Dutch elm disease. As worms and insects ate the decomposing leaves, DDT built up in their bodies. Some robins died from eating these worms and in- sects. Others developed problems with reproduction—either they failed to lay eggs, or they laid eggs with shells so thin that they cracked. Similar prob- lems occurred with fish-eating birds including eagles, ospreys, and gulls.
Rachel Carson’s title, Silent Spring, referred to a widespread decline in bird populations due to these deaths and reproduction problems. Because of dam- age to wildlife and potential threats to human health, in 1972 EPA banned use of DDT in the United States except in the case of public health emergencies.
EFFECTS OF CHEMICAL PROPERTIES
Scientists discovered several chemical properties that help to explain DDT’s unexpected buildup in birds and other wildlife. One of these properties is the slow rate at which DDT breaks down. Through physical, chemical, and bio- logical degradation, compounds decompose into simpler compounds. When exposed to sunlight, moisture, and warmth, some chemicals will degrade quickly. Because DDT degrades extremely slowly, it remains in the environ- ment for many years and continues to make its way into the food of birds and other organisms (Figure 1.3).
Another important property of DDT is that it dissolves much more readily in fat or oil than in water. Once absorbed into the bodies of animals, DDT gets stored in fat and continues to accumulate over the years. This helps to Through
degradation, chemicals break down into simpler forms.
Bioaccumulation causes some chemi- cals to concentrate in animal fat.
bird eating worm DDT=450 ppm*
F I G U R E 1 . 3
Buildup of DDT Concentrations through Diets of Birds
worm eating leaves and soil DDT=140 ppm
soil and decomposing leaves DDT=10 ppm
*ppm = parts per million
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CH A P T E R 4 : EC O L O G I C A L RI S K S
Larger fish such as lake trout DDT = 1 ppm
Aquatic invertebrates DDT = 0.05 ppm Aquatic plants
DDT = 0.01 ppm
Plankton DDT = 0.01 ppm
Lake water DDT = 0.0001 ppm
Eagles and other fish-eating birds
DDT = 10 ppm
Small fish such as minnows DDT = 0.4 ppm F I G U R E 1 . 4
Biomagnification of DDT through Aquatic Food Chains Through biomagnification, these compounds build up to higher concentra- tions at each level of the food chain. In aquatic ecosystems, these processes start with tiny organisms called plankton. When plankton store DDT in their bod- ies, they concentrate it at levels higher than those in the surrounding water.
Minnows and other small fish eat the plankton, and they continue the process of concentration by storing DDT in their fat cells. As these fish get eaten by larger fish, which then get eaten by birds, the concentrations continue to rise (Figure 1.4).
Through
biomagnification, some chemicals build up through the food chain.
Some chemicals adsorb to soil par- ticles.
Chemicals that dissolve in water may leach or wash out of the soil.
DDT was the first pesticide to surprise people with unforeseen environ- mental consequences, but it has not been the only one. In the late 1970s, a pesticide called aldicarb was found in wells used for drinking water by hun- dreds of families on Long Island, New York. Aldicarb was being used to protect potato crops from the Colorado potato beetle but was not expected to leach, or wash out of the soil and into the groundwater. Until the 1940s, most pesticides were compounds of arsenic, mercury, copper, or lead. It is still possible to measure high concentrations of these compounds in the soil of old orchards and farm fields. Instead of breaking down or leaching out of the soil, these compounds adsorb, or get tightly bound to soil particles. Initial tests for aldicarb indicated that it would adsorb to soil and break down into less toxic chemical forms.
How then did aldicarb find its way into the wells of Long Island? The unexpected contamination occurred because Long Island’s light, sandy soils allowed aldicarb to leach downward to groundwater rather than remaining in the topsoil and gradually breaking down.
Another environmental surprise occurred in the discovery of an insecti- cide named toxaphene in the bodies of fish and wildlife in the Arctic and northern regions. During the 1960s and 1970s, toxaphene was heavily used to control insect pests on cotton and other crops in the southern United States and other countries. In 1982 EPA banned the use of toxaphene on U.S. crops because of its effects on human and animal health.
How did toxaphene get hundreds, even thousands of miles from where it had been used? Studies have shown that toxaphene evaporates and gets car- ried by wind, and then rainfall brings it back down to earth. Because it breaks down very slowly, it persists a long time in the environment. The result is that even today, long after its use was discontinued, toxaphene continues to be carried by wind to all parts of the globe (Figure 1.5).
TESTING ENVIRONMENTAL IMPACTS OF NEW COMPOUNDS
Because of what we have learned from experience with pesticides such as DDT, aldicarb, and toxaphene, new pesticide compounds now get subjected to a more thorough scientific review, focusing on questions such as the following:
◗ How rapidly does it break down?
◗ Does it dissolve better in water or in oil?
◗ How quickly does it evaporate?
◗ Does it bind to soil particles or leach out as water percolates through?
◗ Does it tend to build up in fish, birds, and other wildlife?
◗ How toxic is it to humans and to organisms in the environment?
CH A P T E R 4 : EC O L O G I C A L RI S K S
Evaporated and carried away
by wind Pesticide
spray
Carried with runoff into surface water
Taken up into plants and
animals
Degraded by sunlight
Groundwater
Leached into groundwater Soil
Adsorbed to soil
Degraded by microbes or chemical processes F I G U R E 1 . 5
Movement and Fates of Pesticides in the Environment
Act, enacted by Congress in 1976, authorizes EPA to track the 75,000 indus- trial chemicals currently in use in the United States.
EPA screens these chemicals and can require testing of any that appear likely to cause hazards to human or environmental health. Unfortunately, thorough testing is both time-consuming and expensive, and very few of the products currently on the market have gone through this sort of review.
FOR DISCUSSION
◗ Why didn’t scientists anticipate that DDT would cause problems in the environment?
◗ Suppose that you are developing a new pesticide to limit the amount of damage that beetles cause to apple trees. Much of your research will focus on the ability of this compound to protect apple crops, but what else will you need to consider? What properties would you hope to find in a chemi- cal that will be applied to orchards and home gardens?