Climate change presents new challenges for the supply of drinking water

The reservoir Rappbode in the Harz region is the largest drinking water reservoir in Germany, which supplies drinking water to about one million people in areas including the Halle region and the southern part of the state of Saxony-Anhalt. Water temperatures in the basin now have the potential to rise significantly due to climate change. If average global warming reaches between 4 and 6 degrees by 2100, as the current trend suggests, the temperature conditions in the Rappbode basin will become comparable to those of Lake Garda and other lakes south of the Alps. article in Total environmental science magazine, a team of researchers led by the Helmholtz Center for Environmental Research (UFZ) writes that reservoir operators could partially offset the impacts this will have on the drinking water supply – to do so, they would have to change the way the reservoir is managed.

The impacts of climate change can already be seen in the Rappbode reservoir: over the past 40 years, the water surface temperature in the reservoir has increased by around 4 degrees in the summer months. This trend could continue, as has now been demonstrated by a team of researchers led by Dr Karsten Rinke, who researches lakes at UFZ. Working on the basis of a lake model developed by US researchers, the team considered potential reservoir management strategies to predict the impacts that climate change could have on the water temperature and physical structure of the lake, which they control. the stratification and seasonal mixing of the body. of water. Their research looked at three scenarios for future greenhouse gas emissions. The so-called “representative concentration paths” (RCPs) describe whether greenhouse gas emissions will be stopped (RCP 2.6), will continue to increase (RCP 6.0) or even continue to increase non-stop (RCP 8.5) by 2100. According to the Intergovernmental Panel on Climate Change IPCC, the latter case would result in average global warming of over 4 degrees by the end of this century.

For RCP 2.6 and RCP 6.0 scenarios, the study authors predicted that the average water surface temperature of the Rappbode Reservoir should increase by 0.09 degrees or 0.32 degrees, respectively, every decade by 2100. This would correspond to a total increase of about 0.7 degrees (RCP 2.6) and about 2.6 degrees (RCP 6.0) by the end of this century. As expected, the rise in temperatures would be the highest in the RCP 8.5 scenario, which would see the water temperature rise by 0.5 degrees every decade or approx. 4 degrees by 2100.

However, in terms of drinking water use, what happens in the deeper layers of the reservoir, i.e. at a depth of 50 meters and below, is more serious, as this is where the raw water is withdrawn before being treated for prepare it as drinking water. It is true that the impacts by 2100 would be relatively minor in RCP 2.6 and RCP 6.0 scenarios, as the water temperature would continue to be around 5 degrees all year round. However, water temperatures will rise significantly in the RCP 8.5 scenario, by nearly 3 degrees by the end of the century. As a result, the water in the depths of the tank would heat up to about 8 degrees. “This would transform a reservoir in Germany’s northernmost highlands into a body of water comparable to Lake Maggiore or Lake Garda today,” says UFZ scientist Rinke. An increase of this magnitude would have consequences because it would greatly accelerate the speed of biological metabolic processes.

“A rise in temperature to 8 degrees almost doubles the oxygen demand, which is the amount of oxygen that organisms consume during their breathing and degradation processes,” says lead author Chenxi Mi, who is focusing on climate impacts on the Rappbode basin in his doctorate at the UFZ. Increased oxygen consumption will put further pressure on the oxygen balance of the water, as the duration of summer stagnation, the stage of stable temperature stratification in lakes where deep water is closed to oxygen supply by atmosphere, is already extending due to climate change. Plus, the warmer water isn’t even able to absorb as much oxygen. Potential consequences include an intensified dissolution of dissolved nutrients and metals from the sediment, algae growth and an increase in blue-green algae.

In other words, scenario 8.5 would have an impact on the supply of drinking water if it were to occur. The reservoir operators draw raw water from the lower layers for good reason, as the water is cold and contains only low levels of suspended matter, dissolved metals, algae, bacteria and potentially pathogenic microorganisms. If the oxygen content there decreases more rapidly due to rising water temperature, the risk of contamination increases, for example due to substances released from the sediment and increased bacterial growth. Water treatment would therefore require a greater effort on the part of operators, who would have to face greater demands in terms of the treatment capacity they would have to reserve. “This means that preventing deep sea warming is also beneficial from a drinking water supply perspective, and the ideal way to do this is ambitious climate policies that limit warming,” says Rinke.

But the operators are not entirely powerless against the warming of the basin’s deep waters. Simulations of the model developed by Rinke’s team show that some of the heat can be exported using an intelligent system to draw water. This has to do with the water that is released into the downstream waters, i.e. the water that is withdrawn and drained into the water course below the tank in order to keep the discharge conditions stable. This so-called downstream drain should be withdrawn not from the lower layers as it has been hitherto, but rather close to the surface.

“This approach would allow the additional heat caused by climate change to be released again,” Rinke explains. However, he adds, it would be impossible to prevent deep water heating if the air temperature rises above 6 degrees. “Even though operators have had to deal more with water shortages due to the very dry years we have had recently, it is equally important to think about water quality. In terms of reservoir management, we certainly have options and can respond to new conditions caused by climate change. In this way, we can alleviate some negative impacts through climate adaptation measures “.

The operators of the Rappbode reservoir of the Talsperrenbetrieb Sachsen-Anhalt company know this. They worked closely with Karsten Rinke and his team of UFZ researchers for many years to assess the impacts of climate change and discussed potential options for adapting the Rappbode reservoir. The Talsperrenbetrieb is already planning new infrastructures that will allow the implementation of the new management strategies.