RRADIATION ECOLOGYRadiation Ecology or Radioecology is a term that came into common usage in 1956 ppt

More specifically, radiation ecology has come to be recognized as that area of ecology concerned with radioactive substances, radiation and the environment.. Underlying each of these is

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RADIATION ECOLOGY

Radiation Ecology or Radioecology is a term that came into

common usage in 1956 to denote that area of the broad field

of ecology concerned with the assessment of radioactivity

in the environment More specifically, radiation ecology has

come to be recognized as that area of ecology concerned

with radioactive substances, radiation and the environment

The development and subsequent expansion of nuclear

energy for military and peaceful purposes has been

accom-panied by environmental problems, some of which are

typi-cal of other facets of industrialization and some unique to

atomic energy The unique problems primarily concern the

fate and ecological effects of radionuclides released into

the environment

The major environmental problems introduced by the

Atomic Age may be grouped into several areas of

scien-tific and public concern Underlying each of these is the

worry about the effects of ionizing radiation—on man, his

domesticated plants and animals, and on the environment

and its living components, Fallout from weapons testing,

reactor radioactive waste effluents, radioactive waste

dis-posal, nuclear war, and use of nuclear explosives for major

engineering and related technological projects of large scale

comprise the activities which have concerned society and

which, because of potential impact on the environment and

man, have stimulated the development of radiation ecology

Understanding the manner in which our ecological systems

(ecosystems) distribute, assimilate, and affect the

environ-mental behavior of radioactive substances, and the effects of

radiations emitted from those substances, are the concern of

the radioecologist

RADIONUCLIDES OF ECOLOGICAL IMPORTANCE

Radionuclides which are of interest to the ecologist are

listed in Table 1 These radioactive elements represent the

major naturally-occurring and man-made sources of

radia-tion in the environment Principal sources of exposure from

background (natural) radiation are represented by the

ura-nium, thorium and actinium decay series Internal exposure

to man results primarily from 40 K, 14 C, 226 Ra, and 228 Ra and their daughter products that are deposited in the body

Radionuclides such as 222 Rn and 220 Rn and their daughter products represents sources of internal radiation exposure to man from inhalation

Radionuclides produced by the fissioning of uranium (fission products) are of the greatest current concern These man-made isotopes are not essential to organisms, but they constitute the major sources of radiation in the environment whether it be from fallout or waste disposal from reactor operations All of these radionuclides may enter ecosystems where they become part of the flux of systems that are being circulated within and between systems

Some of the fission products which are chemically simi-lar to biologically essential elements are of special inter-est They vary greatly in their physical half-life and in the extent to which they participate in metabolic processes of living organisms The most important radionuclides affecting plants and animals on land are strontium-90, cesium-137, and iodine-131

Strontium-90 remains in the environment for a long time Its half-life is 28 years Strontium is chemically similar

to calcium, and it can enter living organisms as a replace-ment for calcium In weapons fallout strontium-90 is usually deposited in the root systems of plants the amount of 90 Sr that a plant absorbs from soils depends on several factors, particularly on the quantity of calcium in the soil, the relative quantities of calcium and strontium at the depth where the roots are located, and the ability of the plant to discriminate between the two elements The plant is the base for 90 Sr to enter the human food chain This chain is a short and simple one consisting of plants, cows, and man, with cow’s milk being the chief source of entry into man There is consider-able discrimination against the transfer of strontium at each step in this food chain, but the small amount that is trans-ported to man tends to concentrate in bone tissue It remains there, undergoing radioactive decay and emitting its radia-tion Its danger is related to the fact that bone contains blood forming (erythropoetic) tissue In sufficient quantities the radiation can cause leukaemia and bone cancer

