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Forty years of temperature data recorded in two German cities show groundwater is not just heating up, it’s mirroring the same warming pattern seen in the atmosphere.

“Global warming is reflected directly in the groundwater, albeit damped and with a certain time lag,” says Peter Bayer, a senior assistant at ETH Zurich’s Geological Institute.

Bayer and colleagues were able to access uninterrupted temperature measurements of groundwater flows around the cities of Cologne and Karlsruhe, where the operators of the local waterworks have been measuring the temperature of the groundwater, which is largely uninfluenced by humans, for 40 years.

“For us, the data was a godsend,” says Bayer.

Even with some intensive research, they would not have been able to find a comparable series of measurements. Evidently, it is less interesting or too costly for waterworks to measure groundwater temperatures systematically for a lengthy period of time.

“Or the data isn’t digitalized and only archived on paper,” he explains.

Temperature leaps

The data also reveals that the groundwater close to the surface down to a depth of around 60 meters (197 feet) has warmed up statistically significantly in the course of global warming over the last 40 years.

This water heating follows the warming pattern of the local and regional climate, which in turn mirrors that of global warming.

The groundwater reveals how the atmosphere has made several temperature leaps at irregular intervals. These “regime shifts” can also be observed in the global climate, as the researchers write in their study published in the journal Hydrology and Earth System Sciences.

Bayer was surprised at how quickly the groundwater responded to climate change.

Earlier studies

The Earth’s atmosphere has warmed up by an average of 0.13 degrees Celsius per decade in the last 50 years. And this warming doesn’t stop at the subsoil, either, as other climate scientists have demonstrated in the last two decades with drillings all over the world.

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However, the researchers only tended to consider soils that did not contain any water or where there were no groundwater flows.

While the fact that the groundwater has not escaped climate change was revealed by researchers from Eawag and ETH Zurich in a study published three years ago, it only concerned “artificial” groundwater.

In order to enhance it, river water is trickled off in certain areas. The temperature profile of the groundwater generated as a result thus matches that of the river water.

The new study, however, examines groundwater that has barely been influenced by humans. According to Bayer, it is plausible that the natural groundwater flow is also warming up in the course of climate change.

“The difference in temperature between the atmosphere and the subsoil balances out naturally.”

The energy transfer takes place via thermal conduction and the groundwater flow, much like a heat exchanger, which enables the heat transported to spread in the subsoil and level out.

Negative and positive effects?

The consequences of these findings, however, are difficult to gauge. The warmer temperatures might influence subterranean ecosystems on the one hand and groundwater-dependent biospheres on the other, which include cold areas in flowing waters where the groundwater discharges.

For cryophilic organisms such as certain fish, groundwater warming could have negative consequences.

Higher groundwater temperatures also influence the water’s chemical composition, especially the chemical equilibria of nitrate or carbonate. After all, chemical reactions usually take place more quickly at higher temperatures.

Bacterial activity might also increase at rising water temperatures. If the groundwater becomes warmer, undesirable bacteria such as gastrointestinal disease pathogens might multiply more effectively. However, the scientists can also imagine positive effects.

“The groundwater’s excess heat could be used geothermally for instance,” adds Kathrin Menberg, the first author of the study.

Source: ETH Zurich