Environmental Encyclopedia

Подождите немного. Документ загружается.

Environmental Encyclopedia 3

Strategic minerals

Global mineral trade. (McGraw-Hill Inc. Reproduced by permission.)

examples of strategic minerals are tin, silver, cobalt, manga-

nese, tungsten, zinc, titanium, platinum, chromium, bauxite,

and diamonds. The United States must import at least half

the amount of each of these minerals that it uses each year.

Ensuring a constant and dependable supply of strategic

minerals is a complex political problem. In some cases, the

minerals we need can be obtained from friendly nations

with whom we can negotiate relatively easily. Canada, for

example, supplies a large part of the

nickel

, columbium,

gallium, tantalum,

cadmium

, and cesium used by American

industry. Nickel may be obtained from Norway; cobalt and

antimony from Belgium; and fluorspar from Italy.

Other nations on whom we depend, however, are less

friendly, less dependable, or less stable. The southern African

nations of Zaire, Zambia, Zimbabwe, Botswana, and South

Africa, for example, are major suppliers of such strategic

minerals as chromium, gold, platinum, vanadium, manga-

nese, and diamonds. Africa also has large deposits of copper,

cobalt and chromium. When these nations experience politi-

cal unrest, supplies of these minerals may become scarce. A

report issued by the United States Agency for International

Development (USAID) in November 2000 underscored the

1347

point that ongoing American investment in Africa is neces-

sary because of our need for the continent’s strategic min-

erals.

Since 1980, the United States government has tried

to protect American industry, especially defense industries,

from the danger of running out of strategic minerals. Some

analysts have feared that political factors might result in

the loss of certain minerals that are needed by industry,

particularly those used in the manufacture of military hard-

ware. In 1984, the United States Congress established the

National Critical Materials Council (NCMC) to advise the

President on issues involving strategic minerals. The Council

monitored domestic and international needs and trends for

nearly a decade to ensure the nation’s access to a dependable

supply of strategic minerals. In 1993, the NCMC was placed

under the supervision of the newly established National Sci-

ence and Technology Council.

A similar monitoring function has been performed by

the Defense Logistics Agency (DLA), a division of the

Department of Defense dating back to World War II. The

DLA maintains and administers the Defense National

Stockpile Center, or DNSC, which sells and maintains stra-

Environmental Encyclopedia 3

Stratification

tegic and critical materials in order to reduce the country’s

dependence on foreign sources of supply. The DNSC is

presently located in Fort Belvoir, Virginia.

In addition to political unrest, however, the supply of

strategic minerals depends on humankind’s general manage-

ment of the earth’s

nonrenewable resources

. Of the min-

erals in common use in manufacturing and industry, 80 are

in abundant supply. There are, however, about 18 minerals

that are expected to be in short supply around the globe by

2015; these include gold, silver,

mercury

, lead, sulfur, tin,

tungsten, and zinc. It is estimated that the earth’s stores of

these minerals will be 80% depleted by 2040. Strategies for

lowering present levels of use in the developed nations in-

clude

recycling

; inventing new substitute materials and find-

ing new uses for old ones; decreasing the size of products;

and extending the lifespan of items made from strategic

minerals. A recent report to the Canadian government from

the assistant deputy minister for the minerals and metals

sector of

natural resources

stated that competition among

nations for strategic minerals has already begun to transform

the manufacturing processes used by Canadian industries as

of 2002.

[David E. Newton and Rebecca J. Frey Ph.D.]

ESOURCES

OOKS

Anderson, Ewan W., and Liam D. Anderson. Strategic Minerals: Resource

Geopolitics and Global Geo-Economics. New York: John Wiley and Sons,

1997.

Collins, John M. Military Geography for Professionals and the Public. Wash-

ington, DC: National Defense University Press, 2001.

Strategic Minerals: A Resource Crisis. Washington, DC: Council on Econom-

ics and National Security, 1981.

U.S. Bureau of Mines. The Domestic Supply of Critical Minerals. Washington,

DC: U S. Government Printing Office, 1983.

ERIODICALS

Homer-Dixon, Thomas F. “Environmental Scarcity, Mass Violence, and

the Limits to Ingenuity.” Current History 95 (November 1996): 359–365.

THER

Derryck, Vivian L. Remarks to the Foreign Service Association of Northern

California. Presentation given in San Francisco, CA, November 3, 2000.

Kelly, Thomas, David Buckingham, and Carl DiFrancesco, et al. Historical

Statistics for Mineral Commodities in the United States. United States Geolog-

ical Survey Open-File Report 01-006, June 2002.

Minerals and Metals Sector of Natural Resources Canada. Focus 2006:

A Strategic Vision for 2001–2006. Ottawa, Ontario: Public Works and

Government Services Canada, May 2002.

RGANIZATIONS

U.S. Department of the Interior, U.S. Geological Survey, Minerals

Information, 988 National Center, Reston, VA USA 20192 Toll Free: (888)

ASK-USGS (888-275-8747), Email: Contact person: Joseph Gambogi,

jgambogi@usgs.gov, www.minerals.usgs.gov/minerals/

1348

Stratification

The process by which a region is divided into relatively

distinct, nearly horizontal, layers. Sedimentary rock often

consists of various strata because the rock was laid at different

times when different materials were being deposited. Lakes

and oceans also consist of strata where plant and animal life

may be very different depending on the temperature and

amount of light available. The

atmosphere

is divided into

strata that differ in density, temperature, chemical composi-

tion, and other factors. In the lower atmosphere, temporary

stratification, such as temperature inversions, may occur,

often resulting in severe

pollution

conditions.

