A paper in science journal Nature predicts that the number of people lacking clean drinking water will grow by 3 billion by 2050.
German and Dutch researchers studied river basins in southern China, central Europe, North America, and Africa.
They warned they will become “water scarcity hotspots” because of high nitrogen levels caused by fertiliser run-off.
Where we used to worry about the quantity of water available, we should now address its quality, they said.
“For example, agricultural intensification and urbanisation have added excessive pollutants such as nutrients, pathogens, plastics and other chemicals to the water bodies,” they wrote.
Nitrogen run-off causes dead zones where lack of oxygen drives out all aquatic life.
It also causes algal “blooms” in surface water, which produce toxins that contaminate drinking water and cause health issues in humans, such as liver damage, diarrhoea and neurological problems.
In 2010, a quarter of small river basins around the world faced a severe scarcity of clean water.
The researchers now project that, by 2050, an extra 40 million sq km of basin areas may find clean water scarce, affecting 3 billion more people.
The South China Morning Post notes that another peer-reviewed study, published in 2016 by scientists in the Netherlands, found that 4 billion people – half of them in China and India – experience severe water scarcity at least one month each year.
Worldwide, 500 million people face severe water scarcity all year round.
The paper can be downloaded here.