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Everyday, thousands of pollutants are discharged into our environment. Many pollutants lack regulation, and their lingering pres ence threatens biodiversity, affecting individual species or degrading entire ecosystems. Pollutants resist categorization because of their varied forms and effects. Some, such as lead or PCBs, directly toxify the environment, while others, such as fertilizer runoff, are nontoxic but harm aquatic systems by causing excessive plant growth. Noise and light pollution threaten species by disrupting their behavior. Pollutants are also classified by the environment they affect, regardless of their form, such as air, water, and soil pollution. Many pollutants cycle through all of these environments at some stage, entering the air and ending up in water or soil. Classification of pollutants may also derive from where they enter the environment: a so-called point source pollutant enters at a discrete location and is nonmobile, such as effluent from a sewage treatment plant, whereas a nonpoint source pollutant enters from many locations or is mobile, such as surface runoff into the coastal zone from cars (motor oil) or lawns (fertilizers and pesticides). Typically, it has been easier to regulate "point source" pollutants.

What makes something a pollutant? Pollutants tend to persist in the environment. Because of this, even after a pollutant has been banned, its legacy is felt by the environment. Pollutants are often widespread and can be transported over large distances. Pollutants accumulate in an animal's tissues or interfere with vital processes such as the reproductive or immune systems. Some pollutants are toxic in low concentrations and at the extreme will kill an animal. Pollutants can also substantially alter entire ecosystems. Here, we will examine some of the leading pollutants of our environment, including toxic contaminants, organic biostimulants, solid wastes, noise, and light pollution. Because so many pollutants infiltrate our air, water, and soil, it would be difficult to cover them all.

Aerial view of an oil slick spilling into the ocean from the Liberian tanker Ocean Eagle, which split in two at the entrance to San Juan Harbor, Puerto Rico, in 1968 (Bettmann/Corbis)

Toxic Contaminants

Toxic contaminants include trace metals (for example, cadmium, copper, lead, and mercury); biocides/pesticides (for example, DDT, TBT [tributyl tin]); industrial organic chemicals (for example, PCBs, tetrachlorobenzene); and by-products of industrial processes and combustion (for example, polycyclic aromatic hydrocarbons [PAHs] and dioxins). Toxics can be lethal or can interfere with an organism's immune, endocrine, and reproductive systems. Existing toxicity tests for new chemicals rarely reveal the consequences of toxic contaminants on the environment.

Chlorinated hydrocarbons, such as the insecticide DDT (dichloro-diphenyl-trichloro ethane) and PCB (polychlorobenzene), are renowned for their toxic effects on the envi ronment. A particularly troubling characteristic of these pollutants is their ability to persist over long time frames and spread over large areas. When DDT was introduced in the 1940s, it was a marvel; it was cheaper and more effective than any other insecticide. However, its effectiveness came at a price. During the 1950s and 1960s, populations of predatory birds in North America, in particular those that ate fish, including eagles, pelicans, and ospreys, declined rapidly. Analysis of the birds revealed that DDT in their bodies was a million times more concentrated than that in the water where they lived. This discovery led to the concept of bioaccumulation—that is, animals higher up the food chain concentrate contaminants in their bodies. Why is DDT such a powerful toxin? First, it cannot be broken down by the body and is fat soluble, allowing it to accumulate in animal tissue. Second, DDT interferes with calcium deposition in eggs; thus birds were laying thin, fragile eggs that often broke during incubation. Because DDT affected the birds' reproduction, it had an immediate and powerful effect on populations. DDT also disperses readily in the atmosphere and has even been found in organisms of the Arctic and Antarctic (Wania and Mackay, 1996). Even though DDT was banned in 1972 in the United States, it continues to persist in the environment. One notable place is in Palos Verdes, off the coast of California, where DDT manufacturers were allowed to dump their supply of DDT into the ocean—more than 200 tons covering a 20-square-mile area. Manufacturers export DDT and other pesticides that are banned in the United States to developing countries.