Stratosphere

A layer of the

atmosphere

that lies between about 7 and

31 mi (11 and 50 km) above the earth’s surface, bounded

at the bottom by the

tropopause

and at the top by the

stratopause. Scientists became aware of the presence of the

stratosphere with observations of high level dust after the

eruption of

Krakatoa

in 1883. However, the real discovery

of the stratosphere had to await Teisserenc de Bort’s work

of 1900.

The stratosphere is characterized by temperatures that

rise with height. As a result, the air is very stable, not mixing

much vertically, and allowing distinct layers of air, or strata,

to form. There are also persistent regular strong winds, of

which the best known are the intense western winds during

the winter called the polar night jet stream.

The air in the stratosphere has much the same compo-

sition as the lower atmosphere except for a higher proportion

ozone

Absorption

of incoming

solar energy

by this

ozone makes the upper parts of the stratosphere warm and

sets up the characteristic temperature gradient. The relatively

high ozone concentrations are maintained by photochemical

reactions. It has become clear in more recent years that other

chemical reactions involving

nitrogen

and

chlorine

are also

important in maintaining the ozone balance of the upper

atmosphere. This balance can be easily disturbed through

the input of additional

nitrogen oxides

and halogen com-

pounds (from CFCs, or

chlorofluorocarbons

). Transfer of

gases across the tropopause into the stratosphere is rather

slow, but some gases, such as CFCs and

nitrous oxide

, are

sufficiently long-lived in the

troposphere

to leak across

into the stratosphere and cause

ozone layer depletion

Large volcanic eruptions can have sufficient force to drive

gases and particles into the stratosphere where they can also

disturb the ozone balance. High-flying aircraft represent

another source of

pollution

in the stratosphere, but the lack

of development of a supersonic passenger fleet has meant

that this contribution has remained fairly small.

Environmental Encyclopedia 3

Stringfellow Acid Pits

The stratosphere is extremely cold (about −112°F

[−80°C]) in places, so there is relatively little water. Never-

theless, nacreous or mother-of-pearl clouds, although not

frequently observed, have long been known. More recently

there has been much interest in polar stratospheric clouds

which had hitherto received little attention. Stratospheric

cloud particles can be water-containing sulfuric

acid

droplets

or solid nitric acid hydrates. Studies of the Antarctic ozone

hole have revealed that these clouds are likely to play an

important role in the depletion of ozone in the polar strato-

sphere. See also Acid rain; Cloud chemistry; Ozonation;

Stratification; Volcano

[Peter Brimblecombe Ph.D.]

ESOURCES

OOKS

Chamberlain, J. W., and D. M. Hunten. Theory of Planetary Atmospheres.

New York: Academic Press, 1987.

Stray voltage

see

Electromagnetic field

Stream channelization

The process of straightening or redirecting natural streams

in an artificially modified or constructed stream bed. Chan-

nelization has been carried out for numerous reasons, most

often to drain

wetlands

, direct water flow for agricultural

use, and control

flooding

. While this process makes a stream

more useful for human activities, it tends to interfere with

natural river habitats and to destabilize stream banks by

destroying riparian vegetation. When annual flood patterns

are disrupted, fertilizing

sediment

is no longer deposited

on river banks and excessive sediment accumulation can

occur downstream. Perhaps most importantly, wetland

drainage

and the removal of instream obstacles such as

rocks, fallen trees, shallow backwaters, and sand bars elimi-

nate feeding and reproductive habitats for fish, aquatic in-

sects, and birds.

Stringfellow Acid Pits

Aerospace, electronics, and other high-technology busi-

nesses expanded rapidly in California during the 1950s,

bringing

population growth

and rapid economic progress.

These businesses also brought a huge volume of toxic wastes

and the problem of safely disposing of them. A modern-

day reminder of those years is the Stringfellow Acid Pits

1349

located near the Riverside suburb of Glen Avon, 50 mi (80

km) east of Los Angeles.

The Acid Pits, also known as the Stringfellow Quarry

Waste Pits, are located on a 20-acre (8-ha) site in Pyrite

Canyon above Glen Avon. In the mid-1950s, a number of

high-tech companies began to dump their hazardous wastes

into the canyon. No special precautions were taken in the

dumping process; as one observer noted, the companies got

rid of their wastes just as cavemen did: “They dug a hole

and dumped it in.”

Over the next two decades, more than 34 million gal

(129 million L) of waste were disposed of in a series of pan-

shaped reservoirs dug into the canyon floor. The wastes

came from more than a dozen of the nation’s most prominent

companies, including McDonnell-Douglas, Montrose

Chemical, General Electric, Hughes Aircraft, Sunkist

Growers, Philco-Ford, Northrop, and Rockwell-Interna-

tional. The wastes consisted of a complex mixture of more

than 200 hazardous

chemicals

. These included hydrochlo-

ric, sulfuric, and nitric acids; sodium hydroxide; trichlorethy-

lene and methylene chloride;

polychlorinated biphenyls

(PCBs); a variety of pesticides; volatile organic compounds

(VOCs); and

heavy metals

such as

lead

nickel

cadmium

chromium, and manganese.

