Increasing drought, less precipitation, increased demand for water in agriculture - climate change is taking its toll on our groundwater. In Germany and around the world, it is causing regional groundwater levels to drop. When the underground water level is low, polluted surface water from streams and rivers increasingly enters the groundwater. As a result, our drinking water and groundwater ecosystems are at risk, and the quantity problem is also becoming a quality problem. This is what researchers are currently describing in the scientific journal "Water Research". New research approaches and concepts for improving groundwater recharge are urgently needed and must be specifically tailored to the respective regions, they recommend.
We are seeing a direct consequence of climate change, which is endangering our most important water resource - groundwater," emphasizes Hans Jürgen Hahn of the University of Koblenz-Landau, one of the authors of the study. In many areas around the world, groundwater levels are increasingly falling as the rate of groundwater recharge also decreases. At the same time, groundwater withdrawals are increasing due to agricultural irrigation and for drinking water supplies. This results in an additional lowering of groundwater levels, and the landscape water balance changes - the climate impact spiral begins to turn faster and faster. As a result, we are at a tipping point in the landscape water balance in many places," explains co-author Anke Uhl from the Springs and Groundwater Working Group of the German Society for Limnology. Unlike in the past, the lower groundwater level means that in many places the groundwater no longer pushes upward and feeds streams and rivers (exfiltrates); instead, the water from the flowing waters now seeps into the subsurface (infiltrates). As a result of this pressure reversal, pollutants can enter the underground wetlands. This is because not only rainwater and spring water flow in the streams and rivers, but also the effluents from sewage treatment plants. We are increasingly enriching the groundwater with wastewater constituents - with residues of medicines, household chemicals, artificial sweeteners and other pollutants," explains Christian Griebler from the University of Vienna.
Another aspect is that the reversal of the direction of flow between surface water and groundwater is causing wetlands to dry out. ,,Since all current studies predict further declines in groundwater levels in large parts of the world, the problem will become even more acute in the future. As a result, we will be confronted with it especially in the increasingly dry summers," emphasizes Petra Döll of Goethe University in Frankfurt am Main.
The researchers based their conclusions on a worldwide literature study on the consequences of climate change, the effects of groundwater extraction on this resource, and on specialist articles on new pollutants in groundwater. These connections are obvious, but they have not yet been on the scientific radar," says Markus Weiler of the University of Freiburg, explaining the importance of the study’s findings.
Climate change takes place in different regions. Precipitation, groundwater recharge and the amount of groundwater abstraction vary from region to region, as does the degree of interaction between surface water and groundwater - the hydrogeological conditions.
In Austria, for example, the east, northeast and southeast are particularly affected. The areas affected in Germany are spread across the country: Among others, the regions of Upper Rhine, Middle Franconia, Allgäu, eastern Lower Saxony, western North Rhine-Westphalia and southern Hesse are affected, as well as large parts of the new German states.
Adapting concepts to local conditions
The study also shows above all that we need new scientific approaches and models at regional and local level to determine the interactions between surface water and groundwater and, above all, the tipping points in the landscape water balance," explains Markus Noack from Karlsruhe University of Applied Sciences. It is also clear that surface water must be further protected from pollution. This is because the condition of surface waters has direct consequences for the quality of groundwater. There is a solution for minimizing pollutants in the water cycle: "It is high time to reduce water consumption, both industrially and privately, so that less groundwater has to be pumped. In addition, it is important to drastically reduce the input of long-lived pollutants into the water cycle and to consistently expand fourth treatment stages in wastewater treatment plants," says Anke Uhl.
Uhl, A., Hahn, H.J., Jager, A., Luftensteiner, T., Siemensmeyer, T., Doll, P., Noack, M., Schwenk, K., Berkhoff, S., Weiler, M., Karwautz, C., Griebler, C (2022). Making waves: pulling the plug - Climate change effects will turn gaining into losing streams with detrimental effects on groundwater quality, Water Research Volume 220. https://doi.org/10.1016/j.watres.2022.118649