Variation of monthly inventories of ⁷Be fallout in the soils of the sub-basins 3 and 4 in Mato Frio river, a tributary of Serra Azul river

In this study 72 soil samples collected right at the surface were analyzed. They were collected at two work parcels located within the basin of the Mato Frio Creek (total drainage area = 10.6 km²) located in the municipality of Itauna, except for a small fraction in its northern part, which is located in the municipality of Serra Azul. Both municipalities are in the State of Minas Gerais, Brazil. The sampling period covers the time span from May 2014 to May 2015, with the purpose of observing on a monthly basis the variation in the activity of the radionuclide Beryllium-7 (⁷Be, Eγ = 477.6 keV) along a one year hydrologic cycle, stressing the dry and rainy seasons. The objective of this investigation was to establish a baseline for the alteration pattern of the ⁷Be content in the soil in order to use these results for future estimates of rates of erosion or accreation in areas of interest within this basin. In order to measure the ⁷Be activity in the collected samples, a gamma spectrometer was used, composed of a hyperpure germanium detector with a relative efficiency of 50%. The results indicate a net trend towards a marked variation in the activity of ⁷Be in relative to the period of year within which the samples were collected, which in turn results in a reduction or increase in the values of the ⁷Be monthly inventory in the topsoil. (author)