As the resulting runoff moves over and through the soil, it picks up and carries away natural and human-made contaminants, finally transporting them to streams, rivers, wetlands, lakes, and even ground water. Streams, which commonly serve as drinking-water supplies, recreational areas, and biological habitats, are influenced by the landscape through which they flow and the atmosphere from which the water originates. The vulnerability of streams to contamination reflects a complex combination of upstream natural processes, land use, chemical use, and land-management practices. Fish and other aquatic organisms reflect cumulative effects of water chemistry and land-use activities. Fish, for example, acquire some pesticides by ingesting stream invertebrates or smaller fish that have fed on contaminated plants.
Fish also can accumulate some contaminants directly from water passing over their gills.
As water seeps through the soil, it carries with it substances applied to the land, such as fertilizers and pesticides. Water moves through water- bearing formations, known as aquifers, and eventually surfaces in discharge areas, such as streams, lakes, and estuaries. Likewise, surface water can affect the quality and quantity of ground water. Total pesticide use in the United States has remained relatively constant at about 1 billion pounds per year, after growing steadily through the mid-1970s because of increased use of herbicides. Agriculture now accounts for 70 to 80 percent of total pesticide use.