Strontium-90 accumulation normally is greater in children

than in adults, because growing children are building bone at

a greater rate and use a larger amount of calcium A study of

species in the deer family (cervidae) graphically demonstrated

the effects of 90 Sr fallout The levels of 90 Sr in the antlers of

deer rose continually from 1947 through 1955, then remained

constant for 2 yrs, and rose again in 1958 The concentration

of 90 Sr was more than 8 ⫻ as high in 1958 as it was in 1947

With the cessation of major weapons tests in the 1960’s, the

levels began to drop off

Cesium-137 is another major fission product that is

found in fallout and untreated radioactive waste effluents

Cesium behaves chemically very much like potassium and

follows the same metabolic route in plants and animals as

potassium does It enters plants directly through the leaves

after being deposited by rain, and so it appears in plant

tis-sues more quickly than does 90 Sr

From there on the route of 137 Cs is much the same as 90 Sr;

it appears in the milk and muscles of cattle that eat

contami-nated plants, and it enters the human body in food Once in

the human body, it becomes part of muscle tissue and so has

an almost uniform distribution throughout the body It stays

there for only about 4 months Since its half-life, like that of

90 Sr, is about 28 yrs, little of the 137 Cs undergoes radioactive

decay while in the body

The transfer of 137 Cs and 90 Sr from plants to animals

also has been observed in species that are not important as

food sources for man The coconut crab, a land animal that

lives on a diet of fruit and nuts on islands in the Pacific, was

found to have accumulated radioactive materials as a result

of the Pacific test explosions Strontium-90 was found in the

skeleton, and 137 Cs was found in the soft body parts—direct

results of eating contaminated vegetation

The third radionuclide or fission product of importance

in fallout is radio-iodine ( 131 I) The chemistry of radioactive

131 I is exactly like that of natural 127 I, which is not

radioac-tive Therefore, its concentration in the body depends only

on the concentration in the source material Iodine becomes

concentrated in the thyroid glands of vertebrate animals, where it can cause cancer of the thyroid and damage to other tissues Like 90 Sr and 137 Cs, it enters plants as a result of radioactive fallout and then enters humans either by way of the plants themselves, or by way of contaminated milk from cows that eat the plants The radioactivity of thyroid glands removed from certain animals can serve as a sensitive indi-cator of 131 I in the environment, because the concentration of

131 I in the thyroid can be as much as 10,000 ⫻ higher than

the concentration in nature The black-tailed jack-rabbit is a useful animal for such measurements It has a large thyroid gland that is easy to remove and weigh The level of radioac-tivity in each gram of its thyroid tissues varies directly with the fallout on vegetation

Finally there are man-made radionuclides (activation products) which are important because they are the isotopes

of elements which may be essential to plants and animals

Some of these also may enter the environment as activation products resulting from reactor operations or nuclear explo-sives Examples of activation radionuclides are cobalt-60 and zinc-65 In aquatic or marine environments these radio-nuclides have been found to accumulate in food organisms especially shellfish and mollusks Generally 60 Co can be anticipated to be accumulated by organisms or to be retained

in organically enriched materials such as forest floor humus and organic sediment Zinc-65 is of particular concern in marine environments where it is likely to be accumulated in clams and oysters However, being an activation rather than

a fission product its presence depends more on appropriate stable elements present which in turn are exposed to fast neutrons than on fissionable material

RADIOSENSITIVITY OF ECOLOGICAL SYSTEMS

Although there is much current concern about the possible effect of low level, chronic radiation on ecological systems, there is relatively little comprehensive scientific data on

TABLE 1 Radionuclides of ecological importance

Naturally occurring radionuclides

Uranium Thorium Actinium series elements Potassium-40 Carbon-14

Major contributors to background radiation (long half-lives)

Fission products Strontium-89, 90, 91 Yttrium-90, 91

Zirconium-95 Niobium-95 Ruthenium-103, 106 Rhodium-106 Iodine-131 Cesium-137 Barium-137,

140 Lanthanum-140 Cerium-141,

144 Praeseodymium-143, 144 Neodymium-147 Promethium-147

Enter ecological systems through fallout or waste disposal (half-lives ranging from a few hours to 30 yrs)

Radioisotopes of elements essential

to organisms

Hydrogen-3 Cobalt-60 Carbon-14 Sodium-22, 24 Phosphorus-32 Sulfur-35 Potassium-42 Calcium-45 Manganese-54 Iron-59 Copper-64 Zinc-65

