Rising temperatures may impact groundwater quality

As the world’s largest unfrozen freshwater resource, groundwater is crucial for life on Earth. Now, a team of researchers has investigated how global warming is affecting groundwater temperatures and what that means for humanity and the environment ¹. The study indicates that by 2100, more than 75 million people are likely to be living in regions where the groundwater temperature exceeds the highest threshold set for drinking water by any country.

Earth’s climate system is heating up due to the atmosphere’s increased concentration of greenhouse gases, which limits the amount of heat that can be radiated away. The oceans absorb a substantial fraction of this heat, but soil and groundwater also act as heat sinks. However, little is known thus far about the effects Earth’s surface warming has on groundwater over space and time. “To close this gap, we have simulated the projected changes in global groundwater temperatures through 2100,” said Dr. Susanne Benz from the Institute of Photogrammetry and Remote Sensing at Karlsruhe Institute of Technology (KIT), which prepared the study in cooperation with Dr. Kathrin Menberg and Professor Philipp Blum from the Institute of Applied Geosciences at KIT. “We can provide maps showing global groundwater temperatures at various depths beneath Earth’s surface. The maps show that the world’s highest water warming rates can be expected at locations with a shallow groundwater table and/or high atmospheric warming.”

The researchers based their projections on the SSP 2–4.5 and SSP 5–8.5 climate scenarios. These scenarios reflect different socioeconomic development pathways and different trends in the concentration of atmospheric greenhouse gases in the future. SSP 2–4.5 is in the middle range of possible future greenhouse gas concentration trends; SSP 5–8.5 is at the upper extreme.

Millions affected by warmer drinking groundwater

The study indicates that by 2100, groundwater temperatures will rise by 2.1 degrees Celsius in the SSP 2–4.5 scenario and by 3.5 degrees Celsius in the SSP 5–8.5 scenario. “There are already about 30 million people living in regions where the groundwater is warmer than stipulated in the strictest drinking water guidelines. That means it may not be safe to drink the water there without treatment. It may need to be boiled first, for example. The drinking water also gets warmed up in water pipes by heat in the ground,” Benz noted. “Depending on the scenario, as many as several hundred million people could be affected by 2100.” According to the study, the figure would be 77 to 188 million people for SSP 2–4.5 and 59 to 588 million for SSP 5–8.5. The broad ranges are due to spatial variations in climate change and population trends. The researchers projected the lowest warming rates in mountainous regions with deep water tables, such as the Andes and the Rocky Mountains.

Temperature Changes Affect Ecosystems

The temperature of groundwater plays a crucial role in water quality by influencing a number of chemical, biological and physical processes. “Under certain conditions, rising water temperatures can lead to increasing concentrations of harmful substances like arsenic or manganese. These higher concentrations can have a negative impact on human health, especially when groundwater is used as drinking water,” Benz said, adding that warmer water also affects groundwater-dependent ecosystems, aquatic biogeochemical processes, geothermal energy potential, and thermal regimes in rivers. It can also pose a challenge to biodiversity and a risk to carbon and nutrient cycles.

A further effect of increased near-surface and groundwater temperatures is that critical thresholds can be exceeded in water distribution networks. This could have human health implications, e.g. by promoting the growth of pathogens such as Legionella spp. Fish species, especially salmon, are also affected by the changing conditions. Their reproduction could be endangered if groundwater-dependent spawning grounds in rivers become too warm. “Our results show how important it is to take action to protect groundwater and find lasting solutions to counteract the negative impact of climate change on groundwater,” Benz said.