By 1972, residents of Glen Avon had begun to com-

plain about health effects caused by the wastes in the

Stringfellow Pits. They claimed that some chemicals were

evaporating and polluting the town’s air, while other chemi-

cals were

leaching

out of the dump and contaminating the

town’s

drinking-water supply

. People attributed health

problems to chemicals escaping from the dump; these prob-

lems ranged from nose bleeds, emotional distress, and in-

somnia to

cancer

and genetic defects. Medical studies were

unable to confirm these complaints, but residents continued

to insist that these problems did exist.

In November 1972, James Stringfellow, owner of the

pits, announced that he was shutting them down. However,

his decision did not solve the problem of what to do with

the wastes still in the pit. Stringfellow claimed his company

was without assets, and the state of California had to take

over responsibility for maintaining the site.

The situation at Stringfellow continued to deteriorate

under state management. During a March 1978 rainstorm,

the pits became so badly flooded that officials doubted the

ability of the existing

dams

to hold back more than 8 million

gal (30.3 million L) of wastes. To prevent a possible disaster,

they released nearly 1 million gal (3,785 million L) of liquid

wastes into flood control channels running through Glen

Avon. Children in nearby schools and neighborhoods, not

knowing what the brown water contained, waded and played

in the toxic wastes.

Environmental Encyclopedia 3

Strip-farming

When the

Comprehensive Environmental Re-

sponse, Compensation and Liability Act

(Superfund) was

passed in 1980, the Stringfellow Pits were named the most

polluted waste site in California. The pits became one of first

targets for

remediation

by the

Environmental Protection

Agency

(EPA), but this effort collapsed in the wake of a

scandal that rocked both the EPA and the Reagan adminis-

tration in 1983. EPA administrators Rita Lavelle and Anne

McGill Burford were found guilty of mishandling the Su-

perfund program, and were forced to resign from office along

with 22 other officials.

During the early 1990s, citizens of Glen Avon finally

began to experience some success in their battle to clean up

the pits. The EPA had finally begun its remediation efforts

in earnest, and residents won judgments of more then $34

million against Stringfellow and four companies that had

used the site. In 1993, residents initiated the largest single

civil suit over toxic wastes in history. The suit involved 4,000

plaintiffs from Glen Avon and 13 defendants, including the

state of California, Riverside County, and a number of major

companies. See also Contaminated soil; Groundwater pollu-

tion; Hazardous waste site remediation; Hazardous waste

siting; Storage and transport of hazardous materials

[David E. Newton]

ESOURCES

OOKS

Brown, M. H. Laying Waste: The Poisoning of America by Toxic Chemicals.

New York: Pantheon Books, 1980.

ERIODICALS

Gorman, T. “A Tainted Legacy: Toxic Dump Site in Riverside County

Has Sparked the Nation’s Largest Civil Suit.” Los Angeles Times (January

10, 1993): A3.

Madigan, N. “Largest-Ever Toxic-Waste Suit Opens in California.” New

York Times (February 5, 1993): B16.

Mydans, S. “Settlements Reached on Toxic Dump in California.” New

York Times (December 24, 1991): A11.

Strip-farming

In the United States,

soil conservation

first became an

important political issue in the 1930s, when President Frank-

lin D. Roosevelt led a campaign to study the loss of valuable

topsoil

because of

erosion

. It soon became clear that one

source of the problem was the fact that farmers tended to

plow and plant their fields according to property lines, which

usually formed squares or rectangles. As a result, furrows

often ran up and down the slope of a hill, forming a natural

channel for the

runoff

of rain, and each new storm would

wash away more fertile topsoil. This kind of erosion resulted

1350

not only in the loss of

soil

, but also in the

pollution

nearby waterways.

The

Soil Conservation Service

was founded in 1935

as a division of the

U.S. Department of Agriculture

, and

one of its goals was the development of farming techniques

that would reduce the loss of soil. One such technique was

strip-farming, also known as strip-cropping. Strip-farming

involves the planting of crops in rows across the slope of

the land at right angles to it rather than parallel to it. On

gently sloping land, soil

conservation

can be achieved by

plowing and planting in lines that simply follow along the

slope of the land rather than cutting across it, a technique

known as

contour plowing

. On very steep slopes, a more

aggressive technique known as

terracing

is used. Strip-

farming is a middle point between these two extremes, and

it is used on land with intermediate slopes.

In strip-farming, two different kinds of crops are

planted in alternate rows. One set of rows consists of crops

in which individual plants can be relatively widely spaced,

such as corn, soybeans, cotton, or sugar beets. The second

set of rows contains plants that grow very close together,

such as alfalfa, hay, wheat, or legumes. As a result of this

system, water is channeled along the contour of the land,

not down its slope. In addition, the closely planted crops in

one row protect the exposed soil in the more widely spaced

crops in the second row. Crops such as alfalfa also slow

down the movement of water through the field, allowing it

to be absorbed by the soil.

The precise design of a strip farm is determined by a

number of factors, such as the length and steepness of the

slope. The crops used in the strip, as well as the width of

rows, can be adjusted to achieve minimal loss of soil. Under

the most favorable conditions, soil erosion can be reduced

by as much as 75% through the use of this technique. See

also Agricultural pollution; Agricultural technology; Soil

compaction; Soil loss tolerance

[Lawrence H. Smith]

ESOURCES

OOKS

Enger, E. D., et al. Environmental Science: The Study of Relationships. 2nd

ed. Dubuque, IA: W. C. Brown, 1986.

