In May 2019, experts from more than ten national laboratories conducted a large-scale monitoring of the river’s ecological status as part of the fourth Joint Danube Survey (JDS4). More than 2,800 km of the river were surveyed — from its crystal-clear headwaters in the Black Forest to its wide delta branches near the Black Sea. As part of the study, more than 500 samples of water, bottom sediments and biota were collected at 51 locations, covering the urban areas of Bratislava, Budapest and Belgrade, as well as sparsely populated areas of the delta near Kilia and Vylkove in Ukraine. Despite generally good results in terms of organic pollution, the project participants encountered previously unknown challenges: microplastics, bacteria with antibiotic resistance genes, and traces of nearly 100 psychoactive substances.
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Purpose and scope of the study
The first waves of Joint Danube Surveys took place in 2001, 2007 and 2013, and each subsequent survey made it possible to assess the dynamics of changes in the aquatic environment and the effectiveness of policies provided for in the EU Water Framework Directive. JDS4 had several key objectives. First, to identify geographical variations in water quality indicators based on physical, chemical and biological indicators. Second, to assess the degree of organic pollution, in particular through the analysis of biochemical oxygen demand (BOD5). Third, to introduce modern methods for detecting plastic, antibiotic resistance and traces of drugs and narcotics. Given the diversity of sources of pollution — industry, agriculture, urban runoff and shipping — scientists sought to create a comprehensive picture of the risks that could pose a threat to the ecosystem and human health.
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Water purity and swimming safety
Organic pollution was assessed using the BOD5 indicator, which measures the amount of oxygen required by microorganisms to decompose dissolved organic matter over a period of five days. In the upper Danube, where it is fed by crystal-clear mountain tributaries, 91% of samples met the highest “excellent” standard, indicating negligible organic impact. In the middle reaches, where the Danube crosses large urban economic centres and industrial areas, 80% of samples showed a similar level, while in the lower delta, the values dropped to 67%, as the effluents of many systems converge here. Special attention was paid to the microbiological analysis of Escherichia coli and enterococci, which are markers of faecal contamination. At most points, the level of these bacteria did not exceed the norms, and isolated spikes in July and August were recorded in places of mass recreation near city beaches.
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Microplastics in water
FTIR spectroscopy was used to detect microplastics, which allows the chemical composition of plastic to be identified based on its infrared spectrum. Microplastics ranging in size from 0.5 to 5 mm were found in all samples. The concentration varied from 0.2 to 3 particles per litre, with the highest accumulation observed near the treatment plants of large cities — Budapest, Belgrade and Vienna. These particles originate from both domestic and industrial wastewater, as well as from the degradation of larger plastic products. The researchers noted that microplastics can adsorb other pollutants, such as metals or organic toxins, creating complex compounds. In addition, small fragments can accumulate in the tissues of fish, molluscs and aquatic invertebrates, and then enter living organisms, including humans, through the food chain.
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Antibiotic-resistant bacteria
In 30% of samples, genes resistant to fundamental antibiotics, in particular penicillins and tetracyclines, were detected. Genetic analysis using qPCR showed that resistance markers are more common in areas near hospitals and livestock farms, where large doses of drugs are used. This indicates that drug residues are entering the river through wastewater. Resistant bacteria in natural conditions can transfer these genes to other microorganisms, increasing the risk of new strains emerging that are resistant to treatment. This reservoir of resistance makes the Danube a potential source of danger, as river water supplies drinking water to millions of people and is used in agriculture.
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Psychoactive substances
LC-MS/MS analytical methods detected 87 compounds associated with medicinal and narcotic drugs in the samples. Among the most common were metabolites of cocaine, amphetamines, barbiturates and some psychedelics. Although the overall concentrations of these substances (up to 0.2 μg/L) do not pose a direct threat to the health of adults when swimming or even consuming fish in small quantities, their presence indicates that modern treatment facilities are not capable of completely removing fine organic molecules. This poses risks to aquatic and terrestrial life, as the accumulation of such compounds can affect the development of aquatic organisms and biochemical processes in water bodies.
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Fish diversity and the threat to sturgeon
A modern approach using environmental DNA (eDNA) analysis has made it possible to identify traces of genetic material from 72 fish species in the water, including traditional species such as bream, gudgeon and minnow, as well as invasive species such as American bullhead and spined loach. These results indicate a change in the species structure of the biota and the possible pressure of invasive species on the local fauna. Sturgeon species — sterlet and sevruga — were recorded only by DNA traces, and actual individuals were extremely rare. This picture indicates a critically low density of these rare species, which require special conservation programmes and the restoration of natural migration routes.
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Prospects and recommendations
The results of JDS4 showed that important measures to treat wastewater and protect the environment are bearing fruit, as organic pollution has decreased compared to previous surveys. At the same time, the detection of microplastics, antibiotic-resistant genes and traces of psychoactive substances indicates the need to strengthen monitoring of new pollutants. Experts recommend modernising treatment facilities across the basin, introducing protocols for regular monitoring of microplastics and antibiotic resistance, and expanding programmes to restore natural floodplains and river corridors. A key task is to strengthen international coordination within the framework of the EU Water Framework Directive and Black Sea Basin programmes in order to ensure the long-term environmental safety and sustainability of the Danube system.
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