Changes in the Macroelemental Composition of River Waters in Contrast Geological Conditions, Sochi River of the Black Sea Coast of Russia

Abstract

The purpose is to study the main natural factors and patterns determining the formation of the chemical composition of surface waters in highly dissected mountainous, coastal-marine territories with contrasting geological structure, and to determine the diagnostic sign of the geological basement rocks in which the river valley is formed. Materials and methods. Seven key sites have been selected in the Sochi River valley of the Black Sea coast of Russia, characterizing different rocks and anthropogenic load. A three-time selection of river waters was carried out in the spring, summer and autumn periods of the year. The cationic-anionic composition of waters has been determined by traditional hydrochemical methods and their main types characteristic of different seasons and sections of the river valley have been established. Results and discussion. The following types of waters are distinguished: sulfate-bicarbonate-chloride calcium-magnesium-sodium, bicarbonate-sulfate calcium and bicarbonate-sulfate calcium-magnesium. Regular seasonal changes in the hydrogeochemical composition of surface waters have been revealed, which are expressed in a general increase in the concentrations of chemical elements in water during the long summer low water period. The stable geochemical indicator has been established for river waters during the change of rocks. The Ca/Mg ratio changes approximately three times in the direction of increase when neutral mudstones are replaced by subalkaline carbonate rocks. This indicator can be a diagnostic feature of the rocks of the geological base in which the river valley is formed, as well as an indicator that determines the type of river feeding. Conclusion. At Ca/Mg values close to 10 units, when the river is predominantly fed by groundwater, there is a high probability that the increased content of elements in the river waters is due to natural causes associated with the geochemical richness of rocks, and not with the technogenic factor.