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

7Be is a short-lived environmental radionuclide, produced in the upper atmosphere by spallation of nitrogen and oxygen by cosmic rays. After of the production by the nuclear reaction, 7Be diffuses through 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 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. The literature data show that in soils, 7Be 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 into soils due to its short half-life. The maximum mass activity density of 7Be 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 a purely diffusive transport. Many studies applying the advection dispersion models have been reported in the literature in order to modelling the transport of 137Cs 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 7Be 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 7Be profiles and 7Be content in rain water were obtained from measures in a semiarid region at San Luis Province, Argentina during dates corresponding to dry and wet season of the region. (authors)