Moran, J. M., M. D. Morgan, and J. H. Wiersma. Introduction to Environ-

mental Science. 2nd ed. New York: W. H. Freeman, 1986.

Petulla, J. M. American Environmental History. San Francisco: Boyd &

Fraser, 1977.

Strip mining

This technique is used for near-surface, relatively flat sedi-

mentary mineral deposits. How deeply the mining can occur

Environmental Encyclopedia 3

Strip mining

is essentially determined by the combination of technological

capabilities and the economics involved. The latter includes

the current value of the mineral, contractual arrangements

with the landowner, and mining costs, including

reclama-

tion

. Strip mining is used for mining phosphate

fertilizer

in Florida, North Carolina, and Idaho, and for obtaining

gypsum (mainly for wallboard) in western states.

However, the most common association of strip min-

ing is with

coal

. The examples of decimated land in Appala-

chia have motivated calls for prevention, or at least major

efforts at reclamation. Strip mining for coal comprises well

over half of the land that is strip-mined, which totaled less

that 0.3% of land in the United States between 1930 and

1990. This is far less land than the amount lost to agriculture

and urbanization. However, in agriculturally rich areas like

Illinois and Indiana there is a growing concern over the one-

time disruption of land for mineral extraction, compared to

the long term use for food production.

Strip mining has occurred mainly in the Appalachian

Mountains and adjacent areas, the Central Plains from Indi-

ana and Illinois through Oklahoma, and new mines for

subbituminous coal in North Dakota, Wyoming, and Mon-

tana. Important mining is also carried out on Hopi and

Navajo lands, notably Black Mesa in northeastern Arizona.

Despite the small amount of land used in strip mining,

the process radically alters landforms and ecosystems where

it is practiced. Depending on state laws, mining landscapes

prior to 1977 were often left as is, dubbed “orphan lands.”

The 1977 act required the land to be restored as closely as

possible to the original condition. This is a nearly impossible

task, especially when one considers the reconstruction of

the preexisting

soil

conditions and

ecosystem

. Even so,

reclamation is a vital first step in the healing process. Gener-

ally, the steeper the terrain, the greater the impact on the

landforms and river systems, and the more difficult the recla-

mation.

Detailed economic planning precedes any strip mining

effort. Numerous cores are drilled to determine the depth,

thickness, and quality of the coal, and to assess the difficulty

of removing the

overburden

, which consists of

topsoil

and

rock above the resource. If caprock is encountered, expensive

and time-consuming blasting is required, a frequent occur-

rence in the United States. Economic analysis then deter-

mines the area and depth of profitable overburden removal.

Finally, contracts must be negotiated with landowners; strip

mines commonly end abruptly at property lines.

Two kinds of earth removal equipment are typically

used: a front-loading bucket (the classic steam shovel), or a

dragline bucket that pulls the material toward the operator.

Power shovels and draglines built prior to World War II

generally have bucket capacities of 30–50 yd

(23–38 m

Post-World-War-II equipment may have a capacity up to

1351

200 yd

(153 m

). A new development, encouraged by the

1977 reclamation law, is the combination of dozers and

scrapers (belly loaders) more commonly seen on road-build-

ing or construction sites.

After the overburden is removed, mining begins. The

process is conducted in rows, creating long ridges and valleys

in the countryside that resemble a washboard. Coal extrac-

tion follows behind power shovels, leaving a flat, canyon-

like cut. Upon completion of a row, the shovel starts back

in the opposite direction, placing the new overburden in the

now-empty cut.

In hilly terrain, only a few cuts are all that is usually

profitable because the depth of overburden increases rapidly

into the hillside. Since the worst complications as a result

of strip mining occur on hillsides, the environmental price

for a limited amount of coal is very high. Hillside mining

such as this is called “contour mining,” in contrast to “area

mining” on relatively flat terrain. In the latter, the number

of rows are limited mostly by contractual arrangements.

Consequently, the main difference between area and contour

strip mining are the number of rows and the steepness of

the terrain.

Both types of strip mining leave behind four basic land

configurations: l)

spoil

bank ridges; 2) a final-cut canyon

often partially filled by a lake; 3) a high headwall marking

the uphill end of the mining; and 4) coal-haul roads, usually

at the base of the outermost spoil bank and through gaps

in the spoil-bank rows left for this purpose. In some orphan

lands, wilderness-like conditions prevail, where trees popu-

late the spoil banks and aquatic ecosystems thrive in the

final-cut lake. Left alone by man, these may afford a surpris-

ingly rich

habitat

for

wildlife

, especially birds. Deer thrive

in some North Dakota abandoned mines.

Reclamation of area mining is relatively simple com-

pared to contour strip mining. Prior to mining, the topsoil

is removed and stockpiled. The overburden from the initial

cut may be used to fill in the final cut, and the top part of

the headwall is sometimes cut down to grade into the spoil.

The spoil banks are leveled and the topsoil replaced; fertiliza-

tion and replanting, usually with grasses or trees for

erosion

control, and subsequent monitoring of revegetation efforts,

complete the process. In large operations, the leveling and

replanting coincide with mining, which is ideal since this

rapidly rebuilds the vegetation cover.

