Warming Groundwater Affects Millions – Not Only Humans

There is a hidden world beneath our feet: groundwater. Similar to the depths of the ocean, this ecosystem is vast and not well studied. Nevertheless, it boasts an intricate ecology that affects water quality and thus surface ecosystems and human society. And just like the atmosphere and the oceans, groundwater is warming due to climate change.

Christian Griebler, professor for limnology at the Department of Functional and Evolutionary Ecology at the University of Vienna, studies groundwater and its ecology. He recently contributed to a global model of groundwater temperatures published in Nature Geoscience that projects them forward until 2100 based on the IPCC’s climate pathways. The interdisciplinary model that, for the first time, combines international, national, and regional data shows an increase in water temperature in most regions of the world and at different depths and this could affect the quality of drinking water for millions of people. And not only that—the increased temperatures would also affect the extensive habitat of terrestrial groundwater, its ecosystems, and the surface water systems connected to it.

Warmer water already contains less oxygen. Increased microbial activity consumes the remaining oxygen turning the subsurface to low-oxygen and finally no-oxygen regimes. This makes it harder, or even impossible for many organisms to live in that environment. Griebler adds, “This starts a cascade of processes leading to the production of iron, manganese, sulphide, and arsenic species, and even the generation of methane. These severely affect the quality of groundwater for human consumption and when subterranean water mixes with surface water, they can disrupt those ecosystems too.”

Additionally, the warming of groundwater also affects the temperatures of the surface water to which it’s connected. Surface water usually warms quickly and cooler subterranean water seeping into rivers and lakes could help local species to find the lower temperatures they need—for example to spawn eggs. If the groundwater becomes warmer, this refuge is lost. This, in turn, can intensify the stress that climate change places on surface water ecosystems and harm biodiversity.

For an overview and future predictions, Griebler and his colleagues created a model to study the future development of groundwater temperatures on a global scale.

Among other factors, the model calculates the highest annual temperature at the minimum groundwater depth. It shows that in 2020, around 31 million people lived in areas where this temperature already exceeds 34°C - the maximum water temperature recommended in national drinking water guidelines around the world. Following the medium-warming scenario by 2100, these numbers will increase to between 77 million and 188 million depending on the depth of extraction. The extreme-warming scenario projects that between 59 million and 588 million people will be affected(depending on population distributions). 

“Increased groundwater temperatures can lead to the impairment of drinking water, the disruption of critical surface water ecosystems that provide food for millions of people, and even cause underground water distribution networks to heat up enabling the growth of pathogens like Legionella spp.,” warns Griebler. “While these increased groundwater temperatures hold some potential for heating applications, the negative effects outweigh the positive ones. This is especially relevant in developing countries or in poor and rural areas within developed countries where groundwater may be consumed directly without treatment or storage. In the simplest case, people would have to boil the water to make it safe but more complex treatments may be necessary depending on the kind of contamination.” These effects on groundwater contamination would increase the injustice between people causing the climate crisis and people bearing the consequences in lower-income regions of the world. 

Next to combating climate change itself, the main way to mitigate the warming of groundwater is to retain surplus water masses from heavy rains and floods in the landscape to allow its infiltration, as well as a reduction of surface sealing. Soil covered with concrete or asphalt stores more heat and traps it in the ground. Sealed surfaces also do not allow cool rainwater to seep in. Vegetation has a major cooling effect on the ground and under the surface. Griebler adds, “Next to the positive effects on biodiversity and rainwater management, unsealing surfaces is the biggest factor in how we can counteract the effects in climate change on groundwater. Additionally, subsurface infrastructure, especially in cities, create concentrated heat islands.”

In another recent project Griebler and his team studied the effects of surface sealing and pollution on groundwater in Vienna. They collected hundreds of samples all over the city to map biodiversity patterns and microbial and fauna community composition. Griebler says, “My work spans several disciplines and the Environment and Climate Research Hub helps me to connect to many colleagues in other fields.”