Transformation of Dissolved Matter Runoff in the Ural River Mouth

Abstract

The distribution of dissolved major- and trace elements in the mixing zone of the Ural River and the North Caspian waters was studied based on the natural observations data from 2016–2017. Conservative behavior was established for most of major ions (Na, K, Mg, SO4) and some trace elements (Li, Rb, Cs, Sr, Co, Ni, Cu, Zn, Sb, Ga, Y, U, B, F, Cr, Ge, Mo, W) with common parameters of relationships between their concentrations and chloride content for different years. The distribution of components of the carbonate system is controlled by chemogenic precipitation of calcium carbonate at the mouth beach. This process leads to the removal from solution up to 11–17% calcium and 6–8% hydrocarbonates carried by river runoff, and a simultaneous decrease in the pH value. Nutrients are involved in the processes of biological assimilation and regeneration. The mixing zone of the Ural River and Caspian waters is characterized by: (a) phytoplankton consumption of large amounts of silicon and nitrates (up to 58–88 and 61–67%, respectively, of their content in riverine waters), (b) removal of significant part (up to 18–25%) of nitrites, and (c) additional input of phosphates into solution, presumably from pore waters of the surface layer of bottom sediments in amounts 1.5–3 times greater than those removed with river runoff. A distinctive feature of barium migration is an additional input into solution (up to 20%) at the initial stage of salinization owing to the desorption from terrigenous material. The coagulation and flocculation of organic and organo-mineral colloids lead to the removal of significant part (up to 25–100%) of dissolved manganese, iron, aluminum, and rare-earth elements runoff, as well as to the removal of lead, titanium, zirconium, and hafnium from solution in amounts 1.1–6 times greater than their contents in the river water mass.