Reclamation of contour mining presents far greater

difficulties, primarily because of the slope angles encoun-

tered. Research in Great Britain revealed that even well-

vegetated slopes were producing 50 to 200 times as much

sediment

as similar, undisturbed slopes. Furthermore, the

greater slope angles allow much more of the sediment to

reach the channel below, where it eventually flows into

streams and rivers.

Environmental Encyclopedia 3

Strontium 90

Another problem for orphan lands in hilly terrain is

the ecological island left when hills are completely enclosed

by high headwalls. This is not unlike the ecological islands

created in the southwestern United States from

climate

changes and vertical zonation of vegetation. Though far

more recent, ecologists hope these “orphan islands” will allow

interesting case studies of genetic isolation.

The long-term effect of strip mining has been the

subject of research in Kentucky, Indiana, and Oklahoma.

For over a decade the United States

Geological Survey

studied Beaver Creek Basin, Kentucky, obtaining valuable

data before and after contour mining. Their findings were

published in a 1970 report authorized by C.R. Collier and

others. As expected, mining left a degraded landscape, and

resulted in much greater

runoff

, sediment production, and

water quality

problems. By contrast, area mining in Indiana

trapped vast quantities of

groundwater

within the loosened

soil, reducing peak discharges, extending base flow, and

yielding water of acceptable quality.

In Oklahoma, a study conducted by Nathan Meleen

dealt with a mix of both flat and hilly terrain. The findings

were published as a doctoral dissertation in 1977 by Clark

University. Area mining, Meleen found, produced far more

benign impacts than did contour stripping. The worst condi-

tions were encountered where contour mining was adjacent

to streams below, especially around the end of ridges. The

short distances and relatively steep gradients gave ideal condi-

tions for sediment and

acid drainage

into the channels be-

low. Summer runoff decreased while winter runoff increased.

The huge holes left by these unreclaimed operations acted

like reservoirs during the drier summer months, but in winter

when wetter conditions prevailed, they yielded more outflow

than before mining, because of altered

infiltration

rates.

Strangely enough, a 1971 Oklahoma law produced

effects similar to what the Geological Survey found in Ken-

tucky. The incomplete reclamation and lack of sediment

retention or topsoil replacement created ideal erosion condi-

tions, with rates approaching 13% sediment by weight. This

focuses the crucial role of topsoil replacement and rapid

revegetation as the preeminent needs in reclamation. Efforts

to revegetate some orphan lands where topsoil replacement

is impossible will only result in worse conditions, especially

downstream. Given the fact that most impacts from area

mining are retained onsite, and that orphan lands possess

great potential for

recreation

, especially fishing and hiking,

at least some should be left undisturbed. See also High-

grading; Mine spoil waste; Surface mining

[Nathan H. Meleen]

ESOURCES

OOKS

Caudill, H. M. Night Comes to the Cumberlands. Boston: Atlantic-Little,

Brown, 1963.

1352

Collier, C. R., et al. Influences of Strip Mining on the Hydrologic Environment

of Parts of Beaver Creek Basin, Kentucky, 1955-66. USGS Professional Paper

427-C. Washington, DC: U. S. Government Printing Office, 1970.

Doyle, W. S. Strip Mining of Coal: Environmental Solutions. Park Ridge,

NJ: Noyes Data Corp., 1976.

Landy, M. K. The Politics of Environmental Reform: Controlling Kentucky

Strip Mining. Baltimore: Johns Hopkins University Press, 1976.

THER

U.S. Department of the Interior. “Surface Mining and Our Environment:

A Special Report to the Nation.” Washington, DC: U. S. Government

Printing Office, 1967.

U.S. Environmental Protection Agency. “Erosion and Sediment Control:

Surface Mining in the Eastern U.S.: Planning.” Washington, DC: U. S.

Government Printing Office, 1976.

Strix occidentalis caurina

see

Northern spotted owl

Strontium 90

A radioactive

isotope

of strontium, produced during

nu-

clear fission

. The isotope was of great concern to environ-

mental scientists during the period of atmospheric testing

nuclear weapons

. Strontium 90 released in these tests

fell to the Earth’s surface, adhered to grass and other green

plants, and was eaten by cows and other animals. Since

strontium is chemically similar to calcium, it follows the

same metabolic pathways, ending up in an animal’s milk.

When a child drinks this milk, strontium 90 becomes incor-

porated into their bones and teeth. With a

half-life

of about

28 years, strontium 90 continues to emit radiation through-

out the individual’s lifetime.

Student Environmental Action

Coalition

Founded in 1990, the Student Environmental Action Coali-

tion (SEAC) focuses on social justice and on environmental

and political issues throughout the United States. The mis-

sion of SEAC is to unite youth that have exhibited leadership

in their community and to create an environmental move-

ment. It has activists that build allegiances among local,

regional, national, and international communities engaged

in environmental struggles.

The unique aspect of SEAC membership is that there

is not a national office that dictates an agenda to the local

groups. Instead, local groups coordinate a National Council

each year to run SEAC. The local groups are composed of

schools or neighborhood clubs, with each choosing its own

problem areas to focus on. Some topics that are frequently

Environmental Encyclopedia 3

Submerged aquatic vegetation

addressed include combating racism and sexism, and devel-

oping a sustainable living style.

Skills in each local group are developed at conferences

held throughout the year. These conferences are led by stu-

dents and consist of workshops on the strategies, lectures,

and publicity for grassroots organizing.

