Biogeochemical Aspects of 137Cs, 90Sr, Th, and U Migration in the Soil and Vegetation Cover of Steppe and Semidesert Landscapes of Stavropol Krai

The main physical and chemical characteristics of soils, including their particle-size and mineralogical compositions, were determined as a result of comprehensive radioecological studies on two test plots in the northeast of Stavropol krai within the Chograi-Caspian semi-desert landscape and the Chograi-Ragulinskii landscape of grass steppes. Background concentrations of natural radionuclides (NRs) 40K, Th, and U, of technogenic radionuclides 90Sr and 137Cs, and of their isotopic (Sr, Cs) and non-isotopic (Ca, K) carriers were estimated in the upper (0–20 cm) soil layer. Geo-accumulation indices (Igeo) calculated for heavy metals Cs, Sr, Th, and U, were <0 and indicated the absence of anthropogenic contamination of the soils with these elements, as well as the absence of geochemical anomalies. Based on the vertical distribution data for 90Sr and 137Cs in the 0–20-cm layer, the diffusion coefficients and convective transport rates were assessed, and the ecological half-life periods for 137Cs and 90Sr in the 0–10-cm layer were calculated (220 and 77 years, respectively). The activity concentrations of 40K, 90Sr, and 137Cs radionuclides and the mass fractions of their isotopic (Sr and Cs) and non-isotopic (Ca and K) carriers, as well as the mass fractions of HNRs Th and U were estimated in a number of plant species. We calculated the main parameter of the process of biogeochemical migration of the elements in the soil-plant system: the concentration ratio (FV) for the radionuclides and isotopic (Sr and Cs) and non-isotopic (Ca and K) carriers. The FV values depended on the plant species and ranged from 0.08 to 0.22 for 137Cs and from 0.54 to 1.27 for 90Sr. The mean TF values for Th, U, Cs, K, Sr, and Ca could be arranged in the following sequence (n × 10–2): U (0.005) < Th (0.007) < Cs (0.01) << K (57) < Ca (76) < Sr (81).