Vertical and lateral distribution of fallout 137Cs and soil properties along representative toposequences of central Rif, Morocco

Abstract

The purpose of this study is to understand and quantify the relationships between current 137Cs inventories and the soil properties and the physiographic characteristics. A total of 36 cores were taken in seven transects with different slopes, lithology and land use. The analysis focused on the 137Cs mass activity as well as inventories and its relationship with soil properties as grain size and organic matter.

The mass activity of 137Cs and the inventories varied between 3.6 and 63.7 Bq kg−1 and between 521.7 and 3304.4 Bq m−2, respectively. At uncultivated soils, high concentrations of 137Cs are located in the top 10 cm and decreased exponentially in depth. Cultivated soils record disturbed and heterogeneous 137Cs profiles with values of mass activity and inventories up to 38.7 Bq kg−1 and 2510.8 Bq m−2, respectively.

The 137Cs inventories were significantly higher in uncultivated soils (mean: 2086.9 Bq m−2) compared to cultivated soils (mean: 1397.1 Bq m−2). The 137Cs mass activity showed a significant positive correlation with organic matter, silt and clay, but it was negatively correlated with sand and pH respectively. High levels of 137Cs were found at altitudes between 500 m and 640 m a.s.l, and at slopes from 8° to 11°, as well as in dense scrub land and Fersialitic soils. A principal component analysis showed that more than 86.7% of the variance in 137Cs mass activity was explained by the organic matter content and the land use.

The results of this study provide insights into the effects of soil properties and physiographic factors on the behaviour of 137Cs in soils of Mediterranean environments of North Africa and strengthen the reliability of this radioisotope as an erosion tracer.