Climate change heats up groundwater and affects its quality

The rising temperatures resulting from climate change are not only leading to warmer oceans, but are also heating up soils and groundwater. As a result, by the year 2100, several hundred million people could be living in areas where the groundwater is too warm and therefore no longer of drinking quality, according to German and Austrian researchers in a study published in the journal “Nature Geoscience”.

• You might also be interested in: No shortages of drinking water by 2050 [OÖNplus]

While most of the earth’s fresh water (more than two thirds) is frozen in the polar regions and high mountains, groundwater (around 30 percent) is crucial for life on earth. But global warming is also having a serious impact on the water beneath the earth’s surface.

Two scenarios examined

In the current study, changes in groundwater temperature were forecast worldwide up to the year 2100 for two scenarios of socioeconomic global change. Such scenarios (“Shared Socioeconomic Pathways”, SSP) describe various socioeconomic developments and trends in atmospheric greenhouse gas concentrations in the future. “SSP 245” represents roughly the middle of possible future greenhouse gas developments, while “SSP 585” represents the upper end of this development, i.e. with high emissions and temperatures.

The research team led by Susanne Benz from the Institute of Photogrammetry and Remote Sensing at the Karlsruhe Institute of Technology (KIT), which also included Christian Griebler from the Department of Functional and Evolutionary Ecology at the University of Vienna, produced global temperature maps for groundwater at depths of five and 30 meters below the earth’s surface. “These show that the highest groundwater warming rates worldwide can be expected in places with a shallow groundwater table and/or high atmospheric warming,” said Benz in a press release.

Warming has a serious impact on drinking water supplies

Based on the assumptions of SSP 245, groundwater temperatures will rise by 2.1 degrees Celsius by 2100, and by as much as 3.5 degrees Celsius according to SSP 585. This has serious consequences for the drinking water supply, because groundwater that is too warm cannot be drunk without concern and must be boiled. According to the researchers, around 30 million people are already affected by this today.

The new study now shows that this number can rise dramatically: According to SSP 245, 76 to 188 million people will live in areas where the groundwater exceeds the highest limit for drinking water temperature set by the respective country. According to SSP 585, the number will be as high as 59 to 588 million people. The large range of fluctuations is related to spatial differences in global warming and the depth of the groundwater table. The warming will be lowest in mountainous regions such as the Andes or the Rocky Mountains. This also applies to the Alpine region in Austria, Griebler explained to the APA.

The spatial resolution of the current study is not sufficient to make more detailed statements about regional development in Austria. In this context, Griebler refers to a current report from the Federal Environment Agency. According to this, the development of groundwater temperatures “in large parts of Austria shows an increasing trend at a low level, which is linked to the increase in air temperatures caused by climate change.” Based on ten-year time series, around 73 percent of the measuring points show a significantly increasing or slightly increasing trend in groundwater temperatures.

Far-reaching consequences

The consequences of this are far-reaching. In warmer groundwater, microorganisms are more active and consume more oxygen. This causes the entire system to become unbalanced, emphasized groundwater ecologist Griebler. When there is a lack of oxygen, the microbes switch to anaerobic respiration, and dissolved iron and manganese, hydrogen sulfide or methane are produced. “This drastically affects the quality of the groundwater; it can no longer be used as drinking water or only after very expensive treatment,” said Griebler. Heavy metals and nutrients such as arsenic and phosphorus can also be mobilized from the sediment under oxygen-free conditions. The scientist sees this danger particularly where the oxygen concentrations in the groundwater are already very low, for example under large cities. Vienna is another such example.

The scientists say that land use is an “important factor” in protecting groundwater. In urban areas and under large areas of sealed surfaces, the groundwater temperature is on average at least two degrees Celsius higher than in areas with unsealed soil. “No further soil sealing and unsealing are important measures for protecting groundwater,” says Griebler.