GEF

Unit 2  |  Water Science and the Water Cycle 57 Globally, agriculture is the largest source of nonpoint water pollution. Agriculture impacts water quality in several ways. ■■ Farmers remove native vegetation such as trees and shrubs that hold soil in place, accelerating the erosion of topsoil. The eroded soil ends up in water bodies, increasing the sediment load of rivers and lakes. ■■ Modern agriculture also depends on heavy inputs of chemi- cal fertilizers and pesticides. When fertilizers run off into water bodies, they cause eutrophication —the acceler- ated growth of aquatic plants such as algae and other phytoplankton . After these microorganisms die, their decomposition depletes oxygen levels in the water, harming most other aquatic species. ■■ Synthetic agricultural chemicals such as DDT and atra- zine also degrade water quality when they are carried into water bodies. While the impact of these chemicals varies widely, many have been banned due to harmful effects on wildlife and human health. Urbanization is also a major source of water pollution. Asphalt roads do not allow rainwater to percolate into the soil. Instead, water runs off into water bodies via storm drains, pick- ing up a wide range of pollutants along the way. The variety of substances found in urban runoff include petroleum products from roads and parking lots, road salt, lawn fertilizers, pet waste, and a host of industrial chemicals. After large storms, sewage may flow directly into water bodies in some older systems that combine sewage and stormwater flow. In developed areas, buried oil or septic tanks can also leak pollutants into water bodies or groundwater. Scientists have even coined the term urban stream syndrome to describe the inevitable lower water quality in streams near urban areas compared with their rural counterparts. EROSION The process by which soil or rock is removed by water or wind. EUTROPHICATION The process in which a body of water becomes excessively enriched with nutrients resulting in growth of aquatic life and depleting the oxygen availability for other organisms. PHYTOPLANKTON Tiny photosynthesizing organ- isms that inhabit the upper layers of the oceanwhere sunlight is able to penetrate. DICHLORODIPHENYL- TRICHLOROETHANE (DDT) A pesticide that was widely used until 1972 when the U.S. banned it after discovering its detrimental effects on human health and the environment. ATRAZINE A synthetic compound typi- cally used in agriculture to kill plants, such as, weeds. URBANIZATION The development of cities or other human infrastructure. URBAN STREAM SYNDROME The decline in water quality in water bodies near urban development.