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 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