In 1998, SEAC was forced to cut one of its key funding

programs which proved disastrous for the group. Fortu-

nately, they were able to reopen, although with a much

smaller staff and reduction of size and publication of its

magazine, Threshold. Now that SEAC relies primarily on

donations and membership dues, the group has continued

to expand and work on projects such as the Clean Energy

Campaign.

[Nicole Beatty]

ESOURCES

RGANIZATIONS

Student Environmental Action Coalition, P.O. Box 31909, Philadelphia,

PA USA 19104-0609 (215) 222-4711, Fax: (215) 222-2896, Email:

seac@seac.org, <http://www.seac.org>

Styrene

Styrene is an oily organic liquid with an aromatic odor that

is used as a building block for polymers in the manufacture

plastics

, resins, coatings, and paints. Short-term health

effects of styrene exposure include nervous system effects

such as depression, loss of concentration, weakness, fatigue,

and nausea. Potential long-term effects include liver and

nerve tissue damage. Styrene has been designated as a possi-

ble human

carcinogen

. The drinking water standard (Maxi-

mum Contaminant Level, or MCL) for styrene is 0.1

parts

per million

(ppm). Styrene when released into water rapidly

evaporates or is degraded by

microorganisms

. It does not

bind to soils and may leach to ground water. However, its

rapid degradation minimizes its

leaching

potential. Styrene

does not tend to accumulate in aquatic life. It is also found

in the air and in the microgram/cubic meter range.

[Judith L. Sims]

ESOURCES

THER

U.S. Environmental Protection Agency. Consumer Factsheet on: Styrene.

May 22, 2002 [cited June 23, 2002]. <http://www.epa.gov/safewater/dwh/

c-voc/styrene902.html>.

RGANIZATIONS

The Styrene Information and Research Center, 1300 Wilson Boulevard,

Suite 1200, Arlington, Virginia USA 22209, (703) 741-5010, Fax: (703)

741-6010), <http://www.styrene.org>

1353

Subbituminous coal

see

Coal

Submerged aquatic vegetation

Submerged aquatic vegetation consists of a taxonomically

diverse group of plants that lives entirely beneath the water

surface. This diverse group of aquatic plants includes

species

of angiosperm vascular plants, mosses, and liverworts, and

macroalgae (seaweeds). Their underwater growth habit sepa-

rates them from other kinds of aquatic plants that are free-

floating, have floating leaves, or are emergent above the

water surface.

Almost all species of submerged aquatic plants live in

freshwater ponds, lakes, rivers, streams, and

wetlands

, and

shallow marine waters. They can occur in ponds with an

acidic

less than 4 or in alkaline waterbodies with pH

greater than 10, but they tend to be most rich in species at

pHs of 6 to 8. Only a few angiosperm species occur in

brackish

estuarine or marine habitats, including the

eel-

grass

(Zostera marina), widgeon grass (Ruppia maritima),

and turtle-grasses (Thalassia species). No aquatic mosses or

bryophytes occur in saline waters.

Macroalgae, commonly known as seaweeds, are also

classified as submerged aquatic vegetation though they are

not biologically part of the kingdom Plantae. They are mem-

bers of the kingdom Protista and live within the marine

environment

. They, too, are beneficial to the aquatic

habitat

Both grasses and macroalgae absorb nutrients which

can be a source of pollutants in aquatic habitats. Submerged

vegetation also is a major food source for many different

species of organisms including waterfowl,

sea turtles

, and

manatees

. These plants and seaweeds add oxygen to the

water, and grass roots help stabilize shorelines against

ero-

sion

. Additionally, submerged aquatic vegetation provides

shelter and food for organisms. For example, crustaceans,

especially the blue crab, and juvenile or larval fish use sub-

merged vegetation as protective nurseries and a means to

hide from predators. Some organisms like barnacles and

bryozoans attach themselves to plant surfaces to live. Other

species use submerged vegetation as place to lay eggs and

hatch new offspring.

Though communities of submerged aquatic vegetation

help improve

water quality

, their own survival depends on

maintaining good water quality. These plants and macroal-

gae are sensitive to environmental changes brought about

by agricultural

runoff

, industrial waste, and global warming.

Increased temperature,

salinity

, and depth of coastal waters

in particular can contribute to marked habitat change for

Environmental Encyclopedia 3

Subsidence

these species, thus threatening their distribution and abun-

dance.

Submerged aquatic plants are most abundant in rela-

tively shallow water, where they have access to enough sun-

light to engage in

photosynthesis

at a high enough rate to

survive. Waterbodies with poor

visibility

may support few

or no submerged aquatic plants, and only at very shallow

depths. Waterbodies may have poor visibility because of an

excessive abundance of

phytoplankton

, or they may be

highly turbid because of suspended clays, or they may be

brown-colored because of dissolved organic matter leached

from nearby bogs or peaty

soil

. Waterbodies with exception-

ally clear water may have submerged aquatic plants growing

on the bottom as deep as about 19 feet (6 m). Shallow,

moderately fertile, mesotrophic or eutrophic waterbodies

may support especially large populations of submerged

aquatic plants, where they may even be regarded as nuisance

“weeds”.

