FACTORS FOR FORMING THE HYDROCHEMICAL COMPOSITION OF SURFACE WATER IN THE KUDA RIVER BASIN (IRKUTSK REGION)

The increase in population inhabiting the territory of small rivers within the Irkutsk region is responsible for enhancing consumption of river water for daily use. It becomes evident that the anthropogenic load on the ecosystem of rivers is rising and river water quality is changing. Therefore, it is vital to explore the formation and transformation of hydrochemical characteristics under the influence of different factors. The goal of this paper is to analyze the geochemical composition of surface water in the Kuda River basin, and the changes caused by natural and anthropogenic factors. We sampled water at 8 sites of the Kuda River and at its 11 tributaries in different hydrological seasons. In addition, we have analyzed 14 groundwater samples within the basin. The following parameters were measured: pH, TDS (Total dissolved solids), major ion concentrations (Na+, K+, Ca2+, Mg2+, HCO3-, Cl-, SO42-), biogenic components (NO3-, NO2-, NH4+) and microelements. With the received data on hand, we have identified significant spatial and seasonal variability in the hydrochemical properties of surface waters. The pH values vary from 6.8 to 8.3. The highest values of TDS and major ion concentrations were measured for the surface and ground waters of the middle basin. The biogenic components are present in small quantities in biogenic components of the Kuda River and its tributaries. The maximum concentrations of NO2-, NH4+ and Ptotal are determined for the river midstream and of NO3- in its upstream. The lowest concentrations of microelements were detected in the water of the upper part, but Ti, Cu, Zn, Ba, and Pb, for which the average concentrations at that site are higher than at the other ones. The highest concentrations of Li, B, Al, Fe, As, Sr, Mo, and U were determined in the middle, and Mn in the lower part of the Kuda River. The spatial dynamics of major ion concentrations is primarily impacted by geological factors, and it agrees well with the change of geological rocks in the river basin. The cluster analysis was employed to reveal the relationship between surface and ground water in different parts of the watershed. The climate factor effect is reflected in seasonal variations of hydrochemical parameters in the Kuda River water. A long-term frost period causes freezing at some sites on the riverbed. Thus, this is the reason for a poor expression or lack of winter maximum and spring minimum concentrations of some components arriving into the river water through ground feeding. The surface water composition is noticeably transformed in the middle part of the basin due to saline soils and salt marshes bearing SO42-, Ca-2(+), Mg-2(+), Ba, and U. In the studied area, the effluents of industrial and domestic use at the settlements, as well as agricultural enterprises, provide the source of biogenic pollution of surface waters. The drop in pH value and increase in major ion concentrations, primarily sulfate ions, over a long period of time are triggered by the impact of contaminated atmospheric precipitation and the enlargement of arable ploughed lands within the river basin.