Hundreds of millions of people could be living in areas where poor-quality groundwater threatens their health by the year 2100 as a result of rising temperatures.

"This means that the water there cannot be drunk directly without hesitation, but must be boiled," explains Susanne Benz from the Institute of Photogrammetry and Remote Sensing at Germany's Karlsruhe Institute of Technology (KIT) in a press release.

"Depending on the climate scenario, up to several hundred million people will be affected by 2100."

The temperature of the groundwater plays a decisive role in water quality and researchers say this can influence the amount harmful substances such as arsenic and manganese.

"These increased concentrations can have a negative impact on human health, especially if the groundwater is used as a source of drinking water," says Benz.

Pathogens such as legionella could also spread, and rising water temperatures also have an impact on biodiversity. Fish species such as salmon use spawning grounds in rivers that are fed by groundwater.

The researchers say little was previously known about how the warming of the Earth's surface as a result of climate change affects groundwater.

Benz's team has now predicted changes in groundwater temperature worldwide up to the year 2100. The results were published in June in the scientific journal Nature Geoscience.

The study analyses multiple scenarios of greenhouse gas development. In one analysed scenario, the temperature of the groundwater rises by 2.1 degrees, in an extreme scenario by 3.5 degrees.

In these cases, 77 to 188 million people or 59 to 588 million people could live in areas where the groundwater exceeds the highest drinking water temperature limit set by a country.

"The strong fluctuations are related to the spatial variability of climate change and population development," the technology institute says. The lowest warming rates are expected for mountains with a low-lying groundwater table, such as the Andes or Rocky Mountains.

"Even today, around 30 million people live in areas where the groundwater is warmer than the strictest guidelines for drinking water specify," Benz says. "Our results show how important it is to take measures to protect groundwater resources and find sustainable solutions to counteract the negative effects of climate change on groundwater."