Temporal variation of 7Be fallout and its inventory in purple soil in the Three Gorges Reservoir region, China

Abstract

The deposition flux of 7Be was measured in the range from 0.4 to 5.2 Bq m−2 d−1, with a mean value of 2.6 Bq m−2 d−1, during the period of May 31, 2009–May 31, 2010 in the Three Gorges Reservoir region, China. The low annual deposition flux of 7Be was probably due to the adsorption of 7Be within high altitude clouds and the Qinling-Daba Mountain’s barrier and blocking effects on the cold air during winter months. There was a seasonal trend for 7Be deposition, showing a spring maximum and winter minimum, which may be attributed to the seasonal folding of tropopause in spring and little rainfall amount in winter. A strong positive correlation (R2 = 0.67) between 7Be deposition flux and rainfall was observed. The depth distribution of 7Be in undisturbed purple soil profiles suggested that the nuclide was mainly distributed within the top 20.0 kg m−2 (~1.5 cm) of soil in depth and the maximum concentration occurred at a depth of 3.0 kg m−2 (~0.2 cm), then the concentration declined quickly and exponentially with depth. The seasonal inventories of 7Be in soils were consistent with the values calculated from the atmospheric fluxes, which shows a great potential for using 7Be as a tracer to assess soil redistribution in purple soil areas of China.