Answer to Question #5488 Submitted to "Ask the Experts"

Q

I have been trying to find activity levels for radionuclides that are now normal in background soil due to accidents and nuclear testing. The isotopes that I am looking for are ⁴⁰K, ⁵⁴Mn, ⁵⁸Co, ⁶⁰Co, ⁹⁴Nb, ¹³⁷Cs, ¹⁵²Eu, ¹⁵⁴Eu, ²⁰⁸Tl, ²¹⁰Pb, ²¹²Bi, ²¹⁴Bi, ²¹²Pb, ²¹⁴Pb, ²²⁸Ac, ²²⁸Th, ²³⁰Th, ²³¹Pa, ²³⁴Th, ^234mPa, ²³⁴U, ²⁴¹Am, and ²³⁹Pu. I have found some numbers for ⁴⁰K and for uranium, thorium, and plutonium isotopes, but some of the numbers do not seem to match up. If I have a gamma-spectrum printout that says I have, for example, 14 pCi/g of ¹⁵²Eu and 1.7 pCi/g ¹⁵⁴Eu, and 0.8 pCi/g ²⁴¹Am, etc., I could then compare the numbers to see which radionuclides are above the average normal background.

A

The levels of various radionuclides in soil is highly variable throughout the world; there is no one value for what is "average normal background." So-called normal levels can vary by tenfold or more from place to place. Many of the radionuclides you specifically called out are naturally occurring; others are man-made. Some have short half-lives, and their levels in soil will decrease rapidly as a result of radioactive decay. Other processes, such as leaching into the soil, runoff by rainfall, and solubility in water, will also affect the levels in soil. So there is no one answer to the question you pose. All is not lost, however. There are numerous reports published in the scientific literature that characterize the radioactivity in the soil in various regions, and these data are compiled by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), which publishes a comprehensive report of radioactivity in the environment that is updated periodically.

Ron Kathren, CHP