Many pesticides, including DDT and PCB, as well as DBCP (dibromochloropropane), DDE (dichloro-diphenyl-dichloro ethylene), kepone, heptachlor, chlordane, dieldrin, mirex, lindane, toxaphene, dioxins, Bisphenol-A, and phthalates, are endocrine disrupters— that is, chemicals that mimic or inhibit the effects of hormones. Most of these pesticides are long-lived compounds and bioaccumu-late. The toxin tributyltin (TBT) used in antifouling paint on ships interferes with sexual development in some mollusks (for example, females develop male organs), even at concentrations of10 parts per trillion. Declines in marine snail populations have been found along the coasts of North America and Europe because of heavy contamination with TBT (Nehring, 2000).

Atrazine, a common weed killer used heavily on corn crops in the United States, pervades the environment, contaminating runoff and groundwater. Atrazine, even in low doses, has recently been shown to affect frog development

(Hayes et al., 2002). The study found that 20 percent of frogs exposed to doses of just 0.1 part per billion (well below the limit allowed for drinking water) developed abnormal reproductive parts, such as multiple sex organs or both male and female organs. At slightly higher doses of 1 part per billion, 90 percent of males lacked vocal chords, which are essential for attracting mates. Atrazine appears to affect the production of the enzyme aromatase, which converts the male hormone testosterone into the female hormone estrogen.

Sulfur and nitrogen oxides are released into the atmosphere when fossil fuels, such as coal in power plants, or oil in vehicles, or wood, are burned. These combine with water in the atmosphere to create sulfuric and nitric acid, which fall to earth as "acid rain" (these pollutants also create smog in urban areas). Because of prevailing wind patterns and geological characteristics, certain regions (including the northeastern United States, Canada, and northern and central Europe) have been especially affected by these pollutants. Some soils and rock types, however, neutralize or buffer the acid. For example, calcium carbonate in limestone acts as a natural buffer, reducing the damaging effects of acid rain. On the other hand, areas with granite and quartz tend to be very sensitive. Freshwater lakes in those areas are particularly susceptible. Initially the changes affect only some species of invertebrates, but with increasing acidity fewer and fewer species survive, until eventually the lake is dead. That has been widespread in the Adirondacks of New York and lakes of northern Sweden and Canada. Acid rain also dissolves other harmful metals, such as mercury, which plants and animals then absorb. On land, pollution by acid rain and other air pollutants (ozone) tends to affect plants more than animals. Lichens, bryophytes, and fungi suffer the most. Decline of a certain species may be due to acidification of the soil, direct tox-icity, or competition from more resistant species. Animals, such as otter and deer, tend to be indirectly affected by acid rain pollution brought about by changes in their prey or the bioaccumulation of mercury in their tissue, which is released at higher acidities.

Organic Pollutants/Biostimulants Organic pollutants or biostimulants, primarily from agricultural fertilizers and sewage waste, have a major impact on aquatic environments. When these excessive nutrients enter aquatic systems, they stimulate plant growth. Rapid phytoplankton growth or algal blooms create diverse problems. Plant growth is so rapid that animals don't have a chance to eat it. The phy-toplankton then falls to the seafloor, where it decomposes. This decomposition depletes oxygen, creating hypoxic (that is, low-oxygen, less than 2 mg/liter) or even anoxic (no oxygen) environments in which few organisms can survive. Large concentrations of algae also reduce water clarity, preventing light from reaching the bottom and reducing the growth of seagrasses. Changing phytoplankton communities also affect shellfish populations. A long-term increase in excess nutrients into an ecosystem is known as eutrophication. More than 50 percent of the estuaries along the U.S. coast are affected by eutrophication, some—such as the Mississippi River delta, Chesapeake Bay, and the Long Island Sound— severely. Eutrophication is a worldwide phenomenon affecting coastal areas from Europe to Asia.

