MOSAIC 2.0 used isotope hydrology to further understand water partitioning in the city of Berlin. A key insight has been the increased vulnerability of Berlin to drought and water scarcity as the population continues to increase and the consequences of climate change become more apparent. A related issue is the growing importance of re-using treated wastewater to augment river flows and maintain wetlands in the city as the 2018-2022 drought resulted in a prolonged decline in groundwater levels. Effluent from wastewater treatment plants provides most of the flow in many of Berlin’s waterways. Without this augmentation, given the increased frequency and longevity of droughts, rivers like the Panke and Erpe would likely cease to flow in summer. Treated wastewater can also be successfully used to maintain managed urban wetlands habitats that have high biodiversity and amenity value. Our work at Hobrechtsfelde near Buch shows that irrigating wetlands with treated wastewater provides a Nature-based Solution that provides further improvements in water quality along with wider environmental benefits. Isotopic studies show that water flowing into the city in the rivers Spree and Havel which is extracted by bank filtration and sustains public water supplies is the dominant source of urban waste water. Local groundwater is a smaller secondary component.
Rain falling in Berlin is mostly (60%) evaporated back to the atmosphere. Urban green spaces play a key role in this process, with trees and forest areas particularly important as they can tap deeper, older water to sustain transpiration. Only around 20% of rainfall contributes to groundwater recharge. Around 15% of rainfall in Berlin is routed from impervious surfaces via storm drains to streams; isotopic data shows that this dominates flood events in local rivers. This surface runoff can be potentially reduced by a range of engineered storm control measures forming Nature-based Solutions such as ponds, wetlands and infiltration trenches. Such measures are becoming increasingly widespread through urban re-development projects. Our isotope-based modelling studies show that virtually all of this water in storm control measures will contribute to increased evaporation. Groundwater recharge will only be enhanced if measures are designed to route drainage below the rooting zone. Nature-based Solutions for managing storm runoff can create valuable wetland habitats in urban areas. However, the biodiversity of such features depends on the quality of inflow waters.