Migration of dissolved trace elements in the mixing zone between Volga River water and Caspian seawater: Results of observations over many years

Abstract

Based on natural observations over many years, the distribution of dissolved nutrients and trace elements was analyzed in the mixing zone between the freshwater of the Volga River and Caspian seawater. Most of the trace elements (Li, Rb, Cs, B, F, Br, I, Ga, Sc, Y, Co, Ni, Cu, Cd, Ag, V, As, Sb, Bi, Mo, W, and U) show a conservative behavior. During the period of the highest bioproductivity, dissolved phosphates and silica are intensely removed from solution (up to 60–90 and 46–82% of their supply by river runoff, respectively) mostly owing to uptake by aquatic organisms. The distribution of dissolved strontium was assigned to the weakly nonconservative type, because a minor excess of its content above the lines of conservative mixing (8–18%) was observed in some years; perhaps, this is related to different water transformation at the areas of moving and stagnant water in the delta and offshore mouth zone. Barium is characterized by additional input into the solution (up to 52%) in the regions of medium salinity owing to ion exchange reactions in the absorbed complex of river suspended material. The migration of dissolved species of aluminum, manganese, and iron in the mixing zone of Volga and Caspian waters is probably controlled by the coagulation and flocculation of organic and organo-mineral colloids, which is indicated by a sharp decrease in the content of these elements during the initial stage of salinization (59, 91, and 74%, respectively) followed by a plateau. The most complicated distribution was observed for titanium, lead, and rare earth elements (REE), the concentrations of which showed intense removal from the solution (up to 64–88% Ti, 52–87% Pb, and 66–83% REE) followed by a gradual increase, which is probably related to the elevated contents of these elements in the water of the northern Caspian above a local minimum in the zone of active flocculation of colloids.