Zusammenfassung Die Temperatur des Grundwassers hat Auswirkungen auf dessen Qualität und beeinflusst damit unter anderem Grundwasserökosysteme und die Trinkwasserversorgung. Vielfach konnte eine Abhängigkeit der Grundwassertemperatur von der Oberflächentemperatur nachgewiesen werden. Der Fokus dieser Arbeit liegt auf der Ausweitung dieses Ansatzes von Einzelmessungen auf Monatsmittelwerte aus Zeitreihen, was als Voraussetzung gilt, um Erwärmungen von Grundwasserkörpern unter dem Einfluss des Klimawandels raumzeitlich verfolgen zu können. Das Oberrheinische Tiefland dient dabei als Untersuchungsgebiet, da es als grundwasserreiches Gebiet von großer Bedeutung für die regionale Trinkwassergewinnung ist und sich gleichzeitig in einer der wärmsten Regionen Deutschlands befindet. Hier wird der Zusammenhang zwischen satellitengestützten Messungen der Oberflächentemperatur mit den lokalen Grundwassertemperaturen über einen Zeitraum von fünf Jahren betrachtet. Konkret wurden Daten von 19 Grundwassermessstellen und Daten zur Lufttemperatur von vier nahegelegenen Klimastationen für den Zeitraum November 2016 bis April 2022 verwendet und mit Daten des MODIS Terra-Satelliten zur Landoberflächentemperatur kombiniert. Ausgeprägte saisonale Temperaturschwankungen konnten dabei bis in Tiefen von 9 m beobachtet werden, wobei die Temperatursignale gegenüber der Luft- und Landoberflächentemperatur durch einen mit der Tiefe zunehmenden zeitlichen Versatz von zwei bis acht Monaten geprägt waren. Der zeitliche Versatz gegenüber den Lufttemperaturen konnte bei insgesamt 15 Messstellen bestimmt und berücksichtigt werden, sodass sich für die Grundwassertemperaturen stark positive und hochsignifikante ( p < 0,001) Korrelationen sowohl mit der Lufttemperatur, als auch mit den lokalen Landoberflächentemperaturen ergaben. Somit können Monatsmittelwerte zu Luft- und Landoberflächentemperaturen zur Abschätzung von Temperaturentwicklungen der oberflächennahen Grundwasserkörper im Oberrheinischen Tiefland genutzt werden, was großräumige Einsatzmöglichkeiten auch für andere Gebiete eröffnet.

Abstract Quantifying the permissible technical groundwater heat pump (GWHP) use potential is important for informing policy decisions. This study introduces a transferable and efficient methodology to quantify the permissible technical GWHP use potential and its uncertainty using existing hydrogeological data. The methodology is applied to the deep aquifer in the Baar-Zug-Steinhausen area (Canton of Zug, Switzerland). To address the computational demands of strategies based on 3D numerical groundwater flow and heat transport simulations, the aquifer is classified into distinct hydrogeological “clusters”. This clustering is based on key hydrogeological parameters such as aquifer thickness, permeability, and hydraulic gradient. For each cluster, groundwater flow and heat transport are simulated for a single GWHP doublet in 3D box models with homogeneous properties and a constant hydraulic gradient. Small, medium, and large demands, including both balanced and unbalanced energy load profiles are considered. Thermal and hydraulic influence zones are delineated using 0.1 K isotherms and Darcy flow deviation from natural conditions. A spatial packing algorithm is then applied to place the influence zones within their cluster such that they are aligned with hydraulic gradient direction and treating them as non-overlapping hard boundaries. The permissible technical GWHP use potential is defined by the total number of GWHPs that can be accommodated within each cluster without interference. Uncertainty on the permissible technical potential is quantified through additional simulations. This approach also underscores the importance of quantifying and communicating the range of possible permissible technical heat use potentials to stakeholders, guiding future development towards optimized and environmentally sound groundwater heat use.

Abstract Accelerated by urbanization, as well as by agricultural intensification and climate change, increasing nutrient fluxes towards surface waters and increasing water temperatures are increasing the probability for cyanobacterial harmful algal blooms. Significant deterioration of water quality occurs with such events when microcystins (MCs) are released as the most prominent algal toxins. Despite the typical assumption that MCs are removed during percolation, studies have found significant concentrations in groundwater bodies. In this article, advances and challenges associated with monitoring and modeling techniques for characterizing the fate of MCs are discussed. Missing insights in the mechanisms leading to elevated MC concentrations in the subsurface—ultimately limiting the reliability of risk assessments—are identified. An important aspect is the a priori identification of environmental conditions and corresponding events that support bloom formation in surface waters and subsequent transport into groundwater. Challenges associated with required improved monitoring and simulation techniques are formulated. In particular, the role of environmental conditions in urban regions as major drivers for heat and nutrient emissions controlling bloom formation, potentially limiting MC retardation, needs to be further investigated. Based on existing experimental studies on MC reactive transport in porous media, existing monitoring methods need to be refined and combined with suitable modeling approaches. A thorough data analysis can then support efforts for sustainable water management of urban regions under threat by algal toxins.

