Environmental Encyclopedia

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Environmental Encyclopedia 3

Three Gorges Dam

Henry David Thoreau. (The Library of Congress.)

but he did not own it or have to pay taxes or upkeep on it;

he “borrowed” it from his friend Ralph Waldo Emerson

(1803–1882). It is not realistic for most people to try to

emulate Thoreau directly or to take his suggestions literally.

And he agreed, saying “I would not have anyone adopt my

mode of living on any account.” Still, he did figure out that

by working only six weeks a year, he could support himself

and so free the other forty-six to live as he saw fit.

Often characterized as an impractical dreamer, Tho-

reau’s wisdom is repeated widely today, a wisdom that is

commonly of direct applicability and down to earth.

Aldo

Leopold

, for example, is credited with the axiom that wood

you cut yourself warms you twice, but it can be found almost

a hundred years earlier in Thoreau’s chapter on “House-

Warming.” In 1850, he was an advocate of

national forest

preserves, writing eloquently on the subject in his essay

“Chesuncook” in The Maine Woods. He also predicted the

devastation wreaked on the shad runs by

dams

built on the

Concord River.

Thoreau was first and finally a writer. His greatest

contribution remains that his writing still raises the con-

sciousness of the reader, causing people who come in contact

with his work to be more aware of themselves and of the

world around them. As he said “only that day dawns to which

we are awake.” A raised consciousness means an increase in

humility because, as Thoreau teaches, “the universe is wider

1397

than our views of it.” He remained convinced “that to main-

tain one’s self on this earth is not a hardship but a pastime.”

[Gerald L. Young]

ESOURCES

OOKS

Fritzell, P. A. “Walden and Paradox: Thoreau as Self-Conscious Ecologist.”

In Nature Writing and America: Essays Upon a Cultural Type. Ames: Iowa

State University Press, 1990.

Johnson Jr., W. C. “Thoreau’s Language of Ecology.” In Wilderness Tapestry:

An Eclectic Approach to Preservation, edited by S. I. Zeveloff, L. M. Vause,

and W. H. McVaugh. Reno: University of Nevada Press, 1992.

Thoreau, H. D. Walden. New York: A. L. Burt, 1902.

Threatened species

see

Endangered species

Three Gorges Dam

When finished in 2009, the Three Gorges Dam on the

Chang Jiang (Yangtze River) will be the largest dam in the

world. Spanning 1.2 mi (2.0 km), and standing 600 ft (175

m) above normal river level, the dam will create a

reservoir

more than 400 mi (644 km) long. The project was designed

to control floods, ease navigation, and provide badly needed

electricity for China’s heartland. The 26 giant turbines in

the dam’s powerhouse are expected to generate 18,200 mega-

watts of electricity—equal to 10% of the country’s current

supply—to support industrialization and modernization in

the region. The reservoir will flood the scenic Three Gorges,

one of China’s most historically significant areas. It also will

displace at least 1.3 million people and will flood 150 towns

and cities, and more than 1,300 villages.

Proponents of the project argue that China needs the

project’s electricity for modernization. With dangerous rap-

ids drowned under the reservoir, ocean-going ships will be

able to sail all the way to Chongking, nearly 2,000 mi (3,200

km) inland from the ocean. The water stored in the reservoir

also will make possible a long-discussed plan to build aque-

ducts to carry water from southern China to the dry plains

around Beijing. Furthermore, annual

flooding

on the river

caused 300,000 deaths in the twentieth century. In 1998,

the worst flood in history drove 56 million people from their

homes. Planners expect the dam to reduce these floods and

eliminate untold misery for the 300 million people who live

in the Yangtze River Valley.

The Three Gorges Dam was first proposed in 1919

by Dr. Sun Yat-sen, a revolutionary and founder of the

Chinese republic. Later it was championed by Mao Tse-

tung (or Zedong) who led the country from 1949 to 1976.

Environmental Encyclopedia 3

Three Mile Island Nuclear Reactor

Mao saw this project as a way to instill national pride and

demonstrate China’s modernization. Although Mao ordered

a full-scale survey and project design in 1955, prohibitive

costs together with doubts about the safety and feasibility

of the dam have slowed construction. In 1992, Premier Li

Peng pushed through the final vote to authorize the project.

The first stage of construction was completed in 2002, and

flooding of the reservoir will begin in 2003. The water is

expected to reach its maximum height by 2009.

Environmentalists criticize the dam on the grounds

that it will reduce fish stocks, eliminate 78,000 acres (32,000

ha) of important agricultural lands, and threaten habitats of

critically

endangered species

such as the Yangtze river

dolphin, Chinese sturgeon, finless porpoise, and Yangtze

alligator. The reservoir will also fill one of China’s most

important historical and cultural regions, sinking 8,000 ar-

cheological sites and historic monuments. Currently, more

than 250 billion gallons (nearly a trillion liters) of untreated

sewage is dumped into the Yangtze every year. Planners

claim that the new cities into which residents are being

relocated will have better

sanitation

than the old river

towns. How sewage is handled in these new towns, and how

much contamination leeches out of abandoned structures in

the reservoir remains to be seen. Environmentalists worry

that the reservoir will become a stagnant cesspool, dangerous

to both aquatic life and to the millions of people who depend

on the river for their drinking water.