Some of the most widespread and familiar species of

angiosperm submerged aquatic plants include the tape-grass

or wild celery (Vallisneria americana), the waterweed (Elodea

canadensis), the ribbon-leaf pondweed (Potamogeton epi-

hydris), the slender water-nymph (Najas gracillima), the pi-

pewort (Eriocaulon septangulare), the greater bladderwort

(Utricularia vulgaris), and the lake quillwort (Isoetes lacustris).

Examples of bryophytes that are submerged aquatic plants

include an aquatic peatmoss (Sphagnum macrophyllum), an

aquatic moss (Fontinalis antipyretica), and an aquatic liver-

wort (Ricciocarpus natans).

A few species of submerged aquatic plants are used as

ornamentals in aquaria and outdoor water-gardens. There

are even some clubs of aficionados of the aesthetics of these

cultivated aquatic plants. Examples include the Amazon

sword (Echinodorus amazonicus), the tropical hornwort

(Ceratophyllum submersum), the temple plant (Hygrophila cor-

ymbosa), the Java moss (Vesicularia dubyana), and a tropical

aquatic liverwort (Riccia fluitans). Submerged aquatic plants

are also used as

ecosystem

components in the constructed

wetlands that are sometimes used to treat human sewage

and other wastes.

A few submerged aquatic plants, particularly several

non-native invasive species, can be sufficiently abundant that

they are considered serious weeds because of their effects on

the use of waterbodies for

recreation

and

transportation

Some of the most important invasive weeds of this kind in

North America are the Eurasian water-milfoil (Myriophyllum

spicatum), the Brazilian waterweed (Egeria densa), and the

hydrilla (Hydrilla verticillata). Attempts are sometimes made

to reduce infestations of these species using mechanical har-

vesters or herbicides.

[Bill Freedman Ph.D.]

1354

ESOURCES

OOKS

Borman, S., R. Korth, and J. Temte. Through the Looking Glass: A Field

Guide to Aquatic Plants. Madison, WI: University of Wisconsin Press, 1997.

Crow, G. E. and C. B. Hellquist. Aquatic and Wetland Plants of Northeastern

North America. Madison, WI: The University of Wisconsin Press, 2000.

Fassett. N. C. A Manual of Aquatic Plants. Madison, WI: University of

Wisconsin Press, 1957.

Subsidence

To subside is to sink or fall. Subsidence is commonly associ-

ated with the lowering of the earth’s surface due to actions

that have occurred below the surface. Sometimes this is a

natural phenomenon, such as the solubilizing and removal

of minerals by water. When the underground support system

is removed in the process, the surface of the land sinks to

a new level. This often leads to a special topographic form

known as Karst

topography

. Human activities that lead to

the extraction of ores, minerals, and

fossil fuels

often lead

to a weakened mineral structural support and subsidence of

the surface of the earth. In the

arid

portions of the earth,

extraction of water from sub-surface aquifers has led to subsi-

dence of the earth’s surface as well.

Subsoil

The portion of the

soil

that is below the surface. The subsoil

is often referred to as the B

horizon

. Subsoils are generally

lower in organic matter, lighter in color, denser, and often

have a higher clay content than surface soils. Subsoils are

generally not as productive as surface soils for plant growth.

Thus, if surface soils are eroded, future productivity is re-

duced when the subsoil is used for crop production.

Succession

Succession is the gradual transformation or creation of a

biological community

as new

species

move into an area

and modify local environmental conditions. Primary succes-

sion occurs when plant and animal species colonize a pre-

viously barren area, such as a new volcanic island, a sand

dune, or recently glaciated ground. In these cases every living

thing, from

soil

bacteria and

fungi

to larger plants and

animals, must arrive from some adjacent

habitat

. Secondary

succession is the development of communities in an area

that has been disturbed by fire, hurricanes, field clearing,

tree felling, or some other process that removes most plants

and animals. Intermediate successional communities are

known as “seral stages” or “seres.”

Environmental Encyclopedia 3

Succession

As early successional species become established, they

alter their

environment

and make it more habitable for

later seral stages. Usually a disturbed or barren area has low

soil

nutrient

levels, intense sunlight, and no protection from

violent weather. Because precipitation quickly runs off the

bare ground, little moisture is available for plant growth.

Species that can survive under such harsh conditions have

little

competition

, and they spread quickly. As they grow

and thicken, these plants add organic matter to the soil,

which aids moisture retention and helps soil bacteria to grow.

As soil nutrients and moisture increase, larger shrubs and

perennial plants can take root. The shade of these larger

species weakens and eventually eliminates the original pion-

eering species, but it cools the local environment, further

improving moisture availability and allowing species that

demand relative environmental

stability

, such as woodland

species, to begin moving in. Eventually, shade-tolerant spe-

cies of plants will come to dominate the area that sun-loving

plants had first colonized.

One of the most well-documented examples of pri-

mary succession occurred in 1883 when the

volcano

on the

Indonesian island of

Krakatoa

erupted, destroying most of

the island and its life forms, and leaving a new island of

bare volcanic rock and ash. Within a few years several species

of grasses, ferns, and flowering shrubs had managed to arrive,

carried by wind, water, or passing birds from islands 25 mi

(40 km) or more distant. Just 40 years later, more than 300

plant species were growing, and in some areas over 12 in

(30 cm) of soil had developed: soil bacteria, insects, and

decomposers

had managed to reach the island and were

turning fallen organic

detritus

into soil. Over the decades,

the total number of species gradually stabilized, but the

character of the community continued to change as incoming

species replaced earlier arrivals. On Krakatoa succession hap-

pened with unusual speed because the

climate

is warm and

humid and because the fresh volcanic ash made a nutrient-

rich soil.

