Radiocesium distribution in undisturbed soil: measurements and diffusion-advection model.

The profile of 137Cs, due to Chernobyl accident, in undisturbed soil was measured experimentally over the years 1987 to 1994 and was found to remain practically fixed in the upper 30 cm of soil since 1987. Total deposition of 137Cs at the site was 20 kBq m-2, and approximately 80% of that is in the upper 10 cm. The profile has two slopes in semilogarythmic scale, i.e., it appears as the sum of two exponentials. The contribution of weapons fallout is found to be negligible. Pure diffusion and diffusion-advection models of cesium migration are investigated. It is found that the pure diffusion model cannot reproduce the double slope, while the diffusion advection model can.