World Water Day 2022: Groundwater as the basis of life  

Groundwater is the most important resource for our drinking water. In fact, about 90% of drinking water in Bavaria is obtained from groundwater reservoirs via wells and springs. But streams and rivers also "live" from groundwater. It enters the streambed indirectly in varying quantities. If there is no rain for a long period of time, the groundwater component in the stream becomes increasingly important. As researchers from the AquaKlif project team show in a study, the proportion of groundwater in stream headwaters increases to up to 70% when water flow is low. Thus, ecosystems in such streams are increasingly dependent on groundwater inflows in low-flow situations.

This poses a problem that has only recently been explored. As important as the remaining water inflow into the streambed is during dry periods, it can also become problematic for life in the stream, depending on the local situation. If groundwater dominates the water balance of the stream, it has an impact on its oxygen content and thus on life in the stream. Many groundwater sources are low in oxygen and rich in iron, which reacts with the dissolved oxygen in the river water. The resulting iron oxide then visibly colours the stream a rusty brown. Especially in flowing waters where there is a high input of fine sediment, this can lead to the clogging of pores and strong oxygen consumption in the biologically very active exchange zone at the bottom of the stream. This so-called "hyporheic zone" at the bottom of the stream is key to the self-purification capacity of flowing waters and, moreover, to fish nursery and other stream organisms. The stream as a habitat is endangered when there is no oxygen left in the water. Life in the stream has no future if it has nowhere to grow.

These facts should be taken into account for the maintenance and protection of watercourses under changing climatic conditions. "A key goal of future watercourse management must be to prevent the input of fine sediments as much as possible," says Prof. Stefan Peiffer, head of the Department of Hydrology and AquaKlif spokesperson. "Soil eroded from surrounding fields during heavy rainfall exacerbates both the negative effects of higher temperatures in the streams and the oxygen depletion caused by increased groundwater levels. Our results from stream channel experiments show this very clearly."

Another fundamental approach that the researchers in the AquaKlif project agree on is that we have to start actively caring for the groundwater as a basis for life. For example, we must ensure that more rainwater seeps into the ground and thus supports groundwater recharge. In future, the design and management of fields and the management of watercourses, from roadside ditches to rivers, must be geared to this. All measures that allow precipitation to seep into the soil and run off more slowly are welcome. Settlement development can also be designed in a water-sensitive way, for example by greening roofs and façades, unsealing parking areas and by deliberately planning lower-lying areas as water retention areas during heavy rainfall. Heightening groundwater recharge is a key to being able to better cushion against future dry periods.