Distribution of 137Cs in three representative soils of Pernambuco state, Brazil

Contents of 137Cs have been used to quantify soil loss or accumulations, based on values of reference sites with little soil disturbance in the last decades. In Northeastern Brazil no such sites have been evaluated yet. Three areas with flat topography, native vegetation and undisturbed soil were selected in of Goiania, Araripina and Sertania counties, state of Pernambuco. Four points in each area were marked at a distance of 20 to 50 m away from each other and soil samples were collected in 3 cm depth intervals, down to 30 cm. The 137Cs activities were determined using gamma spectrometry with a hyper pure germanium detector. Average contents of 137Cs (Bq m-2) were 71.6 ± 6.3 in Goiania, 64.0 ± 13.8 in Araripina and 95.5 ± 9.8 in Sertania. Values in this range have been reported in Paraiba and Bahia, and higher ones in the Southeastern region, confirming the reduction of stocks in areas closer to the equator. The highest activities occurred in the surface layers, ranging from 0.5 to 1.6 Bq kg-1, and decreased linearly with depth until the detection limits: 18 cm in Araripina, 15 cm in Goiania, and 9 cm in Sertania. In Goiania, the activity was highest (2.06 ± 1.0 Bq kg-1) in an organic horizon above a inorganic horizon. The high activities and stock and lower depth in Sertania could be explained by the predominance of 2:1 clay minerals versus 1:1 clay minerals in the other areas. 137Cs activities were positively correlated with water and KCl pH values and negatively correlated with Al concentrations. The results confirm that, in the reference areas, 137Cs activities are highest in the surface layers, decreasing regularly down the soil profile. (author)