River salinisation has intensified across much of Europe over the past 50 years, with salt levels more than doubling above estimated natural background conditions in rivers across 28 countries.
River salinisation has more than doubled across much of Europe
Over the past five decades, river salinisation has increased sharply across Europe, with rivers in 28 countries recording salt levels more than twice their estimated natural background conditions in some cases. The findings come from the first pan-European assessment of its kind, carried out by an international team of scientists and recently published in Science.
The first pan-European assessment of freshwater salinisation
The research, which also involved the European Commission’s Joint Research Centre (JRC), examined rivers and streams across 35 countries. Researchers assessed concentrations of sodium (Na+), potassium (K+), magnesium (Mg2+), calcium (Ca2+), chloride (Cl-) and sulphate (SO42-) ions in the water, looking for increases above the geogenic background in order to isolate the direct impact of human activities.
The results are striking. Of the 35 European countries examined, 28 have rivers where salinity has more than doubled compared with natural reference levels. In some areas, increases have reached extreme levels, measured through changes in electrical conductivity.
River salinisation versus natural salinity
Measuring the progression of salinisation, rather than simply the absolute salinity of a river, was necessary for two main reasons. Freshwater rivers naturally contain very different concentrations of salts depending on local geography. In karst areas, waters tend to contain more calcium because of the erosion of limestone, while inland watercourses can naturally accumulate sodium and chloride through evaporation.
Another factor is saltwater intrusion, which occurs when denser, heavier seawater moves upstream along the riverbed beneath lighter freshwater.
These physical processes differ, but they have one important feature in common: aquatic ecosystems have evolved to tolerate a particular range of natural salinity. When human pressures or climate-related changes add further salt loads, new hotspots emerge across Europe’s freshwater systems, while areas already considered vulnerable face increasing pressure.
More than 23,000 km of rivers exceed critical levels
According to the study, more than 23,000 km of Europe’s river network, around 2% of the total, exceed the critical threshold of 1 millisiemens per centimetre (mS/cm) for electrical conductivity. This indicates salinity levels well above international standards for protecting aquatic life.
Another 1% of the river network reaches salinity levels that place the water in the “significantly unpleasant” category for drinking-water supply, while 3% exceeds safety limits for irrigation.
Although these percentages may appear relatively small, the rate and extent of river salinisation are particularly concerning. Around 1% of Europe’s river network, approximately 3,600 km, is experiencing extreme salinisation, with electrical conductivity increasing by more than 500% compared with baseline conditions.
A further 9% is undergoing severe salinisation, with increases ranging from 100% to 500%. Another 14% shows moderate increases of between 51% and 100%, while 26% has experienced lower increases of between 11% and 50%.
New river salinisation hotspots are emerging
The research also identified previously unknown river salinisation hotspots. Alongside well-documented problem areas such as the Ebro in Spain, the Meurthe in France, the Werra in Germany and the Oder in Poland, scientists identified new areas of concern in the Bega Veche river basin in Romania, Hungary’s Sió Canal and the Corbones river in Spain.
In Italy, the situation is partly familiar, but no less concerning. Several Alpine watercourses have long been classified among Europe’s severe salinisation hotspots, and Italy is one of nine European countries for which specific data on saltwater intrusion are also available. Some river basins in southern Italy also show high absolute salinisation levels.
Human activity is accelerating river salinisation
Identifying the factors driving the phenomenon is essential. While environmental conditions play a major role in river salinisation across Europe, human activity also has a significant impact, both directly and indirectly.
“Freshwater salinisation across Europe is the result of both natural drivers and human activities,” the JRC explains. “Climate, geology and hydrology influence river salinity, while human impact and land-use changes are strongly associated with increased salinisation, highlighting the importance of managing pressures.”
Beyond the effects of rock weathering, river discharge and soil permeability, agricultural irrigation, mining, urbanisation and even the use of road salt for de-icing all contribute to the problem. In some cases, their impact is expected to increase in the coming years.
Climate change could intensify freshwater salinity
Then there are the effects of climate change. Higher temperatures increase water evaporation, inevitably concentrating salts in rivers and other freshwater bodies.
At the same time, prolonged droughts, combined with the progressive rise in sea levels, are making it easier for seawater to move upstream into rivers and deltas.
One example illustrates the scale of the problem. According to the latest monitoring by Arpae, this summer seawater intrusion in Italy’s Po Delta extended from 24.6 km to 28.1 km inland from the river mouth, completely affecting the Donzella and Tolle branches. Salinity levels there have exceeded thresholds for both drinking-water supply and agricultural use.
