act was amended in 1990 in order to regulate development on coastal lands and to decrease pollution by these activities. In 2002, Congress increased funding for conservation efforts in the Farm Bill in order to decrease pollution in agricultural runoff.
See Also Algal Blooms; Aquatic Ecosystems; Bays and Estuaries; Clean Water Act; Coastal Ecosystems; Environmental Protection Agency (EPA); Groundwater; Groundwater Quality; Industrial Pollution; Industrial Water Use; Insecticide Use; Lakes; Marine Ecosystems; Marine Water Quality; Nonpoint-Source Pollution; Oceans and Coastlines; Precipitation; Water Pollution
Cunningham, W.P., and A. Cunningham. Environmental Science: A Global Concern. New York: McGraw-Hill International Edition, 2008.
Garrison, Tom. Oceanography: An Invitation to Marine Science, 5th ed. Stamford, CT: Thompson/Brooks Cole, 2004.
U.S. Environmental Protection Agency (EPA). “Protecting Water Quality from Agricultural Runoff.” March 2005. http://www.epa.gov/owow/nps/Ag_Runoff_Fact_Sheet.pdf (accessed March 28, 2008).
U.S. Geological Survey. “Earth’s Water: Runoff.” August 30, 2005. http://ga.water.usgs.gov/edu/runoff.html (accessed March 25, 2008).