Effective decision-making in urban water management requires integrating outputs from specialized models. Berlin’s drinking water supply relies on induced bank filtration and managed aquifer recharge from the Spree and Havel rivers. However, river inflows into Berlin are declining -e.g., in summer 2019, the Spree’s inflow was half that of an average dry summer year- and are expected to decrease further over the next decade due to the ending of coal sump water discharge into the Spree. Long-term impacts from climate change are anticipated to exacerbate this trend. Additionally, an analysis of streamflow data and bank filtrate rate-corrected groundwater extraction has identified regions where maximum monthly extractions from drinking water wells already exceed the lowest monthly river flows in Berlin. This imbalance, combined with increasing water demand driven by population growth, leads to a higher proportion of treated wastewater in Berlin’s streams. As a result, risks to drinking water quality intensify, and the complexity and costs of water and wastewater treatment escalate. Furthermore, higher extraction levels are associated with increased bank filtrate fractions, amplifying system stress and emphasizing the need for sustainable water management practices.

In collaboration with the Belin Waterworks (Berliner Wasserbetriebe), we applied a well-calibrated FEFLOW© model of the Berlin-Friedrichshagen waterworks to simulate bank filtrate rates under various recharge and groundwater extraction scenarios. The model was run under three historical well configurations (2010, 2015, and 2019) and then well pumping rates were adjusted in the same relative configuration under three groundwater recharge scenarios.

A review of prior investigations revealed groups of well galleries exhibiting similar changes in bank filtrate fractions in response to extraction levels; our results complement these former investigations. Bank filtrate behavior across well galleries was found to depend on several factors, including well depth, distance to the riverbanks, the presence of opposing riverbanks, and regional groundwater heads. Relating bank filtrate change groups to site characteristics and bank filtrate fractions in other Berlin develops a city-wide understanding of changes in bank filtrate.

Future FEFLOW© modeling scenarios, including commissioning and decommissioning of well galleries, and implementing managed aquifer recharge will be essential to address remaining uncertainties. Outputs from this modeling effort contribute to regional dynamic water balance modeling for Berlin’s semi-closed water cycle in order to support sustainable water management decision-making amid evolving climatic and regulatory challenges.

The trophic index is often used to monitor the primary production of lakes. In Brandenburg, Germany, lakes are sampled several times every three years between April and October. The trophic index is then calculated from the values for phosphorus concentration, turbidity and chlorophyll-a content. This is usually only done for lakes that are monitored according to the Water Framework Directive (area > 50 ha). The low temporal resolution in combination with natural annual variations makes trend analysis of trophic levels very difficult and a high proportion of lakes are excluded from this monitoring.
Satellite images can be used to obtain information on chlorophyll-a and turbidity. Phosphorus, as a nutrient for algae, also has an indirect effect on water color. There are already many indices based on the Copernicus Sentinel-2 program, such as the Normalized Difference Chlorophyll Index, which can be used for real-time water monitoring. In addition, annual data are essential for lake management to identify long-term trends. The trophic index is a widely used and easily interpreted indicator in this regard.
The AD4GD project explored i) which bands of the Sentinel-2 images are best suited for estimating trophic state, ii) how the data can be temporally aggregated within a season, and iii) whether one pixel within a lake is sufficient to reliably describe the trophic state of the lake. Especially the latter was necessary to apply the method to small lakes where regular monitoring is not available.
The developed Normalized Difference Trophic Index (NDTI), aggregated over the months of April to October, best represented the trophic index based on measured values. It was developed and validated using 294 lakes in Brandenburg with trophic data between 2018 and 2022 and is defined for a satellite image as
NDTI_image=(B5-B2)/(B5+B2)
Band 5 describes the near infrared reflectance at 705 nm, band 2 the reflectance of blue light at 490 nm. In oligotrophic lakes, band 2 reflectance usually dominates and the index is below zero. The trophic index based on in-situ measurements is best calculated from monthly values. Similarly, NDTIimage is first averaged monthly and then seasonally (April to October in Germany).
The resulting NDTIseason was found to be highly correlated with the in-situ data for the available years (Pearson correlation coefficient between 0.83 and 0.92). Thus, it allows a comparison of the trophic state of lakes in the Brandenburg region. The data are available at an annual resolution, which is three times more frequent than the conventional analysis. This allows a much more reliable trend analysis, which can be used to monitor the success of water quality improvement measures or to identify water quality problems more quickly. Small lakes can be included in the monitoring without much effort.
A first sensitive analysis has shown that the classification of eutrophic water bodies is more reliable than that of oligotrophic water bodies. Further factors influencing the accuracy of the method will be investigated in a subsequent sensitivity analysis.

Per- und polyfluorierte Alkylsubstanzen (PFAS) stellen auf-grund ihrer Persistenz und Toxizität ein wachsendes Risiko für Wasser-ressourcen dar. In einer achtmonatigen Messkampagne wurde Regen-wasserabfluss eines Berliner Industriegebiets auf 26 PFAS und andere Industriechemikalien untersucht. Zusätzlich wurde ein urbaner See beprobt, der ausschließlich durch Regenwasserabfluss und Grundwasser gespeist wird. PFAS-Konzentrationen im Regenwasserabfluss lagen zwischen 5 und 35 ng/L, PFOA und PFHxA waren am häufigsten nachweisbar. Die Konzen-trationen lagen im Bereich vorgeschlagener Umweltqualitätsnormen für Oberflächengewässer mit Maximalwerten deutlich darüber. Im See wurden deutlich höhere Konzentrationen (bis 99 ng/L) gemessen, die vermutlich durch Altlasten des benachbarten Flughafens und nicht primär durch Regenwasserabfluss verursacht werden. Im Vergleich zu Kläranlagenab-läufen waren die gemessenen PFAS-4-Konzentrationen im Regenwasser-abfluss in dieser Studie um den Faktor 3-10 niedriger. Für Gewässer sind Kläranlagenabläufe auch durch die größeren Volumina als Eintragspfad von PFAS wahrscheinlich von größerer Relevanz als Regenwasserabflüsse. Dennoch ist Regenwasserabfluss insbesondere in Schwammstadt-konzepten mit Versickerungssystemen als potentiell relevanter Eintrags-pfad für PFAS zu betrachten. Die Ergebnisse zeigen die Notwendigkeit eines besseren Verständnisses urbaner PFAS-Quellen für ein effektives Wasserschutzmanagement.