Written by Grant Hamilton

Cs-137 is a radioactive isotope of cesium. Its occurrence is the result of anthropogenic nuclear fission. Most of the Cs-137 present in the environment is the result of fallout from nuclear weapons testing during the Cold War. Cs-137 first reached detectable levels in 1954. Additional fallout occurred with the Chernobyl and Fukushima nuclear power plant disasters. Cs-137 is distributed primarily by rainfall, and is readily absorbed by soil. Cesium-137 decays to barium-137 which then decays to a non-radioactive form of barium. Cs-137 has a half-life of 30.17 years. The measure of Cs-137 concentration is the SI unit of radioactivity, the Becquerel (Bq) per kg or m².

Soil loss can be studied using a technique called radiological fingerprinting to determine the source of suspended sediment in a watershed or the source of a soil deposit. This method can also be used to estimate erosion and deposition rates. Despite regional variations in precipitation, the worldwide distribution of Cs-137 was uniform after levels peaked in the late 1980s following the Chernobyl meltdown (the impact of the Fukushima disaster on global Cs-137 levels is a subject of current study). Thus an arid environment's baseline Cs-137 level would be the same as that of a wetter region. Higher levels of Cs-137 in a soil sample indicate areas where deposition has occurred and lower levels indicate areas where erosion has occurred.

Samples can be collected from the soil using a bucket corer or as a suspension in waterways. Sampling sites should be georeferenced using a GPS. In the laboratory samples are air dried or baked, disaggregated, and sieved through 2 mm mesh. Cs-137 concentrations are measured using a gamma-ray spectrometer. Cs-137 levels are compared with locations throughout the study area to determine sources of deposits. This method has been used in a variety of settings throughout the world.

Nearing et al. (2005) used this method to compare the erosion rates in two watersheds within USDA Agricultural Research Service (ARS) experimental ranges in Arizona. In a follow up study, Ritchie et al. (2009) used the Diffusion and Migration Model for Erosion and Deposition on Undisturbed Soils to account for the redistribution of Cs-137 within one of those watersheds.

Nearing, M.A., Kimoto, A., Nichols, M.H., Ritchie, J.C. (2005). Spatial patterns of soil erosion and deposition in two small, semiarid watersheds. Journal of Geophysical Research. Vol. 110. doi: 0.1029/2005JF000290

• Compared erosion rates of two Arizona watersheds using Cs-137 inventory.

Ritchie, J., Nearing, M.A., Rhoton, F.E. (2009). Sediment budgets and source determinations using fallout Cesium-137 in a semiarid rangeland watershed, Arizona, USA. Journal of Environmental Radioactivity. doi: 10.1016/j.jenvrad.2009.05.008. www.tucson.ars.ag.gov/unit/publications/PDFfiles/2012.pdf‎

• This is a follow up to the study listed above.

U.S. Environmental Protection Agency. (2012). Cesium. http://www.epa.gov/radiation/radionuclides/cesium.html

• General information about the element cesium.

Walling, D.E. and He, Q. (1999). Improved models for estimating soil erosion rates from cesium-137 measurements. Journal of Environmental Quality. 28(2), pp. 611-622. doi:10.2134/jeq1999.00472425002800020027x

• Presents the Diffusion and Migration Model for Erosion and Deposition on Undisturbed Soils.

Yusheng, Z. (n.d.). Applications of Caesium-137 in Soil Erosion and Sedimentation Studies (an introduction). Department of Geography, University of Exeter. http://people.exeter.ac.uk/yszhang/caesium/welcome.htm

• Brief overview of the Cs-137 process with an extensive list of the literature on the subject.

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