SCALE AND LONG-TERM DYNAMICS OF OKA RIVER BASIN POLLUTION

Abstract

Introduction. This article addresses the long-term dynamics of Oka River basin pollution. The basin serves as the main source of drinking water and a receiver of wastewater from a number of regions in European Russia. Methods. We assessed the water quality by 12 main hydrochemical indicators and constructed maps of their distribution with a breakdown into two periods (1990–1999 and 2000–2017). The anthropogenic load along the section in the city of Gorbatov was determined. Spearman’s rank correlation coefficients and their statistical significance were calculated. Results. For 18 gauging stations with 25 and more years of observations, the magnitude of the linear trend (%/year) was estimated using the Theil–Sen estimator, and the statistical significance of the linear trend (Mann–Kendall test) was assessed for individual stations and the entire basin, using a modified Walker test. The runoff of pollutants from the urban territory was estimated between the sections upstream and downstream the cities in the upper reaches of the Oka River basin. The volumes of pollutants in the Oka River from the cities of Orel, Belev and Kaluga were determined for the period of 1990–2017. The calculations of the pollutant runoff, performed between the sections upstream and downstream the cities, made it possible to determine the role of the cities in the formation of point pollution in the upper reaches of the Oka River. The anthropogenic load along the length of the river in terms of the influx of chemicals varies from “low” to “high”. The load is largely due to the intake of pollutants since water bodies and watercourses serve as receivers of both treated and insufficiently treated wastewater from various enterprises. Conclusion. Almost throughout the basin, the water quality is under stress. The statistical analysis showed the existing relationship between a certain type of land use and the concentration of substances in surface waters. It was revealed that the self-cleaning capacity of the river is sufficient to prevent pollutants from accumulating along it.