Potentially toxic elements in floodplain soils the Don River basin of Southern Russia

Large rivers and their deltaic parts and adjacent coastal zones are subjected to strong anthropogenic influence and are often considered as hotspots of environmental pollution. The Don River basin is a highly urbanized area with developed agriculture and industry which negatively affect water quality, aquatic ecosystems and soils. The main objectives of the proposed research were to determine the levels of potentially toxic elements (PHEs) in soils of various aquatic landscapes of the study area, as well as to reveal the relationships between the content of exchangeable PTEs and the physical-chemical properties of floodplain soils.

Depending on the soil-landscape and hydrological conditions and taking into account the intensity of anthropogenic influence, the following zones were identified: the lower Don floodplain from the Tsimlyansk Reservoir to the source of the Mertvy Donets River, Don Delta, the coastal zone of the Taganrog Bay, the mouths of small rivers flowing into the bay, and Taganrog city, an industrial port center on the northern coast of the bay.

The floodplain and coastal landscapes of the study area are dominated by Fluvisols. Solonchaks, Arenosols and Haplic Chernozems which are background soils of the region are less common. Soil samples were collected in summer 2020 from the surface soil horizon (0–20 cm deep). The particle size analysis was conducted using the pipette method; the total organic carbon content in the soils was determined using the dichromate oxidation; the pH was measured by potentiometry in the supernatant suspension of soil and water in a ratio of 1:2.5. The total concentrations of Cr, Mn, Ni, Cu, Zn, Cd, and Pb were determined by X-ray fluorescence analysis using a Spectroscan MAX-GV spectrometer (Spectron, Russia), and the content of exchangeable forms extracted from the soil by NH4Ac buffer solution with pH 4.8 and soil/solution ratio of 1:10 for 18 h was determined by atomic absorption spectrophotometry.

The obtained results showed that soils of the Lower Don and Taganrog Bay coastal zone are rather contrasted in terms of properties and metal contents, which indicates the variability of landscapes, natural and anthropogenic processes in the studied systems. High CV values for Pb, Zn, Cd and Cr indirectly indicate strong anthropogenic influence on these environments. The results of PCA analysis showed that there are two association of metals in terms of geochemical behaviour and sources. The first one included Cr-Zn-Pb-Cd, the elements of anthropogenic origin, the second Mn, Ni, and Cu, which are probably of mixed origin. The obtained results showed that the coastal zone is a diverse and complex system subjected to anthropogenic activities, which is pronounced in the enrichment of aquatic soils with a number of metals and higher proportions of exchangeable forms from different types of sources.

This work was funded by the Russian Science Foundation, grant no. 20-14-00317.