Sediment

and

silt

accumulation is another problem

to be solved, since tremendous volumes of sediment will

accumulate as the silt- and sand-laden Yangtze River slows

in the reservoir. Elsewhere in China silt and sand accumula-

tion has decreased reservoir storage capacity nationwide by

14%. Simply controlling river-bottom gravel may require

dredging

as much as 200,000 m

of material each year. To

reduce sediment accumulation, dam operators plan to let

spring floods, which carry much of the annual sediment

load, flow through silt doors at the bottom of the dam. They

hope this flow will scour out the bottom of the reservoir.

Geologists worry about catastrophic dam failure be-

cause the dam is built over an active seismic fault. Engineers

assure us that the dam can withstand the maximum expected

earthquake

, but China has a poor record of dam safety.

More than 3,200 Chinese

dams

have failed since 1949, a

failure rate of 3.7% compared to a 0.6% failure rate for the

rest of the world. Probably the worst series of dam failures

in world history occurred in Henan Province in 1975, when

heavy

monsoon

rains caused 62 modern dams to fall like

a line of dominoes. Some 230,000 people died in the massive

flooding that followed. Even if the dam is able to withstand

earthquakes, giant waves spawned by upstream landslides

could easily cause a calamitous dam failure. In 1986, a

land-

slide

just a few miles upstream from the dam site dumped

1398

15 million cubic meters of rock and

soil

into the river.

Witnesses reported an 260 ft (80 m) wave rolling down the

river. If a wave that size hits the upstream side of the dam

with a full reservoir, one geologist predicts a flood “of biblical

proportions” as a wall of water races downstream through

lowlands where hundreds of millions of people live.

The Chinese government is installing an early warning

system to predict landslides, and has banned timber cutting

and farming on steep upstream hillsides in an effort to control

both landslides and sediment loads in the river. Critics claim

that there would be safer and cheaper ways to store water

and generate electricity than a single huge dam. Original

estimates were that the Three Gorges Dam would cost $11

billion. By 2002, the costs for construction, relocation, and

landscape stabilization had risen to $75 billion, and the

project is not yet finished. A series of relatively small dams

on tributary streams might have been much cheaper and less

disruptive than the current project.

In 2002, the first stage of the dam was completed and

water started filling the reservoir. By late 2003, the water is

expected to reach a height of 438 ft (135 m) above normal

river level. Most of the new cities above the final 175 m

watermark have already been finished, and relocation of

residents is well underway. Destruction and decontamination

of cities that will be flooded has also begun.

[William P. Cunningham Ph.D.]

ESOURCES

OOKS

Dai Qing. The River Dragon Has Come! Armonk, New York: M. E.

Sharpe, 1998.

Hessler, Peter. River Town: Two Years on the Yangtze. New York: Harper-

Collins, 2001.

Leopold, Luna. Sediment Problems at Three Gorges Dam. Berkely, CA:

International Rivers Network, 1996.

Ryder, Grainne, ed. Damming the Three Gorges: What the Dam Builders

Don’t Want You to Know. Toronto: Probe International, 1990.

ERIODICALS

Sullivan, L. R. “The Three Gorges Project: Dammed If They Do?” Curent

History 94 (1995): 266–70.

Three Mile Island Nuclear Reactor

The 1970s were a decade of great optimism about the role of

nuclear power

in meeting world and national demands for

energy. Warnings about the declining reserves of

coal

petro-

leum

, and

natural gas

, along with concerns for the environ-

mental hazards posed by

power plants

run on

fossil fuels

fed the hope that nuclear power would soon have a growing

role in energy production. Those expectations were suddenly

and dramatically dashed on the morning of March 28, 1979.

Environmental Encyclopedia 3

Three Mile Island Nuclear Reactor

An aerial view of Three Mile Island Nuclear Reactor in Harrisburg, Pennsylvania. (AP/Wide World Photos. Reproduced

by permission.)

On that date, an unlikely sequence of events resulted

in a disastrous accident at the Three Mile Island (TMI)

Nuclear Reactor at Harrisburg, Pennsylvania. As a result of

the accident, radioactive water was released into the Susque-

hanna River, radioactive steam escaped into the

atmo-

sphere

, and a huge bubble of explosive

hydrogen

gas filled

the reactor’s cooling system. For a period of time, there

existed a very real danger that the reactor core might melt.

News of the accident produced near panic among resi-

dents of the area. Initial responses by government and indus-

try officials downplayed the seriousness of the accident and,

in some cases, were misleading and self-serving.

The first official study of the TMI accident was carried

out by the President’s Commission on the Accident at Three

Mile Island. Chaired by John G. Kemeny, then president

of Dartmouth College, the Commission attempted to recon-

struct the events that led to the accident. It found that the

accident was initiated during a routine maintenance opera-

tion in which a water purifier used in the system was replaced.

Apparently air was accidentally introduced into the system

along with the purified fresh water.

1399

Normally the presence of a foreign material in the

system would be detected by safety devices in the reactor.

However, a series of equipment malfunctions and operator

errors negated the plant’s monitoring system and eventually

resulted in the accident.

At one point, for example, operators turned off emer-

gency cooling pumps when faulty pressure gauges showed

that the system was operating normally. Also, tags which

hung on water pumps indicating that they were being re-

paired blocked indicator lights showing an emergency condi-

tion inside the reactor.

The Kemeny Commission placed blame for the TMI

accident in a number of places. They criticized the nuclear

industry and the

Nuclear Regulatory Commission

for be-

ing too complacent about reactor safety. They suggested that

workers needed better training. The Commission also found

that initial reactions to the accident by government and

industry officials were inadequate.

