Elemental Composition of the Sochi River Waters on the Black Sea Coast of Russia During Flood and Low-Water Periods

The elemental composition of the Sochi River waters undergoes sharp changes with shifts in hydrological phases. During floods, it is relatively rich, while during low-water periods, it is comparatively poor. In flood conditions, the river waters are enriched with 19 chemical elements relative to average continental river water concentrations (Clarke values), whereas only 7 elements show enrichment during low-water periods. However, the total concentrations of excess chemical elements are higher during low-water periods due to the presence of Ca and Mg, whose absolute concentrations are two orders of magnitude higher than those of the dominant flood macroelements (Fe and Al). This feature affects the overall water mineralization. During low-water periods, the average total mineralization of river water (109 mg/L) is 1.4 times higher than during floods (80 mg/L). During floods, the river waters become significantly enriched with rare earth elements (REEs), as well as Sn, Fe, Al, Mn, Zr, Cs, Pb, Th, Be, Ti, Co, Cu, Tl, V, Bi, and As. The active influx of these elements is attributed to abundant soil runoff and their ability to form organic and inorganic complexes, bind strongly with iron colloids, and adsorb onto soil colloid surfaces. In contrast, during the drier low-water period, when groundwater contributes more significantly to river flow, the concentrations of most elements, including REEs, decrease. Under the contrasting geological conditions of the studied area — where neutral argillites repeatedly alternate with alkaline marls — the low-water period is more influenced by geochemically depleted carbonate rocks. A comparative analysis of water composition during floods and low-water periods revealed a second group of elements — Ba, Sb, B, S, Na, U, Mo, K, Sr, Ca, Mg, Li, Re, Rb, Si, and Zn—whose influx into the river is associated with groundwater input and, consequently, the specific lithology. Studying variations in the elemental composition of the Sochi River waters across different hydrological phases allowed us to identify element groups whose elevated concentrations in a given phase are linked not with anthropogenic load but with the composition of the bedrock (in low-water periods) or intense soil runoff (in floods).