Restored floodplains could remove 38.000 tons of nitrate pollution in the Danube river basin

The Danube river basin, flowing through 10 countries in central and eastern Europe, has long been affected by elevated nitrate (NO₃) levels, from agricultural run-off and other sources, causing eutrophication. Though levels are improving, nutrient pollution means that more than a fifth of surface-water bodies in the Danube river basin are at risk of failing good ecological status by 2027, as required by the Water Framework Directive. Floodplains offer a vital ecosystem service in permanently removing some of this nitrate (the main nitrogen compound) pollution through denitrification. However, 70–80% of the river basin’s floodplains have been converted to arable land, or disconnected by dykes, engineering works for navigation and hydropower dams.

Restoration of floodplains has the potential to increase the capacity of this ecosystem service, and is considered in the 2021 management plan for the Danube river basin¹. Yet, while some studies have measured rates of denitrification in alluvial soils (soils deposited by surface water in rivers, floodplains and deltas) in specific locations, large-scale estimations are lacking. Researchers, therefore, sought to estimate the quantity of nitrates that are currently removed by large Danube floodplains, and by how much restoration measures could increase this. First, they looked at nutrient emissions and in-stream retention in six river systems, then coupled this data to models of denitrification and flooding.

Nitrogen emissions in the Danube river basin are currently estimated at around 500 000 tons per year, with 44% deriving from agriculture, 30% from urban areas and 23% from forests and natural areas. About 340 000 tons enter the Black Sea, into which the Danube drains. The river basin contains 7 845 km² of active floodplains (inundated at least every 100 years), and 1 440 km² which could be reactivated by hydromorphological river restorations (which consider the physical character and water content of water bodies), amending dykes, or changing land use. In this study, the researchers concentrated on six sections of the Danube and its tributaries – Sava, Tisza and Yantra – comprising 3 842 km² of active and 1 298 km² of potential floodplains larger than 500 hectares (ha).

They investigated nitrogen emissions from both diffuse and point sources (single, identifiable sources) in the whole river basin using a model of nutrient fluxes in river systems (MONERIS)², considering relevant physical and chemical parameters.

Average duration of floods³, water nitrate concentrations (derived from MONERIS) and soil properties were used in another model to quantify denitrification in floodplains – a key process in which nitrogen is removed from these plains. They also estimated the removal of nitrates in stream ecosystems as a function of nutrient and hydraulic loads, and water temperature.

Most of the basin’s active large floodplains are only rarely inundated, the researchers found, but, overall, they currently remove about 33 200 tonnes of nitrates from the river system each year (6.5% of total nitrogen emissions, dominated by nitrate), through a combination of in-stream processes and denitrification in soils.

Due to high nitrate inputs, the lower Danube floodplains (in Romania and Bulgaria) currently remove the greatest absolute quantity of nitrate. As a fraction of emissions, meanwhile, the Yantra and Tisza floodplains (chiefly in Bulgaria/Romania and Hungary, respectively) currently have the potential to remove the largest share of nitrate. This ability is promoted by flat terrain, extensive area and soil with high organic carbon and fine material; this means that floodplain restoration here would be likely to have the greatest impact on water quality.   

Overall, reconnecting floodplain water bodies (e.g. oxbow lakes – located in an abandoned curve or bend of a meandering river channel) could increase in-stream removal by 9.2%, removing about 2 350 tons more nitrates per year. The greatest effect is expected from restoring the previously meandering Yantra and Tisza Rivers and the upper Danube which was formerly braided (i.e. a network of river channels separated by small, often temporary, islands). Reconnecting potential floodplains (e.g. by removing dykes) could remove an approximate additional 2 500 tons a year, representing a rise in potential denitrification of 32%. This would be especially notable in the Yantra region and the middle Danube. If both water bodies and floodplains were reconnected, a total of 38 000 tons of nitrate could be retained by river-floodplain systems, per year.

The researchers acknowledge that their estimates are subject to uncertainties, especially as modelling relied on wide-ranging input data. Nevertheless, the results agree with those from other similar studies⁴, and they conclude that increasing connectivity between water bodies, and expansive, frequently inundated areas in the Danube river basin could make a useful contribution to improving water quality, particularly in the river’s upper sub-basins. However, the researchers also state that reconnection measures will not be able to compensate for all excessive nitrogen emissions in the river basin but rather complement the efforts in agriculture⁵ and waste-water treatment. They suggest that improving understanding of water-quality functions in the region could further aid strategic and integrated planning to achieve environmental targets in the European Green Deal, Water Framework, Habitats and Floods Directives.

Further Information

Denitrification reduces nitrates and forms nitrogen (N₂) gas which returns to the atmosphere and is not harmful to the climate or humans.

Footnotes:

1. International Commission for the Protection of the Danube River (ICPDR), 2021. Danube River Basin Management Plan 2021. IC–231, International Commission for the Protection of the Danube River, Vienna, Austria. Available from: https://www.icpdr.org/main/publications/danube-river-basin-management-plan-drbmp-update-2021
2. Venohr, M., Hirt, U., Hofmann, J., Opitz, D., Gericke, A., Wetzig, A., Natho, S., Neumann, F., Hürdler, J., Matranga, M. and Mahnkopf, J. (2011) Modelling of nutrient emissions in river systems–MONERIS–methods and background. International Review of Hydrobiology, 96(5): 435–483.
3. From Schleuter (2016) using satellite-based data provided by Copernicus, the European Union's Earth observation programme:

Schleuter, M. (2016) Calculation of flood duration in floodplains by means of a universally applicable formula. Available from: https://asnevents.s3.amazonaws.com/Abstrakt-FullPaper/25266/56778058d89c3-1046-564e91e004326-schleuter-25266_REV2overREV1-RE-WORK5.pdf 20 January 2022.

4. For example, Natho et al. (2020) found significant increases in nitrate retention when reconnecting large side arms of the upper Danube river: Natho, S., et al. (2020) Modeling the effect of enhanced lateral connectivity on nutrient retention capacity in large river floodplains: how much connected floodplain do we need? Front. Environ. Sci. 8: 74.
5. International Commission for the Protection of the Danube River (ICPDR) (2021). Guidance Document on Sustainable Agriculture in the Danube River Basin (No. IC 244). ICPDR, Vienna. Available from: https://www.icpdr.org/main/icpdr-publishes-guidance-document-and-policy-paper-sustainable-agriculture [Accessed 8 December 2022].

Source:

Tschikof, M., Gericke, A., Venohr, M., Weigelhofer, G., Bondar-Kunze, E., Kaden, U.S. and Hein, T. (2022) The potential of large floodplains to remove nitrate in river basins–The Danube case. Science of the Total Environment, 843: 156879. Available from: https://doi.org/10.1016/j.scitotenv.2022.156879

To cite this article/service:

“Science for Environment Policy”: European Commission DG Environment News Alert Service, edited by the Science Communication Unit, The University of the West of England, Bristol.

Notes on content:

The contents and views included in Science for Environment Policy are based on independent, peer reviewed research and do not necessarily reflect the position of the European Commission. Please note that this article is a summary of only one study. Other studies may come to other conclusions.