No one was killed in the TMI accident, but a 1990

study showed that there were no statistically significant in-

Environmental Encyclopedia 3

Threshold dose

creases in cancers for residents downwind of the accident.

A study completed in the mid-1990s, however, refutes those

findings stating that “cancer incidence, specifically, lung

can-

cer

and

leukemia

, increased more following the TMI acci-

dent in areas estimated to have been in the pathway of

radioactive plumes than in other areas.” This study also

indicates that the radiation levels may have been higher than

originally reported, and that cancer rates (lung/leukemia)

were 2–10 times higher than upwind rates. Cleanup opera-

tions at the plant took over six years to complete, and with

a $1 billion price tag, cost more than the plant’s original

construction.

Probably the greatest effect of the TMI accident was

the nation’s loss of confidence in nuclear power as a source

of energy. Since the accident, not one new nuclear power

plant has been ordered, and existing plans 65 others were

eventually canceled. See also Carcinogen; Radiation exposure;

Radioactive waste; Radioactivity

[David E. Newton]

ESOURCES

OOKS

Dresser, P. D. Nuclear Power Plants Worldwide. Detroit, MI: Gale Re-

search, 1993.

Ford, D. Three Mile Island: Thirty Minutes to Meltdown. New York: Pen-

guin, 1983.

Gray, M., and I. Rosen. The Warming: Accident at Three Mile Island. New

York: W.W. Norton, 1982.

Moss, T. H., and D. L. Sills, eds. The Three Mile Island Nuclear Accident:

Lessons and Implications. New York: Academy of Sciences, 1981.

ERIODICALS

Booth, W. “Post-Mortem on Three Mile Island.” Science (December 4,

1987): 1342–1345.

Stranahan, S. Q. “Three Mile Island: It’s Worse Than You Think.” Science

Digest (June 1985): 54–57+.

THER

President’s Commission on the Accident at Three Mile Island. Report of

the President’s Commission on the Accident at Three Mile Island. Washington,

DC: U.S. Government Printing Office, 1979.

Threshold dose

In radiology, the threshold dose is the smallest dose of radia-

tion that will produce a specified effect. In the larger context

of toxic exposure, threshold dose refers to the dose below

which no harm is done. There is a threshold below which

relatively little damage occurs from exposure, and above

which the damage increases dramatically. For noncarcino-

genic

toxins

, there does seem to be a safe dose for some

substances, a threshold dose below which no harm is done.

With carcinogens, however, just one change in the genetic

materials of one cell may be enough to cause a malignant

1400

transformation that can lead eventually to

cancer

. Although

there is some evidence that repair mechanisms or surveillance

by the immune system may reduce the incidence of some

cancers, it is generally considered prudent to assume that

no safe or threshold dose exists for carcinogens.

Tidal power

In looking for

alternative energy sources

to meet future

needs, some common physical phenomena are obvious can-

didates. One of these is tidal power. Twice each day on

every coastline in the world, bodies of water are pulled onto

and off of the shore as a result of gravitational forces exerted

by the moon and sun. Only on ocean coasts is this change

large enough to notice, however, and therefore, to take ad-

vantage of as an energy source.

The potential of tidal power as an energy source is

clearly demonstrated. Pieces of wood are carried onto a beach

and then off again every time the tide comes in or goes out.

In theory, the energy that moves this wood could also push

against a turbine blade and turn a generator.

In fact, the number of places on the earth where

tides are strong enough to spin a turbine is relatively

small. The simple back-and-forth movement seen on any

shoreline does not contain enough energy by itself. Geo-

graphical conditions must concentrate and focus tidal action

in a limited area. In such places, tides do not move in

and out at a leisurely pace, but rush in and out with the

force of a small river.

One of the few commercial tidal power stations in

operation is located at the mouth of La Rance River in

France. Tides at this location reach a maximum of 44 ft

(13.5 m). Each time the tide comes in, a dam at the La

Rance station holds water back until it reaches its maximum

depth. At that point, gates in the dam are opened and water

is forced to flow into the river, driving a turbine and generator

in the process. Gates in the dam are then closed, trapping

the water inside the dam. At low tide, the gates open once

again, allowing water to flow out of the river, back into the

ocean. Again the power of moving water is used to drive a

turbine and generator.

Hence the plant produces electricity only four times

each day, during each of two high tides and each of two

low tides. Although it generates only 250 megawatts daily,

the plant’s 25% efficiency rate is about equal to that of a

power plant operating on

fossil fuels

One area where tides are high enough to make a power

plant feasible is on Canada’s

Bay of Fundy

. Experiments

suggest that plants with capacities of 1–20 megawatts could

be located at various places along the bay. So far, however,

the cost of building such plants is significantly greater than

Environmental Encyclopedia 3

Tigers

the cost of building conventional

power plants

of similar

capacity. Still, optimists suggest that serious development

of tidal power could provide up to a third of the electrical

energy now obtained from hydropower worldwide at some

time in the future.

Despite the drawbacks and expense of tidal power,

China is one country that has used it extensively. Since

energy needs are often modest in China, low capacity plants

are more feasible. As of the late 1980s, therefore, the Chinese

had built more than 120 tidal power plants to provide elec-

tricity for small local regions. See also Dams; Energy and

the environment; Wave power

[David E. Newton]

ESOURCES

ERIODICALS

Greenburg, D. A. “Modeling Tidal Power.” Scientific American 257 (No-

vember 1987): 128–135.