Used as tracers in both radionuclide cycling and radiation effects studies on organisms and ecological systems

this problem Much more is known about the

radiosensi-tivity of organisms exposed to radiation doses which are

much higher than we expect to contend with in the normal

environment In general, higher animals are far more

sensi-tive to radiation than are lower animals, and the very young

and the aged are more sensitive than mature, healthy

ani-mals For example Table 2 gives estimated acute doses of

gamma of X-radiation necessary to kill 50% or more of

the adult members of several groups of organisms These

data should be considered only as an indication of

rela-tive radiosensitivity as they represent generalized ranges

Half the humans exposed to a single dose of 500 R will

die For other mammals the lethal dose ranges from less

than 300–1200 R Frogs and newts can survive higher

radiation levels, depending on their body temperatures at

the time of exposure Insects can survive doses of up to

100,000 R in a few instances; most have lethal doses in the

10,000–20,000 R range These kinds of data do not reflect

the more complex responses of organisms subjected to

ion-izing radiation under natural conditions For example, most

organisms go through several stages of development from

egg to adult These stages may take place in different parts

of the ecosystem Likewise, the radiosensitivity of these

organisms may differ in different stages of the life cycle

In radioresistance groups such as insects, 10% or less of

the lethal dose to adults may be effective at juvenile or egg

stages For example, in the bagworm a dose of 450 R is

sufficient to kill 50% of 1-day old eggs, whereas a dose of

approximately 10,000 R was required to produce the same

effect in the larvae

Effects other than lethality also may be produced by

radi-ation, especially in ecosystems where all organisms are linked

through various interactive processes Aside from genetic or

reproductive effects, changes in number, growth, disease

resis-tance, life span or response to physical environmental factors

are of interest to the ecologist Radiation-induced changes

can affect the role of organisms or populations within the

ecosystem Predator–prey relationships, food chain transfers,

and other ecological processes which depend on the

continu-ing interaction between different organisms may be altered

by the impact of ionizing radiation

The effect of ionizing radiation on plants has been studied both outdoors and in greenhouses One indoor test field consists of 10 acres of land with a 60 Co source located

at the center It is installed in a vertical tube, which can be raised to different heights for irradiation and then lowered

by remote control into a lead case when not in use Various species of plants grow in the soil in concentric circles around the source Each species is arranged in a wedge-shaped area

so that the plants are located at various distances from the source and receive various intensities of radiation Plants are exposed to radiation for 20 hr a day

Radiation effects on plants are complex and depend on a number of factors, including the plant species, the maturity of

a plant, its physical condition, the parts of the plant exposed

to radiation, the kind and amount of radiation, and the rate

at which the radiation dose is applied Woody plants gener-ally are more sensitive to radiation than are herbaceous plants (Table 2) Gymnosperms are more sensitive to radiation than angiosperms A pine tree shows severe growth inhibition at a level of about 10 R/day, while the same degree of inhibition in

a gladiolus plant requires about 5000 R/day Some ecologists have speculated that radiation from a nuclear attack would destroy all pine trees and other gymnosperms in irradiated areas, leaving other plants relatively unharmed

It is possible to predict some radiation effects in plants

The meristematic or growth regions in plants are the most radiosensitive tissues It is the absorption of radiation energy

in these regions that alters plant growth and development

Ecologists and botanists have shown that the response of plants to ionizing radiation is directly proportional to the interphase chromosome volumes in meristematic tissues

That is, plant species with large chromosomes are more sen-sitive; those with small chromosomes are more resistant to radiation In general, this is an extremely useful concept, and

it has been applied to predict and assess probable radiation effects on vegetation (natural and agricultural) from military uses of nuclear devices

Seeds are far less sensitive to radiation than are growing plants A stand of pine trees exposed to a total of 12,000 R of gamma radiation was 90% destroyed, yet 95% of the seeds taken from cones on the same trees were viable The high resistance of seeds to radiation damage is probably associated with their low water and oxygen content The sensitivity of dry seeds varied widely among species, however Lily seeds show practically no ability to sprout after receiving a dose of

2000 R Yet the seeds of other plants seem to be stimulated to sprout more vigorously than normal under the same amount

of radiation or more Such differences favor the growth of certain species over others in areas where radiation is a factor

in the environment

EFFECTS OF RADIATION ON ECOSYSTEMS

The effects that large scale ionizing radiation such as from

a nuclear attack would have on plants and animals living together in an ecosystem have concerned radioecologists ever since the first use of atomic bombs Several studies have

TABLE 2 Comparative radiosensitivity of groups of organisms Group Lethal dose range a (rads)

