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

TORRES ASTORGA, R.; VELASCO, R. H.; VALLADARES, D. L.; LOHAIZA, F.; JURI AYUB, J.; RIZZOTTO, M.

Barcelona

Congreso; International conference on the radioecology and Environmental radioactivity; 2014

International Union of Radioecology

Beryllium-7 is a short-lived environmental radionuclide, produced in the upper atmosphere by spallation of nitrogen and oxygen by cosmic rays. After that, this radionuclide diffuses through the atmosphere until it attaches to atmospheric aerosols. Subsequently, it is deposited on 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 and Cartwright, 2005). It is adsorbed onto clay minerals after its deposition does not penetrate deeper into soils due to its short half-life. The 7Be maximum activity is usually found at the surface of the soil column, showing a decreasing exponential profile with soil depth, typical of a purely diffusive transport (Wallbrink and Murray, 1996). In this work, in a semi-arid environment with marked seasonality in the rainfall regime, a dispersive transport model with linear sorption is used to analyse the vertical migration process of 7Be in undisturbed soil. The deposition history is approximated by pulse-like input functions. A time dependent analytical solution of the differential equation is obtained. The proposed model fits acceptably the experimental data