Holloway, T. “Tidal Power.” Sea Frontiers 35 (March-April 1989): 114–49.

“The Potential of Tidal Power—Still All At Sea.” New Scientist 126 (May

19, 1990): 52.

Webb, J. “Tide of Optimism Ebbs Over Underwater Windmill.” New

Scientist 138 (April 24, 1993): 10.

Tigers

Tigers (Panthera tigris) are the largest living members of the

family Felidae, which includes all cats. Siberian tigers (P. t.

altaica) are the largest and most massive of the eight recog-

nized subspecies. They normally reach a weight of 660 lb

(300 kg), with a record male that reached 845 lb (384 kg).

Several of the subspecies have had their populations totally

decimated and are probably extinct, mostly through direct

human actions. The species’ range, overall, has been greatly

reduced in historic times. Currently, tigers are found in

isolated regions of India, Bangladesh, Nepal, Bhutan, South-

east Asia, Manchuria, China, Korea, Russia, and Indonesia.

Tigers are designated “endangered” by the U. S.

Fish and

Wildlife Service

and by

IUCN—The World Conservation

Union

. They are also listed in Appendix I of the

Convention

on International Trade in Endangered Species of Wild

Fauna and Flora

Unlike their relatives, the lion and the cheetah, tigers

are not found in open habitats. They tend to be solitary

hunters, stalking medium- to large-sized prey (such as pigs,

deer, antelope, buffalo, and gaur) in moderately dense cover

of a variety of forest habitats, including

tropical rain forest

moist coniferous and deciduous forests, dry forests, or man-

grove swamps. Within their

habitat

both males and females

establish home ranges that do not overlap with members of

their own sex. Home ranges average 8 mi

(21 km

) for

1401

females, but vary from 25–40 mi

(65–104 km

) for males

depending on prey availability and to allow for the inclusion

of several females in his range. Tigers that live in areas of

prime habitat raise more offspring than can establish ranges

within that habitat, therefore, several are forced to the pe-

riphery to establish territories and live. This creates an im-

portant condition in that this series of peripheral individuals

helps promote genetic mixing in the breeding population.

Tigers are often labeled “maneaters.” Although most

tigers shy away from humans, some have been provoked

into attack. Others, having been encountered unexpectedly,

attack people as a defense, and a very few are thought to

hunt humans consciously. An estimated 60–120 people fall

victim to tigers each year. Tigers’ typical prey includes larger

mammals such as deer and buffalo, and the cats will actively

search for this prey, instead of waiting in ambush. Tigers

hunt alone, and, even though they are highly skilled preda-

tors, are rarely successful more than once in every 15 at-

tempts. With a

scarcity

of habitat for large prey and reduced

cover, many tigers will opportunistically attack domesticated

livestock, thus, themselves becoming targets of humans.

Population pressures from humans, habitat loss,

poaching

, and

overhunting

have led to the

extinction

probable extinction of four subspecies and large reductions

in the populations of the other four subspecies of tiger.

The Balinese tiger (P. t. balica) has been extinct for several

decades. The South China tiger (P. t. amoyensis), which was

the target of an extermination campaign, may be extinct in

the wild. There has been only one unconfirmed report of

the Caspian tiger (P. t. virgata) in the last 30 years, thus it

is probably extinct. The Javan tiger (P. t. sondiaca) is also

probably extinct, as its population was reduced to four or

five individuals, and there has not been a confirmed sighting

since the late 1970s. Little is known of the population levels

of the Indochinese tiger (P. t. corbetti) but the number is

thought to be between 1,000 and 1,500, with 60 in captivity.

The Sumatran tiger (P. t. sumatrae) has a population in the

wild of about 500 individuals. The Siberian tiger, whose

population dropped to 20–30 individuals in 1940, has a

population today of over 500 in the wild. The Bengal tiger

(P. t. tigris) has been the target of renewed poaching efforts

since mid-1990. The bones of these tigers bring a handsome

price on the black market. Tiger bones are ground up, dis-

solved in a liquid, and used in Chinese medicine. Early

estimates are that 500 or more Bengal tigers have been

poached in the last three years for this purpose.

[Eugene C. Beckham]

ESOURCES

OOKS

Grzimek, B., ed. Grzimek’s Encyclopedia of Mammals. Vol. 4. New York:

McGraw-Hill, 1990.

Environmental Encyclopedia 3

Tilth

Nowak, R. M. Walker’s Mammals of the World. 6th ed. 2 vols. Baltimore:

Johns Hopkins University Press, 1999.

ERIODICALS

Angier, Natalie. “The Cat Comes Back.” New York Times Upfront (January

31, 2000): 24.

Luoma, J. R. “The State of the Tiger.” Audubon 89 (1987): 61–63.

“Man Saving Endangered Tigers.” Xinhua News Agency (March 15, 2001).

THER

The Tiger Foundation. [cited May 2002]. <http://www.tigers.ca>.

Tilth

Tilth is the ability of the

soil

to facilitate tillage, resist weeds,

and allow plants to take root. It can also refer to the act of

tilling, or cultivating soil. Some have related soil tilth to the

physical condition of the soil created by integrating the

effects of all physical, chemical, and biological processes

occurring within a soil matrix. Soil tilth is usually used as a

general descriptive term (good, moderate, or poor) rather

than a precisely defined scientific quotient. See also Agricul-

tural chemicals; Arable land; Soil fertility

Timberline

The elevational or latitudinal extent of forests. Above upper

timberline, either at high elevations or at the Arctic or Ant-

arctic limits, atmospheric and

soil

temperatures are too cold

for forest development. Below lower timberline, conditions

are too dry to support forests. Timberlines are more common

and pronounced in the western than in the eastern United

States because of more extreme topographic and climatic

conditions in the West.