Herbaceous plants 5,000–70,000

Deciduous trees 4,000–10,000

a Estimated acute whole body gamma radiation doses required to kill 50% or more of the adult organisms

been conducted involving small ecosystems in an attempt to

determine what would happen on a large scale In one study,

10,000 acres of land surrounding a nuclear reactor where

exposed to radiation ranging from lethal levels to levels no

higher than the natural background radiation The ecosystem

on this land consisted mainly of an oak-hickory-pine climax

forest The forest was exposed to a mixture of gamma

radia-tion and neutrons, with an intensity similar to that expected

from fallout after a nuclear attack The radiation reached

about 37,00 trees, plus many more herbaceous species and

many more shrubs Ecologists examined thousands of plants

in order to differentiate between the effects of ionizing

radia-tion and the effects of frost, disease, insect damage, drought,

and other natural factors In still another study, a community

of spring and summer annuals was exposed to gamma

radia-tion for nearly 4 months during one growing season and then

was observed over the next 3 yrs In still another study, gamma

radiation was applied daily throughout one winter and spring

to a forest and to an open field with a well-established cover

of annual plants

On the basis of these and other studies, radioecologists

have formulated a scenario depicting how ionizing radiation

would affect the plants and animals of our forests and fields

if a nuclear attack occurred during the summer growing

season People emerging from shelters several weeks after the

attack would find little change in their surroundings, except

in areas of extremely high radiation All plants and animals,

both large and small, would have been killed in these high

radiation areas, and as the plants died they would subject

the surrounding areas to further danger from fire However,

most fields and woodlands would appear unchanged by

radiation when viewed from a distance Closer inspection

would reveal more clearly the extent of the damage The

ground would be littered with the bodies of birds and

ani-mals killed by the radiation Inspection of lakes, streams,

ponds and marshes would show that the lower animals had

fared better Fish, frogs, toads and salamanders would be

alive and healthy The sound of insects would be heard as

before Among the plants the damage would be least

seri-ous to those that appear early in a natural succession pattern

Mosses and lichens would be undamaged, annuals would be

somewhat affected, shrubs more so, and trees most of all The

damage to pine trees would be most apparent Pines nearest

the radiation zone would have turned a brilliant red brown

within a few days after the attack Other plants in the forest

and fields would undergo little change during the remainder

of the summer

In the autumn the oaks, hickories and other hardwood

trees would lose their leaves earlier than usual—perhaps as

much as 7 weeks earlier in areas nearest the high-radiation

zone The following spring these areas would remain in their

state of winter dormancy 7 or 8 weeks longer than usual

Examination of the hardwoods (oaks, hickories, etc.) at this

time would reveal severe damage to the buds, resulting in the

development of fewer leaves and of abnormal leaves Near

the high-radiation zones, the trees might be leafless The

distribution of annuals in the open fields and on the forest

floor would also be changed from previous years Certain

species would grow in greater numbers, partly as a result

of the stimulation of their seeds by radiation and partly as

a result of the radiation in seed germination among other competing species The delay in development of leaves on the trees would give these annuals an extra long growing season In the abundance of sunshine, weeds would grow on the forest to heights of 8 ft or more The absence of a leafy canopy would also cause changes in the forest soil With greater wind flow through the bare trees and higher tempera-tures from direct sunlight, the soil would become drier and harder during sunny weather In rain storms the harder impact

of rain drops would wash away topsoil in areas not covered

by weeds or shrubs Throughout the first summer follow-ing the attack, birds and animals from outside the irradiated areas would move in to replace those that were killed

RADIOACTIVE TRACERS

With the threat of nuclear war receding, and nuclear reac-tors being equipped with ever more elaborate safe-guards

to reduce radioactive releases to the environment, the thrust

of radioecology is changing Activation products in con-trolled quantities are now being used as radioactive tracers

to follow the pathways of chemical elements in the bodies

of organisms and in the complex interactions of ecosystems

The radioactive materials have the advantage of being easily detected and quantitatively measured in biological materi-als without elaborate chemical separation of the elements otherwise necessary

