Mobility of natural radionuclides and selected major and trace elements along a soil toposequence in the central spanish pyrenees

Natural gamma-emitting radionuclides (U, Ra, Th, Pb) and selected major and trace elements (Ca, K, Mg, Na, Fe, Al, Mn, Pb, Ba, Zn, Sr, Li, Co, Ni, Cu, Cr, Cd) were determined in a soil toposequence along a mountain slope of the Tertiary Flysch landscapes in the Central Spanish Pyrenees. A variety of basic properties were also measured from the soil profiles. Mean radioisotope activities (Bq kg) range from 22 to 33 for U; 25 to 32 for Ra; 23 to 33 for Pb, and 37 to 46 for Th. The radionuclides showed different patterns in their depth distribution, thus U and Pb had largest differences in concentrations down the soil profiles, whereas Ra and Th exhibited uniform depth distributions. U was depleted in all upper soil layers and enriched in deeper layers. Pb exhibits very different depth profiles along the soil toposequence and accumulated at upper layers in three sites. U/Ra activity ratios indicate disequilibrium in the U decay chain and reflect the leaching of U in contrast with the lack of mobility of Ra. The values of Th/Ra indicate that the initial proportionality in the U and Th decay chains has not been maintained in this toposequence. The relationships between soil properties and gamma-emitting radionuclides suggested the association of Ra and Th with Fe and Mn oxides. Al, Ca, Fe, and K were the most abundant elements, followed by Mn, Ba, Pb, Sr, Li, and Zn, whereas Co, Cu, Ni, and Cr are as trace elements and Cd was not detected. Correlations between elements suggest association with carbonates (Ca, Sr), silicates and clay minerals (Al, K, Na) and with Fe and Mn oxides (Cr, Cu, Ni, Co, Zn). Along the soil toposequence, Ca, Sr, Mg and Na, K, Al increase at the bottom slope positions, due to highest carbonate contents and abundance of finer soil fractions (clay and silt), respectively. Fe and Mn decrease at the bottom slope because highest contents of Fe and Mn oxides are at upper slope positions. This research is of interest to describe the geochemical cycling of elements in the environment and to assess the processes that affect their mobility in the ecosystems.