EVALUATION OF CHEMICAL INDICATORS OF ANTHROPOGENIC INFLUENCE IN THE LOWER DANUBE BASIN

Abstract

The article explores the nature and trends of hydrochemical parameter connection and changes in the Lower Danube basin. The research examines nitrogen and phosphorus compounds, dissolved oxygen, and biochemical oxygen demand as indicators of anthropogenic influence and surface water quality alterations. The study was conducted between 2015 and 2023 at three sampling points: the Reni and Vilkovo water intakes on the Danube River and the Yalpuh Reservoir, which is connected to the Danube. The study employed mathematical statistics approaches. The research established changes in annual median values, investigated correlation links between chemical components by Spearman correlation, and implemented factor analysis by minimizing residuals. A spatiotemporal analysis of chemical components indicative of anthropogenic impact on the Lower Danube surface water revealed notable changes in the basin during 2019-2021. These changes were manifested as a sharp increase in ammonium concentrations and phosphate growth since 2020-2021, an abnormal decrease in dissolved oxygen concentration in 2019, a decrease in BOD within Yalpuh to the Danube BOD level, and a shift in trends from decreasing to increasing for nitrites. The water quality class also changed. Currently, the Danube River water is of the third quality class for nitrates, phosphates, and ammonium. In the Yalpuh Reservoir, it is of the 2nd class for nitrogen compounds and the 3rd class for phosphates. Two principal groups of chemical components are identified based on their underlying causes and sources, as revealed by correlation and factor analysis. A consistent relationship is observed between ammonium and phosphates across all monitoring sites, suggesting their role as pollutants entering surface waters via wastewater discharge and agricultural runoff and representing the main anthropogenic impact factor. The Danube River displays a specific relationship between nitrates and dissolved oxygen, reflecting the seasonal fluctuations in their concentrations and their underlying causes of changes, which stem from both anthropogenic and natural processes.