Oxygen consuming substances in European rivers (Indicator)

Key messages: Biochemical oxygen demand (BOD) levels in European rivers halved between 1992 and 2022. However, levels have remained steady at around 2.7mg O₂/l since 2010. Ammonium concentrations fell to 20% of the 1992 level. After 2014, the level has stabilised around 110µg NH₄-N/l. The decrease in BOD and ammonium concentrations is due to improvements in wastewater treatment.

Biochemical oxygen demand and ammonium in European rivers

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Organic pollution of rivers from municipal and industrial wastewater negatively affects aquatic ecosystems, causing oxygen loss and changes in species composition (i.e. deterioration of ecological status). Severe organic pollution may lead to rapid deoxygenation of river water, high concentrations of hazardous ammonia, and the disappearance of fish and aquatic invertebrates.

The amount of oxygen and ammonium in water can tell us a lot about how much it is polluted with organic matter. The parameter ‘biochemical oxygen demand’ (BOD) identifies how much oxygen biological organisms need to break down the organic matter in the water. If the amount of biologically degradable organic matter increases, BOD goes up.

Ammonium levels also go up when there is more organic material that can be broken down by biological organisms. Therefore, levels of BOD and ammonia in river water are informative parameters about the health of rivers.

In European rivers, BOD levels have generally been decreasing between 1992 and 2022. A notable decrease is evident at 48% of the river sites, with an additional 3% of rivers showing a small decline. A significantly increasing BOD trend is recorded at 10% of the sites. The shorter, more representative time series of 2000–2022 closely follows the longer one.

Annual ammonium concentrations decreased by 11.1µg/l per year (-2.2%) on average over the period 1992-2022. Significantly decreasing concentrations were observed at 71% of the sites, with an additional 5% of the sites showing a marginal decrease. No change has been observed at 22% of the river monitoring sites. The shorter, more representative time series of 2000–2022 shows higher concentrations, but a similar trend of overall decrease.

Please consult the relevant indicators and signals below for a more comprehensive overview on the topic.

The key indicators for the oxygenation of water bodies are BOD and ammonium. BOD is most commonly expressed in milligrams of oxygen consumed per litre of sample during five days of incubation at 20°C. This is the amount of oxygen needed by microorganisms for aerobic decomposition of the organic matter in the water. Ammonium, when oxygenated by bacteria to nitrate, is toxic to aquatic life at certain concentrations. This depends on the water temperature, salinity and pH.

The indicator illustrates spatial variations in the current state, and temporal trends of BOD and ammonium concentration in rivers.

Annual mean concentrations are used as a basis in the indicator analyses. The EEA aggregates to annual mean concentrations unless a country has reported aggregated data only.

Automatic quality control procedures are applied to both the disaggregated and the aggregated data, and data failing certain tests are excluded from further analysis. In addition, a semi-manual procedure is applied, focusing on suspicious values that have a major impact on the country’s time series and the most recently reported data.

For time series analyses, only complete series after inter/extrapolation are used. This ensures that the aggregated time series are consistent, i.e. the same sites are included throughout. Inter/extrapolation of gaps up to three years is allowed to increase the number of available time series. The selected time series are aggregated to country and European level by averaging across all sites for each year.

Trends were analysed using the Mann-Kendall method (Jassby et al., 2017) in the free software R (R Core Team, 2020) with the wql package. The size of the change is estimated by calculating the Sen slope (Theil et al., 1992; Sen, 1968). Absolute and relative Sen slopes are summarised across Europe and individual countries by averaging. For the trend analysis, the same time series are used as for the original time series analysis, but without gap filling. Calculations of the mean concentration over the last 3 years are used for analysis of the current state.

More information is available on the EEA indicator page (EEA, 2024). 

References and footnotes

Jassby, A. D., et al., 2017, 'Exploring water quality monitoring data' (https://cran.rproject.org/web/packages/wql/vignettes/wql-package.html) accessed March 25, 2024.
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R Core Team, 2020, 'R: The R Project for Statistical Computing' (https://www.r-project.org/index.html) accessed March 25, 2024.
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Theil, H., 1992, 'A Rank-Invariant Method of Linear and Polynomial Regression Analysis', in: Raj, B. and Koerts, J. (eds), Henri Theil’s Contributions to Economics and Econometrics: Econometric Theory and Methodology, Springer Netherlands, Dordrecht, pp. 345–381.
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Sen, P. K., 1968, 'Estimates of the Regression Coefficient Based on Kendall’s Tau', Journal of the American Statistical Association 63(324), pp. 1379-1389 (https://www.tandfonline.com/doi/abs/10.1080/01621459.1968.10480934) accessed 25 March 25 2024.
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EEA, 2024, European Environment Agency, Oxygen consuming substances in European rivers Online (Oxygen consuming substances in European rivers | European Environment Agency's home page), Accessed 08 January 2025
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EC, 2024, ‘Urban Wastewater’ (https://environment.ec.europa.eu/topics/water/urban-wastewater_en), accessed 5 August 2024.
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Oxygen consuming substances in European rivers (Indicator)

Biochemical oxygen demand and ammonium in European rivers

Zero Pollution Action Plan 2030 target or policy objectives

Other relevant indicators and signals

References and footnotes