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In European rivers, oxygen consuming substances decreased over the period 1992 to 2020. Biochemical oxygen demand (BOD) fell to half of the 1992 level but remained steady at around 2.5mg O2/l since 2010. Ammonium concentrations fell to 23% of the 1992 level. After 2014, the level stabilised at around 100µg N/l. The decrease in BOD and ammonium concentrations is a consequence of the improvement in wastewater treatment. The 1990s economic crisis in central and eastern European countries also resulted in decreasing pollution from manufacturing industries.
Organic pollution of rivers from wastewater, both municipal and industrial, as well as diffuse runoff from agriculture, negatively affect aquatic ecosystems, causing loss of oxygen and changes in species composition (i.e. deterioration of ecological status). Severe organic pollution may lead to the rapid de-oxygenation of river water, high concentration of hazardous ammonia and disappearance of fish and aquatic invertebrates. In addition, it can have negative effects on the use of the water for human purposes such as drinking, bathing and recreation. Without treatment, organic pollution is slowly diluted and degraded naturally along the river course. Biochemical oxygen demand (BOD) and ammonium are key indicators of organic pollution in water. BOD is the amount of dissolved oxygen needed by aerobic biological organisms to break down organic matter present in a given water sample at a certain temperature over a specific time period. The BOD value is most commonly expressed in milligrams of oxygen consumed per litre of sample during 5 days of incubation at 20°C. Ammonium is toxic to aquatic life at certain concentrations in relation to water temperature, salinity and pH. BOD and ammonium increase with higher loads of biologically degradable organic matter.
Key sources of organic pollution are municipal wastewater; industrial wastewater, especially from paper or food processing industries, and agricultural emissions, specifically from surface runoff of silage, manure and slurry from intensive livestock farms.
In European rivers, BOD levels have generally been decreasing between 1992 and 2020 (Figure 1a), with an average annual decrease in BOD of 0.06mg/l (0.7% per year). The BOD reached its lowest level (2.4mg/l) in 2010 and has only surpassed 3.0mg/l once since then, in 2015. A significant decrease is evident at 44% of the river sites, with an additional 7% of the rivers having a marginally decreasing trend. A significantly increasing BOD trend is recorded at 12% of the sites.
Annual ammonium concentrations decreased by 10.93µg/l per year (-2.6% per year) on average over the period 1992-2020 (Figure 1b). Significantly decreasing concentrations were observed at 74% of the sites, with an additional 4% of the sites showing a marginal decrease. No change has been observed at 20% of the river monitoring sites. A significant increase was evident at 1% of the sites.
The current mean concentration of BOD for the period 2018-2020 is 3.1mg O2/l for 24 European countries (10,776 sites). Almost two thirds of the river monitoring sites have less than 3 mg/L, which is also the recommended water quality for salmonid fish according to the Fish Directive 2006/44/EC and recommended for water intended for the abstraction of drinking water (according to the Directive concerning the quality required of surface water intended for the abstraction of drinking water 75/440/EEC).
Countries with the highest share of river sites with less than 1.4mg/l are Slovenia (100%), Ireland (91%), and Cyprus (74%). The share of monitored river sites with BOD equal to or higher than 3mg/L is particularly high (more than 50%) in Albania, Kosovo under UNSCR 1244/99, and Portugal. High BOD concentration is observed in agriculturally and industrially developed lowlands of Europe, such as the Po valley and lower BOD in the highlands of Europe such as Scotland, the Alps, and the Dinaric Alps.