Popular Science: Most of the nitrogen in the Elbe comes from wastewater, based on isotopic composition
Although the overall quality of water in surface streams monitored since the 1960s has improved significantly, contamination with nitrogen compounds (especially nitrates) remains high. Researchers have analysed the monthly data of nitrate concentrations and river flow at 138 CHMI monitoring stations in 2000-2008. They also analysed water samples collected in 2008-2009 for nitrogen isotopes at 63 selected locations on rivers in the Elbe river basin: Berounka, Ohře, Vltava and the Elbe upstream of the confluence with the River Vltava. The aim of the study was to find out the origin of the nitrates.
Natural nitrogen is composed of two stable isotopes, 14N and 15N, and due to isotope fractionation, these isotopes occur in different proportions. For example, in the nitrogen cycle food chain, lighter isotopes (14N) are excreted while heavier ones (15N) remain in the bodies of organisms, leading to more heavy isotopes being excreted by animals positioned higher in the food chain. Therefore, the 15N/14N nitrogen isotope ratio in surface waters can be used to determine the isotopes’ origin. This ratio is given relative to the ratio in atmospheric nitrogen, known as standard, and is denoted as δ15N.
Isotopic information thus reflects different types of land use: δ15N below 3.5‰ indicates the dominant use of commercial synthetic fertilisers, 3.5-9‰ is typical for mineral soil nitrogen and δ15N above 9‰ represents the origin of nitrogen in animal and human faeces.
If we calculate the values per area and population, the nitrate load in the Elbe river basin is similar to other river basins in Europe. For the whole Upper Elbe basin, it is about 706 kg/km2/year, so the annual dose of nitrates per capita is about 5.7 kg. Approximately 76% of nitrates in the Elbe basin have the isotopic composition corresponding to animal and human origin, and the remaining 24% suggest their origin in mineral fertilisers and soil resources.
The results of the study also showed that the three sub-basins reviewed (Berounka, Ohře and Vltava) have a similar nitrate load, while the Elbe basin upstream of the confluence with the Vltava river has, in terms of area, a much higher load. Per capita, it is 8.2 kg for the Elbe, which is significantly more than in the Berounka (5.5 kg), the Vltava (4.4 kg) and the Ohře (4 kg). The origin of the pollution in the Elbe river basin upstream of the confluence with the Vltava is also different: 84% comes from human and animal sources, while in the Ohře and Berounka river basins it is only 52% and 64%, respectively. According to this one-off sampling, only a small part of the pollution comes from commercial fertilisers: in the Elbe upstream of the confluence with the Vltava it is therefore 16%, in the Vltava 25%, in the Berounka 36%, and in the Ohře it is as high as 48%.
However, due to the methodology used, the results are debatable. A large portion of the water in the rivers reviewed comes from groundwater where the concentration of nitrates is significantly lower. Mr Bůzek from the Czech Geological Survey who studies nitrates in water monitored the outflow of streams in the Elbe and the Jizera River throughout the year. He found that, during spring, higher outflows bring more nitrogen from the surface than they do in winter or autumn. Therefore, the proportion of nitrogen from animal and human waste presented in the study can be overestimated.
Nevertheless, the described variability of the isotopic composition of nitrates reflects both the different land use and the availability of wastewater treatment plants in the respective river basins. In 2009, only 30% of wastewater treatment plants removed nitrogen compounds – mainly in larger municipalities. The size of the settlement thus affected nitrate contamination, so much so that wastewater from human settlements was one of the main sources of nitrates in the entire Elbe basin. Furthermore although the number of wastewater treatment plants has increased since 2009, still only a small part of them is able to remove nitrogen and phosphorus.
Mayo, A. L., Ritter, D. J., Bruthans, J., & Tingey, D. (2019). Contributions of commercial fertiliser, mineralised soil nitrate, and animal and human waste to the nitrate load in the Upper Elbe River Basin, Czech Republic. HydroResearch, 1, 25-35.