Hydrochemical assessment of transboundary rivers (on the example of the Narva border river)

The purpose of the study is to establish the regularities of the formation of the biogenic regime of the Narva River. Based on experimental data on the determination of concentrations of chemical elements for the long-term observation period from 1992 to 2019, a trend of reducing the content of biogenic elements over the past 14 years has been revealed. This is a consequence of reducing the anthropogenic load due to the reconstruction of treatment facilities on the Estonian and Russian sides. Analysis of intra-annual changes in the content of biogenic elements showed a steady increase in the content of nitrogen compounds during high water with maximum water runoff and an increase in phosphorus content in the autumn period. This indicates a significant influence of the waters of Lake Peipus and the Narva reservoir, which determine the intra-reservoir processes that intensify at the end of the warm season. The calculation of runoff based on experimental data of average annual water consumption and monthly determinations of concentrations of mineral, biogenic and organic substances for the period from 1992 to 2019. The calculated values of the runoff indicators are: ionic runoff 43,1 t/km2, total phosphorus 9,8 kg/km2, total nitrogen 142 kg/km2, organic carbon 2,75 t/km2, according to the background values of the runoff indicators of the Baltic region. It is important to note that for the period from 1993 to 2006; significantly higher runoff rates for phosphorus phosphates and nitrogen nitrates corresponded to the literature data on runoff from reclaimed agricultural areas, which also indicates a significant decrease in the biogenic load on the Narva River in the last decade from 2006 to 2019.