Present distribution and variability of water flow, suspended sediment load, and geochemical runoff in the Volga River delta

Environmental changes in the Volga River delta attract attention of researchers due to increasing anthropogenic pressure on the river catchment, global climate changes, and natural fluctuation cycles of the Caspian Sea level. These changes significantly affect the hydrological regime and erosion-accumulative processes in the Volga delta.

In 2018-2020, the authors conducted series of field hydrological and geochemical studies within the Volga River delta, which covered all major systems of the deltaic water streams. We have determined water discharge, suspended sediment concentration, content of dissolved and suspended chemical elements (ICP-MS/ICP-AES). The results obtained in the study provide a comprehensive view of the current spatial and temporal distribution of the water flow, suspended sediment yield, and geochemical flows along the main branches, numerous channels and rivers during the both high and low water periods, and also allow us to compare them with long-term trends established by previous studies.

We have found that the present distribution of water flow and suspended sediment load within the Volga delta differs from the second half of the XX century. It determines the ongoing restructuring of the water balance, and transformation of the rate of erosion and accumulative processes within the main systems of the deltaic branches. The study allowed delineating the Volga Delta by zones of erosion, transition, and accumulation of suspended matter in different phases of the water regime in relation to the modern basis of erosion, which is the current level of the Caspian Sea.

The distribution of geochemical runoff within the Volga Delta is determined by water and sediment runoff, as well as concentrations of dissolved and suspended forms of chemical elements. The largest geochemical runoff passes through the Bakhtemir and Buzan systems. During the low-water period, the volumes of geochemical runoff of these branches are similar. During the flood period, despite the increasing share of Buzan water runoff, the prevailing fluxes of matter pass through the Bakhtemir system due to higher concentrations of elements and larger solid discharge.

The ratio of dissolved and suspended elements in flows is determined mainly by the properties of chemical elements. According to the percentage ratio of dissolved and suspended forms, we divided elements into 3 groups: 1) migrating mainly in dissolved forms (Na, Ca, Sr, Mg, Mo, U, K, Li, Ba, As, Sb), 2) migrating mainly in suspended forms (Pb, Y, Zr, Ti, Mn, Fe, Al, REE), 3) elements with changeable behavior depending on geochemical and hydrological conditions (Ni, Cd, Zn, Co, Cu, et al.).

The research was supported by RFBR project No. 18-05-80094 and №17-05-41174-RGS.