Times Beach

More than 2,000 residents of this Missouri town, located

about 30 mi (48 km) southwest of St. Louis, were evacuated

after it was contaminated with

dioxin

. In 1971, chemical

wastes containing dioxin were mixed with oil and sprayed

along the streets of Times Beach to keep down the dust.

The spraying was done by Russell M. Bliss, whose company

collected and disposed of waste oils and

chemicals

from

service stations and industrial plants. Much of this toxic

waste oil was sprayed on roads throughout Missouri.

Shortly after the spraying around Times Beach, horses

on local farms began dying mysteriously. At one breeding

stable, from 1971 to 1973, 62 horses died, as well as several

dogs and cats.

Soil

samples were sent to the Centers for

Disease Control (CDC) in Atlanta for analysis, and after

1402

three years of testing, the agency determined that dangerous

levels of dioxin were present in the soil.

A decade after the spraying, the town experienced the

worst

flooding

in its history. On December 5, 1982, the

Meramec River flooded its banks and inundated the town,

submerging homes and contaminating them and almost ev-

erything else in Times Beach with dioxin. This compound

binds tightly to soil and degrades very slowly, so it was still

present in significantly high levels ten years after being used

on the roads as a dust suppressant. Following a CDC warn-

ing that the town had become uninhabitable, most of the

families were temporarily evacuated. The U.S.

Environmen-

tal Protection Agency

(EPA) eventually agreed to buy out

the town for about $33 million and relocate its inhabitants.

At the time, the CDC considered soil dioxin levels of

one part per billion (ppb) and above to be potentially hazard-

ous to human health. Levels found in some areas of Times

Beach reached 100 to 300 ppb and above. Dioxin is consid-

ered a very toxic chemical. Exposure to this chemical has

been linked to

cancer

, miscarriages, genetic mutations, liver

and nerve damage, and other health effects, including death,

in humans and animals.

Indeed, the contamination seemed to take a serious

toll on the health of Times Beach residents. Town officials

claimed that virtually every household in Times Beach expe-

rienced health disorders, ranging from nosebleeds, depres-

sion, and chloracne (a severe skin disorder) to cancer and

heart disease. Almost all of the residents tested for dioxin

contamination showed abnormalities in their blood, liver,

and kidney functions.

By 1983, federal and state officials had located about

100 other sites in Missouri where dioxin wastes had been

improperly dumped or sprayed, with levels of the compound

reaching as high as 1,750 ppb in some areas.

A decade after the evacuation of Times Beach, debate

over dioxin’s dangers continues, and some CDC officials

now say that the agency overreacted and that the town should

not have been abandoned. But by then, Times Beach had

become a household name, joining

Love Canal

, New York,

and

Seveso, Italy

, on the list of municipalities that were

ruined by toxic chemical contamination. See also Carcinogen

[Lewis G. Regenstein]

ESOURCES

ERIODICALS

“Dioxin Cleanup: Status and Opinions.” Science News 141 (January 11,

1992): 30.

Felton, E. “The Times Beach Fiasco.” Insight 7 (August 12, 1991): 12–19.

Gorman, C. “The Double Take on Dioxin.” Time 138 (August 26,

1991): 52.

Kemezis, P. “Times Beach Cleanup Begins.” ENR 225 (August 2, 1990):

37–8.

Environmental Encyclopedia 3

Toilets

Tipping fee

The fee charged by the owner or operator of a

landfill

for

the acceptance of a unit weight or volume of

solid waste

for disposal, usually done by the truckload. The tipping fee

is passed back along the chain of waste acceptor to hauler

to generator in the form of fees or taxes. Tipping fees rise

as the volume of available landfill space is depleted, or as it

becomes harder to open new landfills due to public opposi-

tion and stricter environmental regulations. See also Garbage;

Municipal waste; Transfer station; Waste management;

Waste stream

Tobacco

Tobacco (Nicotiana tabacum) is an herbaceous plant culti-

vated around the world for its leaves, which can be rolled

into cigars, shredded for cigarettes and pipes, processed for

chewing, or ground into snuff. Tobacco leaves are the source

of commercial nicotine, a component of many pesticides.

The tobacco plant is fast-growing with a stem from 4–8 ft

(1–3 m) in height.

Native to the Americas, tobacco was cultivated by

Native Americans, and Christopher Columbus found them

using it in much the same manner as today. American Indi-

ans believed it to possess medicinal properties, and it was

important in the ceremonies of the plains tribes.

Tobacco was introduced into Europe in the mid-1500s

on the basis of its purported medicinal qualities. Tobacco

culture by European settlers in America began in 1612 at

Jamestown, and it soon became the chief commodity ex-

changed by colonists for articles manufactured in Europe.

The leading tobacco-growing countries in the world

today are China and the United States, followed by India,

Brazil, and Turkey, as well as certain countries in the former

Soviet Union. Although about one-third of the annual pro-

duction in the United States is exported, the country also

imports about half as much tobacco as it exports. The leading

tobacco-growing state is North Carolina, followed by Ken-

tucky, South Carolina, Tennessee, Virginia, and Georgia.