Early successional species, those most able to quickly

establish a foothold, are known as pioneer species. Usually

pioneer species are opportunists, able to find nourishment

and survive under a great variety of conditions, quick to grow,

and able to produce a great number of seeds or offspring at

once. Pioneer species have very effective means of dispersing

seeds or young, and their seeds can often remain dormant

in soil for some time, sprouting only after conditions become

suitable. Dandelions are well-known pioneers because they

can quickly produce a seed head with hundreds of seeds.

Each tiny seed has a lightweight structure that allows the

wind to carry it long distances. Dandelions are quick to

invade a lawn because of this effective seed-dispersal tactic,

because they can survive under sunny or shady conditions,

and because they grow and reproduce quickly. In open sun-

1355

shine, they are competitive enough to establish themselves

despite the presence of a thick mat of turf grass.

Later successional species tend to be more shade toler-

ant, slower to grow and reproduce, and longer lived than

pioneer species. Their larger seeds do not disperse as easily

(compare the size of an acorn or walnut with that of a

dandelion seed), and their seeds cannot remain dormant for

very long before they lose viability. Seedlings of some late

successional species, such as the Pacific Northwest hemlock,

require shade to survive, and most require considerable mois-

ture and soil nutrients.

We usually think of succession occurring after a cata-

strophic environmental disturbance, but in some cases the

gradual environmental changes of succession proceed in the

absence of disturbance. Two outstanding examples of this

are bog succession and the invasion of prairies by shrubs

and trees.

In cool, moist, temperate climates, plant succession

often turns ponds into forest through bog succession. Water-

loving plants, mainly rush-like sedges and sphagnum moss,

gradually creep out from the pond’s edges. Often these plants

form a floating mat of living and dead vegetation. Other

plants—cranberries, Labrador tea, bog rosemary—become

established on top of the mat. Organic detritus accumulates

below the mat. Eventually the bog becomes firm enough to

support black spruce, tamarack, and other tree species. As

it fills in and dries, the former pond slowly becomes indistin-

guishable from the surrounding forest.

Many

grasslands

persist only in the presence of occa-

sional wildfires. During wet decades or when human activity

prevents fires, woody species tend to creep in from the edges

of a

prairie

. If sufficient moisture is available and if fires

do not return, open grassland can give way to forest. Fire

suppression has aided the advance of forests in this way

across much of the United States, Canada, and Mexico.

Grazing or browsing animals can also be important forces

in maintaining biological communities. Adding or removing

grazers can initiate successional processes.

Longer, slower disturbances than fire or field clearing

can also initiate succession. Over the course of centuries,

climate change can cause significant alteration in the charac-

ter of biomes. Minor variations in rainfall or temperature

ranges can alter community structure for decades or centu-

ries. The end of the last glacial period about 10,000 years

ago allowed

tundra

, then grasslands, then temperate forests

to advance northward across North America. Even geologic

activity, such as mountain building or changes in sea level,

has caused succession. More recently, human introductions

of species from one continent to another have caused signifi-

cant restructuring of some biological communities.

Turnover from one community to another may occur

in just a few years or decades, as it did on Krakatoa. Stages

Environmental Encyclopedia 3

Succession

Five stages of primary succession on a terrestrial site. (McGraw-Hill Inc. Reproduced by permission.)

in bog succession may take a hundred years or more. Often,

succession continues for centuries or millennia before a stable

and relatively constant biological community emerges. This

is especially likely in areas whose climate is moist and mild

enough to support the high species diversity of temperate

tropical rain forests

. However, simpler ecosystems such

as arctic tundra may take centuries to recover from distur-

bance because low temperatures and short summers make

growth rates almost universally slow.

Generally, successional processes are understood to

lead ultimately to the emergence of a

climax

community,

a group of species perfectly suited to a region’s climate, soil

types, and other environmental conditions. Climax commu-

nities are said to exist in equilibrium in their environments;

where early successional species groups tend to facilitate the

development of later stages, climax communities, sometimes

called mature communities, tend to have self-perpetuating

characteristics or mechanisms that help maintain local envi-

ronmental conditions and help the community to persist. In

the 1930s, F. E. Clements, one of the best-known plant

ecologists in the United States, identified a list of just 14

climax communities that he claimed were the final result of

1356

succession for all the various sets of environmental conditions

in the country.

More recent evaluations of climax communities, how-

ever, have concluded that climax forests are often a patch-

work of stable and unstable areas. In an

old-growth forest

in the Pacific Northwest, hemlock and spruce may compose

the local climax community, but every time a large tree falls,

it creates a sunny opening that makes way for other seral

species to grow for a time. Occasional fires disturb larger

patches of forest, and succession starts again from the begin-

ning in disturbed areas. The entire

biome

is more a shifting

and changing mosaic than a uniform and unchanging forest.

Often, slight gradations in soil types, slope, exposure, and

moisture cause a variety of stable communities to blend in

an area. The term “polyclimax community” was developed

to recognize the complexity of such assemblages.

In some cases, climax communities may be difficult

to distinguish at all. For instance, in a biome regularly dis-

turbed by fire, “climax” species usually require fire to aid

seed germination and eliminate competitors. Definitions

here become somewhat blurred: Is a climax community one