Aquaculture operations also produce organic waste through uneaten food, feces and urine, and dead fish. Although still a minor organic pollutant, it can have a major local impact. Areas with offshore salmon pen farming (such as L'Etang Inlet, New Brunswick, Canada; and Puget Sound in Washington state) have significant nitrogen and phosphorous inputs brought about by aquaculture. Directly beneath the pens, there is often an anoxic area that extends 30 to 150 m from the caged area. Effluent from pond aquaculture (such as that used for shrimp and catfish) also contaminates nearby waterways. Besides releasing organic nutrients, aquaculture is also a source of chemical and biological pollutants. Antibiotics, parasiticides, pesticides, hormones, anesthetics, pigments, minerals, and vitamins are added to the feed for various types of pen and pond aquaculture systems. Especially in pen aquaculture, which is completely open to the surrounding water, uneaten food enters the water, where it can contaminate wild species. Similarly, escaped fish are a form of biological contaminant. Farm-raised fish have been bred with certain traits; when they escape they can reproduce with and alter the wild population.

Our ecosystems have been fundamentally changed by pollution. It was long thought that it is normal for temperate forests to lose nitrogen into soil and stream waters in inorganic forms such as nitrate and ammonium. However, recent studies of ancient and unpolluted temperate forests in Chile and Argentina reveal that it may be an artifact of pollution (Perakis and Hedin, 2002). South American forests are dominated by the release of dissolved organic nitrogen. Their North American counterparts (in this study the Smokey Mountains of Tennessee and Tionesta National Forest in Pennsylvania) release high levels of inorganic nitrogen. The cycling of nitrogen in North American temperate forests appears to be a consequence of excessive fertilizer use and nitrogen deposition from acid rain.

Solid Waste

Solid waste is generated from household and industrial sources, and it includes everything from food to plastics. Solid waste is usually disposed of in landfills. Landfills take up space and, if not properly contained, can leach toxins into the soil and poison groundwater. In countries with limited space, solid waste is burned at high temperatures. But incineration is expensive, creates very hazardous ash, and pollutes the air with toxic chemicals. Solid waste can be minimized through recycling and composting. Certain materials, such as metals, glass, and paper, are, in fact, easier to recycle. The composting of organic materials, such as food and paper, is an effective way to reduce solid waste— and it produces fertilizer.

Mining is a major source of solid waste. In the United States, mining produces more than 1.7 billion tons of waste, compared with the 180 million tons produced by all municipalities combined. Extraction of minerals, coal, and oil destroys and fragments habitat, and it is very polluting; in the worst instances it can lead to catastrophic spills. Open-pit mining is an extremely wasteful process. Metals, such as gold or copper, or mineral substances, such as coal, are extracted from ore found close to the surface. Most ore contains only small amounts of the target metal; the remaining excavated rock is wasted. The amount of waste depends on the metal and the region being mined, but typically it is huge. Some 3 tons of ore are needed to produce enough gold for just one ring. Copper mining is also wasteful; for every ton extracted, 99 tons of waste rock are produced. New technologies to extract and process minerals found at low concentrations in ore are increasing the waste produced by the mining industry and making it possible for new areas to be exploited. Much mining waste is also hazardous, polluting the environment with heavy metals, acid-producing sulphides, and other contaminants. Additional waste, known as tailings, is also produced during processing. Tailings are also highly toxic, being made up of heavy metals and chemicals, such as cyanide and sulfuric acid. Mining waste and tailings are stored in special containment areas or ponds near the mining site. Pollutants often leach from these sites into soils, groundwater, and nearby lakes and streams. If these sites are not well maintained, disasters may occur. For example, in southwestern Spain in 1998, a mining accident released 5 billion liters of toxic sludge into the Guadalquivir River. Contamination spread over a huge area downstream, damaging the wetlands of Coto Donana and the Donana National Park.

