Superficial soil erosion assessment in agricultural land and bare land using 7Be fallout

Geologic and hydrologic phenomenon monitoring presents great environmental and financial interest and several radioisotopes, natural and artificial, have been used for this purpose. The more used are 137Cs, 210Pb not supported and 7Be. In the present work, 7Be was used to determine the soil erosion in three areas: one with soy ploughed at the direction of the slope, one with it perpendicular to the slope and one in an area with bare soil. 7Be is a cosmogenic radionuclide, with half-life of 53.3 days, produced by spallation of oxygen and nitrogen by cosmic rays in the troposphere and stratosphere. 7Be deposition occurs by dry and wet deposition, although wet deposition contributed by 95%. This can be verified through the measures of the 7Be inventory correlated with the precipitation, which resulted in a good linear adjustment. The experimental set up consisted of two HPGe detectors: one with 66% of relative efficiency and one with 10% of relative efficiency, both detectors coupled to standard gamma ray spectrometry nuclear electronic chain. Soil samples were packed in 1 liter Marinelli beckers. Sampling was accomplished until the depth where 7Be was present and it was possible to verify that its penetration in the soils could be very well adjusted by an exponential type function. The maximum beryllium-7 penetration in the bare soil without sign of soil erosion was 3 cm, that is, beryllium-7 is a useful tool as tracer for superficial soil erosion determination. The constant of mass relaxation h0 was determined as 4.71 ± 0.36, result that is in agreement with other works in the international literature. It was verified that when the soy is ploughed perpendicular to the slope, the soil redistribution rate is smaller, resulting in economic advantage. The bare soil is very exposed to the erosion, because does not exist any barrier to contain the soil that flows at the direction of the slope, such fact was verified in this work, where it was determined that the bare soil area presented the largest soil erosion among the tree studied areas. (author)