Chemical differentiation of recent fine-textured soils on the Caspian Sea coast: A case study in Golestan (Iran) and Dagestan (Russia)

Abstract

On the recent fine-textured coasts located to the northwest (Russia) and southeast (Iran) of the Caspian Sea at altitudes of 25–26 m below sea level, the difference in elemental composition between two coasts as well as the vertical differentiation of elemental composition (18 chemical elements, 48 samples) in soil pits were studied. The soils of both key sites are close in particle size distribution (the average contents of sand, silt and clay were 10%–30%, 60%–74% and 13%–16%, respectively) and pH (7.4–8.8). Soils on the Iranian coast affected by shallow, highly saline groundwater (the water table is 1.5–2.5 m, solute concentration ~100 g/l) have a uniform vertical distribution of most elements except for As, P and Pb. In soils on the Russian coast with deeper and less saline ground water (2.5 m, ~50 g/l), the total content of the majority of elements (Al, As, Ca, Co, Fe, K, Mn, Ni, P, Pb, Sr, Ti, V and Zn) except for Cr, Cu, Mg and Si changes significantly (P value < 0.05) with depth. The contents of Ni, Si, V and Zn in topsoil, as well as the contents of Al, Cr, Fe, K, Mn, Ni, P, Si, Ti, V and Zn in subsoil, are higher on the Russian coast. The contents of As, Cu, Cl−, SO42− and HCO3− in subsoil are higher on the Iranian coast. Based on weathering indexes (R, PWI, Si/R, Si/Ses and Si/Fe), the recent soils on the Russian coast are significantly more weathered than the soils on the Iranian coast. In Russia, the contents of the potentially toxic elements (As, Cr, Co, Cu, Ni and V) do not exceed the thresholds given in the national soil quality guidelines. In Iran, there is an excess of Co, Cu and Ni according to the national soil quality guidelines for soils with pH > 7. The use of the most stringent international thresholds has shown that the contents of potentially toxic elements on both coasts of Russia and Iran exceed those limits by a factor of 1.5–3.