Radiocesium and plutonium: still together in “background” soils after more than thirty years

Soil samples were collected from sites near Denver, Colorado, in the summer of 1994 as part of the Rocky Flats Plant “Background Soils Characterization Program.” Analysis of the soils for plutonium and radiocesium give a mean activity ratio of radiocesium to plutonium of 38±4 (as of 1 July 1994). This ratio strongly suggests that these soils received their plutonium and radiocesium from world-wide fallout from the testing of nuclear weapons, over 30 years ago, and that these two chemically different elements are still fixed together in the sampled soils in the same ratio as in soils analyzed over 20 years ago. Thus, the radiocesium/plutonium activity ratio in background soils is predictable and deviations may indicate contamination with radiocesium and/or plutonium from a source other than weapons fallout.