For example, the radioisotope 137 Cs was added to the upward flow of water in trunks of yellow poplar trees in Tennessee about 18 years ago In the ensuing years radio-ecologists have followed the movement of this relatively inert tracer into leaves of the trees, into leaf-eating insects, into the insect eating birds, into the forest litter as the dead leaves fell, into soil insects, and so forth Periodic sam-pling has confirmed the recycling of natural materials in this forest ecosystem The radioecologists, in concert with systems analysts, are currently developing computer simula-tion models to mimic the ecological cycles revealed by this cesium tagging experiment Comparable information on the exchange of materials from one component of the ecosys-tem to another could never have been obtained without the knowledgeable use of radioactive materials by these trained radioecologists

Other activation products such as calcium-45 or phosphorus-32 find use in studies of metabolic processes

in organisms, populations or communities Such studies lead to an understanding of regulatory processes and struc-tural characteristics of living systems Other examples of experimental use of radioactive tracers may range ecologi-cally, from studying the uptake of 45 Ca tagged fertilizer by corn, to following the pathways of 32 P in a stream, includ-ing its distribution in the non-livinclud-ing as well as the livinclud-ing components

Radiation ecology is now an area of ecological research and teaching that encompasses far more of the impacts of

man on his environment than the atom bomb and nuclear

reactors Understanding of nearly all pollutant chemicals

in the environment is being enhanced by use of

tech-niques and principles of ecological cycling developed by

radioecologists

CURRENT DEVELOPMENTS IN RADIATION

ECOLOGY

The last several years have witnessed a major decrease of

interest in, and hence support of, research in radiation

ecol-ogy In the United States the research programs and projects

initiated primarily under the Atomic Energy Commission

(AEC) have been mostly dismantled The rationale behind

these policy shifts is difficult to comprehend; however, it

seems to have been associated with a perception that most

of the scientific challenges associated with the ecological

aspects of radiation are either sufficiently understood or can

contribute little to those practical issues related to

radia-tion protecradia-tion that are still of concern Despite this

ratio-nale, there has been little change in the long-standing public

fear of ionizing radiation and its potential consequences In

addition, the recent major accident at the Russian nuclear

power station at Chernobyl (1986), in which 50–100 Mci

was released into the environment, not only raised or

exacer-bated fears in those public sectors already concerned about

radiation problems associated with nuclear power but also

served to galvanize resistance in large groups (e.g., the Soviet

public and other East European populations) that hitherto

had either accepted nuclear power or manifested little if any

public resistance

The Chernobyl accident underscored both the

inef-fectuality of political boundaries against environmental

contamination and the role of food chains, both natural

and agricultural, in exposing humans and other organisms

to potentially harmful levels of radionuclides Likewise,

Chernobyl focused interest on the direct consequences of

radiation on ecosystems in the zones of high contamination

(within a radius of 18 km of the reactor site) The release of

large quantities of 134 Cs and 137 Cs resulted in the

contamina-tion of lakes, streams, and forests in the path of the plume

The need to understand the rates of transfer and patterns of

bioaccumulation of these radionuclides in different

ecologi-cal pathways became manifest in many European countries

located thousands of kilometers away from the reactor In

Sweden, for example, high concentrations of 137 Cs were

found in reindeer and moose (1,000 to 10,000 Bq/kg) and

in several species of freshwater fish The relatively rapid

buildup of radionuclides in these organisms was the result

of processes which can affect both the rate and extent of

bio-accumulation in food chains Thus the Chernobyl accident

has emphasized an increased need for additional research in

radiation ecology

Food chains are the ecological pathways by which many

substances are moved in terrestrial and freshwater

envi-ronments In the case of radionuclides, these pathways are

important in the assessment of radiation exposure to critical

population subgroups and human populations Until recently the uptake and transfer coefficients used in regulatory models were mainly generic default values intended for use in lieu

of site-specific information The Chernobyl accident demon-strated the importance of and need for geographic-specific data on individual radionuclide behavior in terrestrial and fresh-water pathways

Unlike the United States, most other countries are involved in extensive radioecological research This research is aimed at obtaining data for predicting exposure resulting from transport of radionuclides in agricultural food chains The processes of interest in terrestrial envi-ronments are those involving atmospheric deposition onto soils and vegetation; resuspension and leaching from these surfaces; uptake from soils by the edible portions of vegeta-tion; and transfer into meat, milk, and other animal products utilized by humans In the aquatic environment the key pro-cesses involve the bioaccumulation of radionuclides from sediments, water, and algae into the edible components of aquatic biota

