After applying conventional methods to fight soil erosion, Tunisia is using cesium-137 (137Cs) to measure their efficiency. The innovative program is fitting for a country where desertification threatens nearly 52% of land suitable for agriculture, forestry, and pasture farming.
“Conventional methods require several field visits and many years of measurements, while the fallout radionuclide method…requires only one sampling. This can be done in a few days, making the method time and resource efficient,” said Emil Fulajtar, a soil scientist at the Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture. (FAO is the Food and Agriculture Organization of the United Nations, and IAEA is the International Atomic Energy Agency.)
“The method is based on the origin and behavior of fallout radionuclides in soil. The radionuclides fall from the atmosphere and accumulate in the uppermost layer in soil. When part of this layer is removed by erosion, the fallout radionuclides are removed together with the soil,” explained Fulajtar. “We then compare their distribution in the area with a so-called reference site, which was not affected by erosion and deposition. The areas where we have less radionuclides are eroded.”
Fallout from Nuclear Weapons Tests
Cesium-137, which has a half-life of nearly 30 years, is one of the more common fission products of uranium-235 and other fissionable isotopes in nuclear reactors and nuclear weapons.
Fulajtar said the cesium-137 used to measure soil erosion in Tunisia comes from atmospheric nuclear weapon tests carried out in the 1950s and early 1960s. Atmospheric circulation and precipitation patterns have made this nuclear fallout present in soils and sediments around the world. Fallout has previously been used to quantify soil degradation in riparian areas, but the Tunisian experiment is the first time such methodology has been used to measure agricultural soil conservation techniques.
Although cesium-137 dissolves easily in water and bonds to topsoil and concrete, it and other radionuclides are strongly fixed to soil and are not taken up by plants. This fixation means the fallout does not risk contaminating crops. The measuring process is also not dangerous: Exposure to extremely large amounts of 137Cs can cause radiation sickness, but the minute levels of nuclear fallout in world soil does not pose a risk to human health.
“Its deposition in the atmosphere was later stopped because atmospheric nuclear weapon tests were forbidden. The cesium-137 released during such tests spread to the stratosphere, circulated around the globe, and came down to soils mainly as a result of rain and snowfall,” Fulajtar said.
Ridges to Reduce Erosion
According to Fulajtar, the cesium-137 method of measuring soil erosion provides long-term mean soil redistribution rates, representing the period from its release until the time of sampling. “Such information about spatial and temporal distribution of erosion is crucial for developing soil conservation programs in many parts of the world,” he said.
In Tunisia, 137Cs was used to measure the effectiveness soil ridges, created to conserve both soil and water. According to the IAEA, 80-centimeter-high soil ridges are situated about 40 meters from one another. They break the effective slope length and create a barrier for surface water runoff. Results from the study showed soil ridges reduced erosion by 41%.
The Tunisian Ministry of Agriculture is evaluating the results of the study and deciding how they will inform further research or policy. Scientists suggest studying erosion rates after placing soil ridges closer together or with no-tillage land management.
Johan Six, a soil scientist with the Swiss Federal Institute of Technology in Zurich who was not involved in the Tunisian project, said the fallout radionuclide technique is useful in areas that have significant differences in erosion rates. “137Cs gives you an order of magnitude of erosion rate, so you need to have a big difference in erosion rate to be able to detect it with 137Cs,” he said.
Six also emphasized the benefits of the method, noting that such low amounts of 137Cs pose no real risk to agriculture and the technique does not disrupt production. “There are no side effects to its use to determine erosion.”
—Issa Sikiti da Silva (@sikitimedia), Science Writer