Pollutants may enter both surface water and underground water supplies that we access with wells. Once contaminants are present, they can be extremely difficult to get rid of. The primary sources of water pollution are industrial and agricultural chemicals, residential sewage, and nonpoint sources.

▸ Industrial and Agricultural Chemicals One industrial pollutant is a class of organic chemicals called PCBs that were widely used in industry until the 1970s. After several large-scale contamination events, PCBs were banned. However, because PCBs often enter mud and sand beneath bodies of water, they can be difficult, if not impossible, to eliminate. Parts of the Great Lakes and some coastal areas, for example, are still polluted with PCBs. Other harmful industrial pollutants are heavy metals like cadmium, lead, mercury, and zinc.

Large-scale monoculture has increased the use of pesticides and insecticides. These chemicals can enter the water supply in the form of runoff after heavy rains, or they can seep directly into groundwater. Pesticides can be very dangerous pollutants. DDT, which is both cheap and long lasting, effectively controls agricultural pests and disease-carrying mosquitoes. But when DDT gets into a water supply, it has disastrous effects on the organisms that directly and indirectly rely on that water—a function of a phenomenon called biological magnification.

Biological magnification occurs if a pollutant, such as DDT, mercury, or a PCB, is picked up by an organism and is not broken down or eliminated from its body. Instead, the pollutant collects in body tissues. Primary producers pick up a pollutant from the environment. Herbivores that eat those producers concentrate and store the compound. Pollutant concentrations in herbivores may be more than ten times the levels in producers. When carnivores eat the herbivores, the compound is still further concentrated. Thus, pollutant concentration increases at higher trophic levels. In the highest trophic levels, pollutant concentrations may reach 10 million times their original concentration in the environment, as shown in Figure 6–8.

These high concentrations can cause serious problems for wildlife and humans. Widespread DDT use in the 1950s threatened fish-eating birds like pelicans, osprey, falcons, and bald eagles. It caused females to lay eggs with thin, fragile shells, reducing hatching rates and causing a drop in bird populations. Since DDT was banned in the 1970s, bird populations have recovered. Still a concern is mercury, which accumulates in the bodies of certain marine fish such as tuna and swordfish.

In Your Notebook In your own words, explain the process of biological magnification.

FIGURE 6–8 Biological Magnification In the process of biological magnification, the concentration of a pollutant like DDT—represented by the orange dots—is multiplied as it passes up the food chain from producers to consumers. Calculate By what number is the concentration of DDT multiplied at each successive trophic level?

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