Radionuclide Transport in Lakes
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
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.
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.
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?
- One of the pioneers of the science of radioecology
was Dr. Eugene Odum, who founded the Savannah River Ecology Laboratory
- Radionuclides have a rate of decay called a
half-life. These vary from a few seconds to many years and are specific
to individual radionuclides. For example, barium-137 has a half life
of 2.6 minutes. So it loses half its radioactivity every 2.6 minutes.
Cesium-137, on the other hand, has a half life of 33 years. And plutonium
has a half life of 24,000 years.
- Because all radionuclides decay over time, and
because most radionuclides are bound in sediments, simply leaving a
contaminated reservoir alone may be the least ecologically disruptive
and most cost- effective cleanup strategy.