The 137Cs technique applied to steep Mediterranean slopes (Part I): the effects of lithology, slope morphology and land use

Abstract

Concentrations in the soil of anthropogenic and natural radionuclides have been investigated in order to assess the applicability of the 137Cs technique in an area of typical Mediterranean steep slopes. This technique can be used to estimate net soil redistribution rates but its potential in areas with shallow and stony soils on hard rock lithology have not been evaluated so far. In this research, the validity of using this technique in stony shallow soils at very steep slopes is discussed together with the relations between radionuclide concentrations and other soil properties, lithology, slope morphology and land use in a Mediterranean environment. Both natural Potassium-40 ( 40K), Uranium-238 ( 238U), Thorium-232 ( 232Th) and anthropogenic Caesium-137 ( 137Cs) radionuclides have been determined in samples taken along slope transects on uncultivated serpentinite soils and cultivated gneiss soils. In addition to the radionuclide concentrations, parameters such as slope position, slope angle, aspect, soil depth, surface stone cover, moss, litter, vegetation cover, soil crust, stone content and bulk density have been quantified. All the natural radionuclides 40K, 238U, 232Th show significantly higher concentrations in the gneiss than in the serpentinite soils, opposed to the 137Cs concentration, which is found significantly higher in the serpentinite soils probably because of the difference in clay mineralogy. The exponential decreasing depth distribution of 137Cs and its homogeneous spatial distribution emphasise the applicability of the 137Cs technique in this ecosystem. Lithology determines the concentration of natural and anthropogenic radionuclides. Land use determines the relations between 137Cs concentration/inventory and some soil characteristics. Higher 137Cs concentration and inventory are associated with higher percentages of vegetation cover, higher percentage of stones in the soil and higher values of soil bulk density in cultivated gneiss soils. Slope morphology and land use influence the soil redistribution at slope scale. The gneiss slopes show a zonation of four to five areas of differential erosion/accumulation processes corresponding with more regular slopes and soil redistribution due to water erosion and to tillage translocation and erosion. The serpentinites, as an example of a more unstable slope type, show more erosion areas with less accumulation downslope and soil redistribution due to water erosion.