Solid waste originating on land also pollutes the marine environment (Coe and Rodgers, 1997). Plastics and fishing gear threaten many marine species. Turtles appear to confuse plastic bags with jellyfish, one of their main prey animals. The plastic blocks their digestive track, killing the turtles. Studies of stranded sea turtles off the coast of Brazil found that the most common debris ingested were transparent and white plastic bags; the turtles also showed evidence of damage on their carapaces from fishing activities. Recent studies of thirty remote island sites around the world revealed that floating marine debris is mostly made up of plastics. In addition to harming the marine mammals that swallow them, these plastics act as rafts, spreading invasive species, like barnacles and mollusks, around the globe (Barnes, 2002). Lost or discarded fishing gear, another major source of marine pollution, can remain a danger for many years, entangling turtles, seals, seabirds, and fish. Gear also damages the reef and benthic habitats that support marine life.

Noise Pollution

Transportation (cars, trains, airplanes, shipping) and industry (construction or factory) are the leading sources of noise pollution. Animals rely on hearing to communicate, avoid predators, and obtain food. To avoid noise, wildlife may alter their behavior, possibly leaving critical habitat or forage areas, though responses will vary with the kind of noise and the species. Waterfowl, for example, are particularly disturbed by low-flying aircraft. Noise can cause hearing loss and interfere with communication, and long-term exposure may have physiological effects because of increased heart rate and metabolism.

Many studies have examined the effects of noise on wildlife. Magnificent frigatebirds (Fregata magnificens) in the Florida Keys appear to be disturbed by low-altitude aircraft at their nesting sites. Birds flushed from their nests when they hear a noise may even break their eggs or injure the young. Caribou calves exposed to overflights suffer higher mortality rates. Bighorn sheep in the Grand Canyon are particularly sensitive to helicopter passes in summer, apparently because they graze at higher elevations and are closer to the source of the sound. Many desert animals have acute hearing and depend on it for hunting. Desert iguanas and the endangered kangaroo rat experience hearing loss caused by motorcycle noise.

Noise from shipping, fishing, recreation, dredging, military activities, or oil exploration disturbs marine animals. Whales and dolphins, which rely on sound for communication and navigation, appear particularly affected. Whales startled by noise (especially at low frequencies) may dive suddenly, swim faster, or change their vocalizations. At the extreme, noise may even lead to the animals' death. In March 2000, nine Cuvier's beaked whales (Ziphius cavirostris), three Blainville's beaked whales (Mesoplodon densirostris), two unidentified beaked whales, two Minke whales (Bal-aenoptera acutorostrata), and one spotted dolphin (Stenella frontalis) were stranded in the Bahamas, some bleeding from their ears, and at least seven of them died. According to the U.S. Navy and the National Marine Fisheries Service, testing of sonar in the area appears to be linked to the strandings. The marine mammals were confined to a narrow channel during calm conditions, which tend to amplify sound. Recent studies show that whales, like human divers, are susceptible to diving illnesses. Noise from sonar or explosives causes marine mammals to dive deeper. On long, deep dives, more nitrogen enters the blood from the lungs in the form of bubbles; too much nitrogen in the bloodstream can kill an animal. In humans, this illness is known as the bends.

Light Pollution

Satellite images of the planet at night dramatically reveal the extent of light pollution. Urbanization literally lights up the planet every night. The effect of light pollution is well documented in nesting turtles and hatchlings, which normally use the moonlight to guide them back to the ocean, but instead walk toward the brighter artificial lights on land. At night during foggy weather, when visibility is low, migrating birds can become disoriented by radio towers, especially those with heights greater than 200 m. In the United States there are more than 40,000 towers; where bird studies have been conducted, mortality rates have ranged from 375 to 3,285 per tower per year, and sometimes 1,000 birds have been killed on a single night. It is common knowledge that lights attract moths and night-flying insects, but few realize that they may be affecting their populations. The energy that moths spend attracted to artificial lights may prevent them from finding mates or good places to lay their eggs. Declines in moth populations may be linked to the effects of artificial lights on their reproduction. Plants whose reproduction is controlled by the lengths of day and night may not flower as a result of artificial lighting.

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