137Cs in French soils: deposition patterns and 15-year evolution.

Around 2000 surface samples have been taken from French soil at around twenty stations over the last fifteen years. Caesium 137, the only artificial radionuclide in the terrestrial environment currently detectable with gamma spectroscopy, was measured in the samples. The levels of caesium activity were compared to deposition estimates for deposits from atmospheric weapons tests and Chernobyl fallout. Since the start of data acquisition, specific activity in the samples has decreased with a mean effective half-life of between 8 and 11 y, for meadow soil and agricultural soil respectively. These field observations confirm that the conventional soil migration-retention model, which uses distribution coefficient Kd, is insufficient for modelling caesium distribution in the surface soil layer. Although the differences between sites tend to diminish with time, surface specific activity in agricultural and meadow soil varies on average from a few Bq to several tens of Bq per kg of dry matter.