Nicotine occurs in tobacco along with related alkaloids

and organic acids such as malic and citric. Nicotine content

is determined by the

species

, variety, and strain of tobacco;

it is also affected by the growing conditions, methods of

culture and cure, and the position on the plant from which

the leaves are taken. Tobacco is high in ash content, which

can range from 15–25% of the leaf. Flue-cured tobacco is

rich in sugar, and cigar tobaccos are high in nitrogenous

compounds but almost free of starch and sugars.

Most tobacco products are manufactured by blending

various types of leaves, as well as leaves of different origins,

1403

grades, and crop years. Cigarette manufacturers usually add

sweetening preparations and other flavorings, but the prepa-

ration of tobaccos for pipe smoking and chewing is as varied

as the assortment of products themselves. Snuff is made by

fermenting fire-cured leaves and stems and grinding them

before adding salts and other flavorings. Cigars are made by

wrapping a binder leaf around a bunch of cut filler leaf and

overwrapping with a fine wrapper leaf.

In the United States and elsewhere, stems and scraps

of tobacco are used for nicotine extractions. They can also

be ground down and made into a reconstituted sheet in a

process like papermaking, which is used as a substitute cigar

binder or wrapper or cut to supplement the natural tobacco in

cigarettes. See also Agricultural chemicals; Cigarette smoke;

Respiratory diseases

[Linda Rehkopf]

ESOURCES

OOKS

Chandler, W. U. Banishing Tobacco. Washington, DC: Worldwatch Insti-

tute, 1986.

ERIODICALS

Heise, L. “Unhealthy Alliance: With U.S. Government Help, Tobacco

Firms Push Their Goods Overseas.” World Watch (September-October

1988): 19–28.

THER

National Academy of Sciences. Environmental Tobacco Smoke: Measuring

Exposures and Assessing Health Effects. Washington, DC: National Academy

Press, 1986.

Toilets

The origin of the indoor toilet for the disposal of human

wastes goes far back in history. Archaeologists found in the

palace of King Minos on Crete an indoor latrine that had

a wooden seat and may have worked like a modern flush

toilet; they also discovered a water-supply system of terra

cotta pipes to provide water for the toilet. Between 2500

and 1500

B.C.

, cities in the Indus Valley also had indoor

toilets that were flushed with water. The

wastewater

was

carried to street drains through brick-lined pits. In 1860,

Reverend Henry Moule invented the earth closet, a wooden

seat over a bucket and a hopper filled with dry earth, charcoal,

or ashes. The user of the toilet pulled a handle to release a

layer of earth from the hopper over the wastes in the bucket.

The container was emptied periodically. During the eigh-

teenth and nineteenth centuries in Europe, human wastes

were deposited in pan closets or jerry pots. After use, the

pots were emptied or concealed in commodes. The contents

of the jerry pots were often collected by nearby farmers who

used the wastes as organic

fertilizer

. However, as cities

Environmental Encyclopedia 3

Tolerance level

grew larger,

transportation

of the wastes to farms became

uneconomical, and the wastes were dumped into communal

cesspits or into rivers. The flush toilet common in use today

was supposedly invented by Thomas Crapper in the nine-

teenth century; Wallace Rayburn wrote a biography of Crap-

per, titled Flushed with Pride, in 1969.

The development of the flush toilet was primarily re-

sponsible for the development of the modern sanitary system,

consisting of a maze of underground pipes, pumps, and

centralized treatment systems. Modern sanitary systems are

efficient in removing human and other wastes from human

dwellings but are costly in terms of capital investment in

the infrastructure, operational requirements, and energy re-

quirements. The treated wastewater is usually disposed of

in rivers and lakes, sometimes causing adverse impacts upon

the receiving waters.

Sanitary systems require an abundant supply of water,

and the flush toilet is responsible for the largest use of water

in the home. Each flush of a conventional water-carriage

toilet uses between 4–7 gal (15–26 L) of water, depending

on the model and water supply pressure. The average amount

of water used per flush is 4.3 gal (16 L). Since each person

flushes the toilet an average of 3.5 times per day, the average

daily flow per person is approximately 16 gal (60 L) for a

yearly flow of 5,840 gal (22,104 L).

To reduce the volume of water used for flushing, a

variety of devices are available for use with a conventional

flush toilet. These devices include:

Tank insert - a displacement device placed in storage tank

of conventional toilets to reduce the volume (but not the

height) of stored water.

Dual flush toilet - devices used with conventional toilets

to enable the user to select from two flush volumes, based

on the presence of solid or liquid waste materials.

Water-saving toilet - variation of conventional toilet with

redesigned flushing rim and priming jet that allows the

initiation of the siphon flush in a smaller trapway with less

water.

Pressurized and compressed air (assisted flush toilet) -

variation of conventional toilet designed to utilize com-

pressed air to aid in flushing by propelling water into the

bowl at increased velocity.

Vacuum-assisted flush toilet - variation of conventional

toilet in which the fixture is connected to a vacuum system

that is used to assist a small amount of water in flushing.

In addition to modifications to conventional flush toi-

lets, non-water carriage toilets are available to reduce the

amount of water required. They are also used for disposing

of toilet wastes. Types of non-water carriage toilet systems

include:

1404

Composting toilet - self-contained units that accept toilet

wastes and utilize the addition of heat in combination

with

aerobic

biological activity to stabilize human excreta;

larger units may accept other organic wastes in addition

to toilet wastes and requires periodic disposal of residuals.