The assessment of the environmental and health impacts resulting from radiation exposure is dependent on the use

of mathematical models, which, like all other models, are prone to uncertainty The best method for evaluating uncer-tainties in the predictions of dose-assessment models is to test predictions against data obtained under real-world con-ditions The large extent of contamination following the Chernobyl accident has provided exactly this type of oppor-tunity Currently an international cooperative effort known as BIOMOVS (BIOspheric MOdel Validation Study) is under way to test models designed for the calculation of environ-mental transfer and bioaccumulation of radionuclides and other trace substances More than 20 assessment models are now being tested against data collected from numerous sites throughout the Northern Hemisphere Upon completion of the initial model testing effort of the BIOMOVS project, additional long-term testing is being planned and organized

by the International Atomic Energy Agency (IAEA)

Another issue of concern that has not received research attention recently in the United States in the direct effect

of ionizing radiation on populations and communities of organisms This issue invariably arises whenever there is a nuclear-related incident In the case of the Chernobyl acci-dent, radiation exposures in the immediate vicinity of the reactor resulted in 28 human fatalities, with a larger number

or persons (209) suffering varying degrees of radiation sick-ness Pine forests within several kilometers of the reactor site received sufficient contamination to result in an accumulated dose of more than 1000 rads According to Soviet reports, pronounced morphological damage to pine foliage was vis-ible within 5 months after the accident in the zones where the doses ranged from 300 to 1000 rads Lethal effects in the

1000 rad zone were also manifest by this time, and by winter (7 months postaccident) 400 ha of forest was destroyed An ecological preserve has been established in one of the natu-ral areas subjected to high levels of radionuclide contamina-tion The Soviet government has announced its intention to carry out long-term radioecological observations and studies

in this preserve to assess the long-term impacts, if any, on the

resident flora and fauna

To the Soviets’ credit, they have recognized both the

need and the opportunity to obtain data on the long-term

effects of ionizing radiation on plant and animal populations

as manifested through genetic mechanisms To this end,

they have established experimental facilities at the accident

site to carry out this research Radioecologists have long

recognized the need for hard data on the long-term

conse-quences of exposure to chronic radiation to populations of

organisms Little is known about the interaction of ionizing

radiation and environmental stress on populations that are

subject to competitive pressures, predation, and other

fac-tors that affect survival We need to be concerned with the

effects that a buildup of radionuclides in the environment

would have on the eventual fate of the organisms inhabiting

such an environment

Thus, despite the current lack of attention given to

research issues in radiation ecology in the United States,

much can be learned by collaborating with scientists in

Europe and Asia who are now engaged in investigating the

fates and effects of radioactive substances deposited from

the Chernobyl accident

REFERENCES

1 Lansdell, Norman, The Atom and the Energy Revolution, Philosophical

Library, New York, 1958

2 Curtis, Richard and Elizabeth Hogan, Perils of the Peaceful Atom,

Dou-bleday and Co., New York, 1970

3 Bryerton, Gene, Nuclear Dilemma, Ballantine, New York, 1970

4 Ravelle, Roger et al., The ocean Scientific American, September 1969

5 Russell, R Scott et al., Radioactivity and Human Diet Pergamon Press,

London, 1966

6 Auerbach, Stanley I A Perspective on Radioecological Research, J

Soc Radiol Prot 4(3): 100–105, 1984

7 Izrael, Yu, A et al., Ecological Consequences of Radioactive

Contami-nation of the Environment in the Chernobyl Emergency Zone Moscow,

1987

8 Peterson, R C., Jr., et al., Assessment of the Impact of the Chernobyl Reactor Accident on the Biota of Swedish Streams and Lakes, Ambio

15(6): 327–334, 1986

9 BIOMOVS Progress Report No 6 Swedish National Institute for Radi-ation Protection, Stockholm, 1988

10 International Atomic Energy Agency Coordinated Research Project on the Validation of Terrestrial, Aquatic, and Urban Radionuclide Transfer Models and Acquisition of Data for that Purpose IAEA, Vienna

STANLEY I AUERBACH

Oak Ridge National Laboratory

RADIOACTIVE WASTE