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

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

Barcelona

Conferencia; 3rd international conference on radioecology & environmental radioactivity; 2014

7 Be is a short-lived environmental radionuclide, produced in the upper atmosphere by spallation of nitrogen and oxygen by cosmicrays. After of the production by the nuclear reaction, 7 Be diffusesthrough the atmosphere until it attaches to atmospheric aerosols. Subsequently, it is deposited on the earth surface mainly as wet fallout. The main physical processes which transport 7 Be in soil are diffusion and advection by water.Migration parameters and measurements confirm that sorption is the main physical process, which confines 7 Be concentration to soil surface. The literature data show that in soils, 7 Be is concentrated near the surface (0?2 cm) as it is adsorbed onto clay minerals after its deposition on the soil surface and does not penetrate deeper intosoils due to its short half-life. The maximum mass activity density of 7 Be is found at the point of input of the radionuclide, i.e. at the surface of the soil column, showing a exponential distribution profile typical of apurely diffusive transport. Many studies applying the advection dispersion models have been reported in the literature in order to modelling the transport of 137 Cs in soils. On them, the models are used to achieve information of the mechanisms that govern the transport, i. e. the model is used to explain the soil profile of radionuclide. The effective dispersion coefficient and the apparent advection velocity of radionuclide in soil are also obtained by fitting the analytical solution of the model equation to measured depth distributions of the radionuclide. In this work, the advective dispersive transport model with linear sorption is used to analyze the vertical migration process of 7 Be in soils of undisturbed or reference sites. The deposition history is approximated by pulse-like input functions and time dependent analytical solution of equation model is obtained. The values of dispersion coefficient and apparent advection velocity obtained by fitting experimental profiles are compared with values that characterized the soil of sites. The experimental data of 7 Be profiles and 7 Be content in rain water were obtained from measures in a semiarid region at San Luis Province, Argentina during dates corresponding to dryand wet season of the region.