Incinerating toilet - small self-contained units that utilize

a burning assembly or heating element to volatilize the

organic components of human waste and evaporate the

liquids; requires periodic disposal of residuals.

Oil-recycle toilets - self-contained unit that uses a mineral

oil to transport human excreta from a toilet fixture to a

storage tank; oil is purified and reused for flushing; requires

removal and disposal of excreta from storage tank periodi-

cally (usually annually).

The wastes from toilets are referred to as “blackwater.”

If the wastes from toilets are segregated and handled sepa-

rately using alternative non-water carriage toilets from the

wastewaters generated from other fixtures in the home (re-

ferred to as “graywater"), significant quantities of pollutants,

especially suspended solids,

nitrogen

, and pathogenic

organisms, can be eliminated from the total wastewater flow.

Graywater, though it still may contain significant numbers

of pathogenic organisms, may be simpler to manage than

total residential wastewater due to a reduced flow volume.

See also Municipal solid waste; Sewage treatment

[Judith Sims]

ESOURCES

ERIODICALS

Love, S. “An Idea in Need of Rethinking: The Flush Toilet.” Smithsonian

6 (1975): 61–66.

Rodale, R. “Goodbye to the Flush Toilet.” Compost Science 12 (1971): 24–25.

THER

U.S. Environmental Protection Agency. Design Manual: Onsite Wastewater

Treatment and Disposal Systems. Cincinnati, OH: Municipal Environmental

Research Laboratory, U.S. Environmental Protection Agency, 1980.

Tolerance level

Tolerance level refers to the maximum allowable amount of

chemical residue, such as a

pesticide

, legally permitted in

food. Tolerance levels are determined by government agen-

cies such as the

Environmental Protection Agency

(EPA)

and the

Food and Drug Administration

(FDA), and are

based on the results of testing, primarily animal testing. This

testing determines the dosage level at which few or no effects

are observed. This dose is then adjusted to a human equiva-

lency dose with a margin of error built in for added safety.

While pesticides play an important role in modern

society, and are used for a variety of purposes, not until

Environmental Encyclopedia 3

Topsoil

recent decades was it clear how dangerous and persistent

many pesticides are. The EPA monitors only a small fraction

of pesticides, and in many cases their probable effects on

the

environment

are unknown. Pesticide tolerance levels

are particularly important worldwide, since pesticides are

found in the tissues of people living even in very remote

areas of the world. For example, as a result of environmental

contamination, the concentration of pesticides in human

breast milk has, at certain times in some areas, exceeded the

tolerance level for cow’s milk.

To establish tolerance levels, scientists conduct a

risk

assessment

, or an evaluation of the hazards to the environ-

ment, including human health, from exposure to the sub-

stance. Data on the toxicity of the substance are combined

with data about exposure. The calculation assesses the theo-

retical maximum residue contribution, or the amount of a

chemical that would be present in the average daily diet if

all foods treated by that chemical had amounts at the toler-

ance level. The highest concentration allowable is called the

maximum acceptable tolerance concentration. For drinking

water, tolerance levels are called the National Primary Drink-

ing Water Requirements. For

air pollution

standards, toler-

ance levels are called the permissible exposure level.

Environmentalists who criticize the government’s tol-

erance levels charge that standards are not safe enough either

for the general population or for workers exposed to

toxins

Critics also claim that tolerance levels do not take into ac-

count either the cumulative effect of residues from a variety

of sources in the environment, the duration of exposure, or

the unanticipated effects of two or more

chemicals

com-

bined. Tolerance levels also are criticized for not recognizing

threats to at-risk populations such as children, the elderly,

and pregnant women.

Animal rights

activists have criticized

the use of animals to determine tolerance levels, and advocate

the use of computer models or in vitro testing (testing on

tissue cultures) to make determinations of toxicity or safety

of chemicals. See also Pesticide residue; Toxic substance

[Linda Rehkopf]

ESOURCES

OOKS

Harte, John, et al. Toxics A to Z. Berkeley: University of California

Press, 1991.

Toluene

Toluene is a potentially hazardous liquid that occurs natu-

rally in crude oil, and can be obtained from the tolu tree.

Toluene is used in manufacturing paints, adhesives, and

rubber

, as well as in the leather-making and printing indus-

tries. People can be exposed to toluene by breathing

automo-

1405

bile

exhaust, fumes from paints, kerosene, and heating oils,

or by drinking well-water that is contaminated with toluene.

At high doses, toluene may causes dizziness, unconscious-

ness, and possibly death. High doses may cause kidney dam-

age, while low doses may cause nausea, hearing and vision

problems, disorientation, and fatigue. Toluene in air is very

reactive so it increases

photochemical smog

and leads to

the production of peroxybenzoylnitrate, a potent eye irritant.

The

Environmental Protection Agency

(EPA) does not

classify toluene as a

carcinogen

, although the EPA has set

a limit of 1 milligram per liter for safe drinking water.

[Marie H. Bundy]

Top predator

see

Predator-prey interactions

Topography

The relief or surface configuration of an area. Topographic

studies are valuable because they show how lands are devel-

oped and give insight into the history and relative age of

mountains or plains. Topographic features are developed by

physical and chemical processes. Physical processes include

the relatively long-term tectonic actions and continental

movement that lead to subduction of lands in some cases

or to the development of high elevation mountains. Earth

surface forms are usually altered more quickly by the action

of water, ice, and wind, leading to the development of deep

canyons, leveling of mountains, and filling of valleys. Chemi-

cal processes include oxidation, reduction, carbonation, solu-

tion, and hydrolysis. These reactions lead to the alteration

of organic and mineral materials that also influence the

topographic forms of the earth.