Abstract As urbanization and transport demands rise, railway systems face higher risks from operational disruptions due to system failures and climate-related impacts, especially in urban areas. Understanding water flow and distribution beneath railway embankments is crucial not only for assessing potential contaminant transport in soil and groundwater but also for identifying possible structural weaknesses. This study implements numerical simulations to investigate these dynamics, focusing on a standard railway embankment designed according to German norms. The model setup includes ballast, a low-permeability protection subgrade surface layer (PSS), a subgrade (embankment base), and a subsurface to a depth of 10 m. Numerical simulations were conducted following a univariate approach. Variation parameters included key hydraulic characteristics of the embankment and subsurface. Further parameters were groundwater recharge rate, groundwater level, existence of a less permeable horizontal layer at multiple depths, and thickness of the embankment. The most sensitive parameters shaping water percolation fronts are identified as empirical coefficients (especially the Van Genuchten shape variable α), groundwater levels and hydraulic conductivity. The results indicate that the PSS layer retains the greatest amount of water from the rail system, particularly beneath the ballast. Low α values (0.8 m –1 to 7.7 m –1 ) led to total retention of water within the embankment, with no visible percolation fronts and the lowest values observed at the PSS layer. Shallow groundwater increased initial water content up to 0.30 at the PSS layer, rapidly approaching near fully water-saturated conditions. Embankments with low hydraulic conductivities (0.048 m d −1 to 1.207 m d −1 ) show the highest water contents, reaching up to 0.46, forming a clogging area prone to retain contaminants.

Abstract The cooling demand of cities is currently increasing due to urban heat island effects which could be covered with renewable energy to reach climate goals. An efficient solution is thermal cooling using groundwater. This approach involves introducing waste heat into the aquifer, which, at the same time, also raises some concerns among water authorities about potential impacts on groundwater quality. Hence, strict regulations were imposed on temperature spreading and maximum injection temperatures, limiting the thermal use of groundwater for cooling. However, regulations vary widely by country, because the influence of such temperature changes on groundwater conditions is still uncertain. In the present study, a summary of country-specific regulations is presented and evaluated with respect to possible impacts. Based on the review, a novel approach for the comparative assessment of the influence of different legal frameworks on city-wide relevant thermal impact areas is presented and implemented in three urban case studies in Germany, Switzerland and Spain, with various hydrogeological conditions. The results demonstrate that in the cities with higher hydraulic conductivities and strict regulations, the thermally impacted areas, defined by a 3K-isotherm, affect only a low percentage of the aquifer, comparable to those with less strict regulations. However, extensive thermal groundwater use by large systems, relatively high temperature spreading and aquifers with low hydraulic conductivities, lead to moderate percentages of impacted aquifer areas, especially by considering real operation data. The presented methodological approach provides a simplified tool to help city managers and local water authorities to define operational thresholds related to thermal-cooling use of groundwater.

Der Klimawandel sowie die deutliche Intensivierung der Untergrundnutzung durch den Menschen setzen hydrogeologische Systeme insbesondere im städtischen Raum zunehmend unter Druck. Thermische und chemische Kontaminationen, bauliche Eingriffe, veränderte Grundwasserneubildungsraten sowie Nutzungskonflikte sind nur einige wenige Beispiele für die Vielfältigkeit der Stressoren urbaner Grundwasserleiter. Angesichts eines prognostizierten urbanen Bevölkerungsanteils von global über 60 % bis zum Ende dieses Jahrzehnts und der veränderten Sicherheitslage steigt zugleich die Relevanz einer hochresilienten und nachhaltigen Bewirtschaftung der Wasserressourcen in diesen heterogenen und hochdynamischen Systemen. Dabei bringt die wachsende Bedeutung urbaner Grundwasserleiter als geothermische Energiequellen, als Speicherlösungen und für Ökosystemleistungen aber nicht nur neue Herausforderungen für Versorger, Planer, Behörden wie auch Wissenschaftseinrichtungen mit sich, sondern eröffnet auch wichtige Chancen für eine nachhaltige Stadtentwicklung. Das Themenheft "Urbane Hydrogeologie" greift diese Komplexität auf und zeigt auf, wie vielfältig und zugleich dringlich der wissenschaftliche sowie planerische Handlungsbedarf ist. Es bietet einen integrativen und differenzierten Überblick über aktuelle Erkenntnisse, methodische Weiterentwicklungen und zentrale Forschungslücken.