Modelling the observed vertical transport of 7Be in specific soils with advection dispersion model

TORRES ASTORGA, R; VELASCO, H; J. JURI AYUB; VALLADARES, D.

Barcelona

Conferencia; International Conference on Radioecology & Environmental Radioactivity; 2014

NRPA - Noruega

Beryllium-7 is a short-lived environmental radionuclide, produced in the upper atmosphere byspallation of nitrogen and oxygen by cosmic rays. After that, this radionuclide diffusesthrough the atmosphere until it attaches to atmospheric aerosols. Subsequently, it is depositedon the earth surface mainly as wet fallout (Papastefanou, 2006).The main physical processes that transport 7Be in soil are diffusion and advection by water.Migration parameters and measurements confirm that sorption is the main physical process,which confines 7Be concentration to soil surface until no more than 2 cm deep (Salisbury andCartwright, 2005). It is adsorbed onto clay minerals after its deposition does not penetratedeeper into soils due to its short half-life. The 7Be maximum activity is usually found at thesurface of the soil column, showing a decreasing exponential profile with soil depth, typicalof a purely diffusive transport (Wallbrink and Murray, 1996).In this work, in a semi-arid environment with marked seasonality in the rainfall regime, adispersive transport model with linear sorption is used to analyse the vertical migrationprocess of 7Be in undisturbed soil. The deposition history is approximated by pulse-like inputfunctions. A time dependent analytical solution of the differential equation is obtained. Theproposed model fits acceptably the experimental data.