Topsoil

The upper portion of the

soil

that is used by plants for

obtaining water and nutrients, often referred to as the hori-

zon. Higher levels of organic matter in topsoil cause it to

be darker and richer than

subsoil

and give it greater potential

for crop production. The loss of topsoil is a critical problem

worldwide. The net effect of this widespread topsoil

erosion

is a reduction in crop production equivalent to removing

about 1% of the world’s cropland each year. Soil material

that can be purchased to add to existing surface soils is also

referred to as topsoil. This kind of topsoil may or may not

be from the surface of a soil. Often it is a dark-colored soil

that is high in organic matter and in some cases may be

more organic soil than mineral soil.

Environmental Encyclopedia 3

Tornado and cyclone

A tornado is a vortex or powerful whirling wind, often visible

as a funnel-shaped cloud hanging from the base of a thunder-

storm. It can be very violent and destructive as it moves

across land in a fairly narrow path, usually a few hundred

yards in width. Wind speeds are most often too strong to

measure with instruments and are often estimated from the

damages they cause. Winds have been estimated to exceed

350 mph (563 kph). Very steep pressure gradients are also

associated with tornados and contribute to their destructive-

ness. Sudden changes in atmospheric pressure taking place

as the storm passes sometimes cause walls and roofs of build-

ings to explode or collapse.

In some places such storms are referred to as cyclones.

Tornados most frequently occur in the United States in the

central plains where maritime polar and maritime tropical air

masses often meet, producing highly unstable atmospheric

conditions conducive to the development of severe thunder-

storms. Most tornados occur between noon and sunset, when

late afternoon heating contributes to atmospheric instability.

Torrey Canyon

The grounding in 1967 of the supertanker Torrey Canyon

on protruding granite rocks near the Scilly Isles off the

southwest coast of England introduced the world to a new

hazard: immense

oil spills

, especially from supertankers. It

exposed current technology’s inability to handle such massive

quantities of spilled oil and the need for ship design that

would help prevent them.

Although oil spills are tragic, each incident provides

insight into the progress being made to prevent them. The

Torrey Canyon accident pointed out needed research in the

area of oil spills. These include the efficacy of storm waves,

oil-consuming

microbes

, and time to heal the

envi-

ronment

This wreck demonstrated the inadequacies of existing

international marine law, especially questions of responsibil-

ity and liability. The international aspects of the Torrey

Canyon case were complex. This American-owned vessel

was registered in Liberia, sailed with a crew of mixed nation-

als, and grounded in United Kingdom waters, also contami-

nating the Brittany coast of France. The British government

set precedent by ordering the bombing of the wreck in a

futile effort to torch the remaining oil.

Efforts to salvage the spilled oil quickly proved useless,

as the escaping oil rapidly thinned and became widespread.

It was later learned that the most volatile parts of the oil

evaporated within several days. Fortunately, this amounted

to a large percentage of the total spill.

1406

A number of crucial lessons were learned as a result

of abatement efforts. Dispersants, which break the oil into

tiny droplets, were inadequate and applied too little and too

late. Once “mousse” (the term for water encased in the oil)

forms under wave action, little can be done to break up this

ecologically hazardous muck. One of the first actions in

response to the 1993 grounding of the Braer near the Shet-

land Islands was the aerial spraying of dispersants on the

rapidly thinning oil.

Use of

detergents

on the intertidal zone of England’s

resort beaches proved deadly to grazing organisms. The most

effective treatment of oil-tainted water and beaches was

discovered to be nature’s storm action combined with meta-

bolic breakdown by

microorganisms

. Efforts to clean oiled

seabirds proved largely futile, as they succumbed to hypo-

thermia, stress, and

poisoning

from ingested oil.

The French discovered an effective emergency tactic

to lessen the damage. They used straw and other absorbent

materials to sop up the incoming oil. They also protected a

100-year-old research section at Roscoff by making a long

boom out of burlap stuffed with straw and wood cuttings,

an operation dubbed “Big Sausage.” To compensate for inad-

equate anchoring, students physically held the boom in place

while the tide rolled in, with new troops periodically relieving

those chilled in the cold water. See also Amoco Cadiz, Clean

Water Act; Exxon Valdez

[Nathan H. Meleen]

ESOURCES

OOKS

Cowan, E. Oil and Water: The Torrey Canyon Disaster. Philadelphia: Lippin-

cott, 1968.

Petrow, R. In the Wake of Torrey Canyon. New York: David McKay, 1968.

Williams, A. S. Saving Oiled Seabirds. Rev. ed. Washington, DC: American

Petroleum Institute, 1987.

ERIODICALS

Walsh, J. “Pollution: The Wake of the Torrey Canyon. Science 160 (April

12, 1968): 167–169.

THER

Office of Technology Assessment. Coping With Oiled Environments. Wash-

ington, DC: U.S. Government Printing Office, 1989.

Toxaphene

Toxaphene was once the nation’s most heavily used

pesti-

cide

and accounted for one-fifth of all pesticide use in the

United States. It was used mainly on cotton and dozens of

food crops and on livestock to kill

parasites

. It is severely

toxic to fish and

wildlife

and has been implicated in massive

kills of fish, ducks, pelicans, and other waterfowl. Because

of this, its ability to cause cancerous tumors and genetic