This research covers only a small area of 180 km out of total 1500 km of the Oka River, which however, has a decisive effect on the state of the Oka River all the way to its confluence with the Volga River. Due to the physical and geographical features of this area such as the presence of karst and direct bedding of Devonian and Carboniferous sediments at the surface of the upper river basin cause a high content of iron compounds, manganese, strontium and aluminum. The beginning of the site studied is characterized by an increased COD (Chemical Oxygen demand) value, which gradually decreases to the recommended level downstream, which can also be characterized as a natural feature.
Anthropogenic load becomes truly noticeable after the confluence of the Oka and Moscow rivers. In this area, the content of nutrients increases hundreds of times and exceeds tens of times in MAC, and the main problem is nitrites (more than 40 MAC) and the decrease in dissolved oxygen to dangerously low levels (less than 6 mg / l).
But the most striking example of human exposure is the microplastic content in water. In the Serpukhov region, more than 0.4 particles per cubic meter of river water were found, and below Kolomna, this number rises to 1.1 particles, which is equivalent to more than 10 tons of plastic per year. Furthermore, in areas with a relatively low population density between Kashira and Kolomna, less than 0.2 particles were detected. Apparently, a significant part of the microplastic that enters the river accumulates in the bottom sediments and in the coastal areas, which will become a problem for future generations.
According to the results, that were derived from the analysis of the presence of nutrients, metals, microplastics and hydrochemical indicators, the Moscow River has a major negative impact on the studied section of the Oka River.