Danube river water quality trends: A qualitative review based on the open access web of science database

Abstract

The Danube is the longest river in the European Union and Europe's second longest river after the Volga. With a total area of 801,463 km2 and covering 10% of Continental Europe, the Danube River Basin features the highest degree of cultural diversity in the world and plays a major role in the continent's socioeconomic, political and cultural life. This review aims to assess the evolution of Danube water quality (WQ) over the past decades. By using keywords, a search protocol and filters on the Web of Science platform, a scientific bank of 41 relevant studies was obtained out of the 124 open access articles that were initially available. These papers were closely reviewed. The review's main conclusions are the following: since the beginning of the 1970s, the Danube water regime has changed because of the construction of water reservoirs, dams and hydropower plants. Between 1960 and 1990, nitrogen discharge into the Danube basin has increased about fivefold, whereas phosphate has doubled due to the increase in anthropogenic inputs. Most recently, reductions of nutrient loads were observed, which is linked to political, economic and WQ management changes. Even if at nowadays Danube WQ has an upward trend, the river still shows signs of degradation (mainly organic pollution) downstream of major cities and in some important tributaries. Also, certain specific areas, polluted with substances listed as a priority in the European Water Framework Directive, as well as with newly emerging contaminants, have been identified, which is a matter that requires urgent measures.