Evaluation of Erosion and Sedimentation Rates using the 137Cs Technique and the Contents of the Chemical Elements Fe, Ti, K, Mn, Zn, Ni and Cr in an Agricultural Field in Dakar, Senegal

Senegal is a sub Saharan country which encounters degradation problems on its land. Of all the types of soil degradation, erosion is the one that contributes the most. However, in Senegal, the availability of erosion data, especially quantitative, is very limited. In this study, the aim is to use, for the first time in Senegal, the fallout of radionuclides to assess soil redistribution. Thus, the use of the anthropogenic radionuclide 137Cs has made it possible to estimate the rates of erosion and soil deposition relative to a period of approximately 60 years, at the level of an agricultural field in the Dakar region. An undisturbed field in the recent past was identified and served as a reference field in this study. The surface activity or the 137Cs inventory corresponding to this reference site is 414 ± 62Bq m-2. The transformation of 137Cs (Bq m-2) inventories into erosion or deposition rates was carried out using the “Mass Balance II†conversion model. The results obtained showed that the average and gross erosion rates were 18.8t ha-1 year-1 and 14.6t ha-1 year-1, respectively, while the average and gross deposition rates were 23.1t ha-1 year-1 and 5.2t ha-1 year-1, respectively. These results lead to a net erosion rate over the entire field of 9.4t ha-1 year-1 and to a rate of mobilized sediment leaving the study site of 65%. Analysis by X-ray fluorescence of the major chemical elements Fe, K and Ti in soil samples from the agricultural field produced average concentrations of 2.74, 0.16 and 0.74g kg-1, respectively. As for the minor chemical elements analyzed, the most preponderant is Cr followed in order by Ni, Mn and Zn.