The atoms of many elements occur in different forms, or isotopes,
based on their sub-atomic structure. Some forms are unstable because they have an excess
amount of energy. Unstable forms are called radioactive. Radioactive isotopes seek
stability by releasing their excess energy in the form of particles (alpha, beta or
neutrons) or gamma rays. This process of energy loss is called radioactive decay. Once a
radioactive element releases its excess energy, it becomes a stable element and is no
longer radioactive.
Radioactive isotopes of elements are called radionuclides. Some radionuclides, such as
uranium and radon gas, occur naturally. Others, like strontium, cesium, plutonium and
tritium, which is a form of hydrogen, are human-made products of nuclear fission (the
splitting apart of atoms).
When small amounts of these radionuclides enter the environment through radioactive
fallout, improper waste disposal or nuclear accidents, they are of special concern. The
energy released by radionuclides as they decay can be very harmful to organisms, and in
sufficient quantities will increase the probability of causing genetic damage and cancer.
RADIONUCLIDE TRANSPORT IN LAKES
Most radionuclides that move into lakes attach to sediments on the lake bottom. These
radionuclides chemically bind to the lake's sediments, particularly the clay part, and
thus the contaminants are largely trapped on the lake bottom.
Some radionuclides, such as tritium, do not effectively bind with sediments and move
through the environment more easily. In the case of tritium, transport occurs rapidly
through the atmosphere or water. Radionuclides that do not bind to sediments are within
the lake's water and can accumulate in aquatic vegetation, fish and other animals.
BIOACCUMULATION
Several factors can affect the movement (bioaccumulation) of radionuclides from sediments
into plants and animals. Those factors include the specific radionuclide, the type of
plant or animal, and the nature of the sediments.
For example, cesium is a radionuclide that behaves similarly to the stable element
potassium. Potassium is a very important nutrient found in all organisms. Cesium mimics
potassium in the environment, thus cesium tends to be taken up by plants and animals as if
it were potassium.
When a plant takes up cesium through its roots, it is then
transported to the leaves and stems. When animals eat the contaminated plant, they ingest
cesium as well. Cesium moves in the animal's body like potassium and ends up in the muscle
tissues. As other animals feed on the contaminated animal, the cesium is transported up
through the food chain, similarly to the pesticide DDT that was of concern in the 1970s.
Cesium is one of the few radionuclides that increases in concentration as you move up the
food chain; most radionuclides do not.
Radionuclides that behave like stable elements are called chemical analogues. Another
example is strontium, a chemical analogue of calcium. Strontium tends to be taken up as if
it were calcium. It typically concentrates in animal bones or shells.
RESEARCH
The study of how radionuclides move through the environment and their effect on ecosystems
is called radioecology. The Savannah River Ecology Laboratory has a rich tradition of
study in radioecology based in the Laboratory's Biogeochemical Ecology Division. Research
on radionuclide transport in lakes and streams on the Savannah River Site is part of the
Laboratory's radioecology program.
Scientists have measured low levels of radioactive contaminants in some streams and lakes
on the site. Research has shown how some of these radionuclides move through the food
chain. This information helps scientists determine the potential impact on the plants and
animals that live in this environment.
In addition, researchers can calculate the risks to humans who might consume contaminated
tissues from fish or wildlife. Researchers also estimate the risk to humans who might live
on the site in the future if it ever reverts to public use.
The instruments used to detect radionuclides are among the most sensitive ever developed
by humans. They are capable of measuring extremely small amounts of radiation, amounts
much smaller than what is considered harmful to plants and animals. This capability has
allowed radionuclides to be used as tracers of environmental phenomena. For example, what
if you wanted to know if plants take up stable potassium faster in sandy or clay soils?
You could set up an experiment where you grow plants in different soil types within a
greenhouse. Because cesium behaves like potassium, a radioecologist could place a tiny
amount of cesium in the soil and determine how long it takes for the cesium to move into
the plants growing in the clay soil versus those growing in the sandy soil. The results
from cesium (which is easier to measure than potassium because of the instrument's
sensitivity) would give you an indication of the movement of stable potassium.
On a larger scale, Ecology Lab scientists have used radionuclides in contaminated lakes as
tracers to study a variety of physical, chemical and biological processes that would
otherwise be difficult to investigate. These include processes such as the movement of
elements between sediments and the overlying water, uptake of elements into plants and
long-term changes in the distribution and fate of elements.
The focus of radioecology research at the Laboratory is turning toward the cleanup and
restoration of contaminated areas. One experimental project involves the application of
different types of fertilizers and chemicals to contaminated soil to see if the movement
to the plants can be reduced. Scientists are learning how to reduce radionuclide movement
and bioaccumulation from studies at the contaminated Par Pond lake on the Savannah River
Site.
DID YOU KNOW?
This fact sheet was produced by the Outreach Program of the Savannah River Ecology Laboratory.
Last review: October 12, 2007